HomeMy WebLinkAboutSLO Basin Public Draft GSPDRAFT
Public Draft of the
San Luis Obispo Valley Basin Groundwater Sustainability Plan
Available for viewing in the August 18, 2021 Agenda Packet: August 12, 2021
Recommended the GSAs to receive and file for public comments:August 18, 2021
Available for public comments on www.slowaterbasin.com: August 19, 2021
Close of public comment period: September 19, 2021
Per the GSC's recommendation on August 18, 2021, the Public Draft of the GSP will be
distributed to the City and County GSAs to receive and file. This draft document is now
posted on the web portal: www.slowaterbasin.com for public comments. Comments from
the public are being collected using a comment form available at www.slowaterbasin.com
by clicking on "Submit Comment". If you require a paper form to submit by postal mail,
please contact your local Groundwater Sustainability Agency (GSA). All comments
submitted will also be posted online for viewing.
San Luis Obispo
Valley Basin
Groundwater
Sustainability Plan
Public Draft
AUGUST 2021 SAN LUIS OBISPO VALLEY GROUNDWATER BASIN GROUNDWATER SUSTAINABILITY AGENCIES
S AN LUIS OBISPO VALLEY GROUNDWATER
BASIN GROUNDWATER SUSTAINABILITY
AGENCIES
San Luis Obispo Valley
Basin Groundwater
Sustainability Plan
AUGUST 2021
Prepared by Water Systems Consulting, Inc.
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies i
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
TABLE OF CONTENTS
List of Figures .................................................................................................................................................... vii
List of Tables ...................................................................................................................................................... xi
Acronyms & Abbreviations ............................................................................................................................ xiii
Executive Summary ........................................................................................................................................1-1
Introduction ............................................................................................................................................... 2
Plan Area .................................................................................................................................................. 2
Outreach Efforts ...................................................................................................................................... 4
Basin Setting ............................................................................................................................................. 5
Groundwater Conditions ........................................................................................................................ 6
Water Budget ....................................................................................................................................... 10
Monitoring Network ............................................................................................................................. 11
Sustainable Management Criteria .................................................................................................... 14
Projects and Management Actions .................................................................................................... 16
Implementation Plan............................................................................................................................. 18
1 Introduction To The SLO Basin GSP ........................................................................................................1-1
1.1. Purpose of the Groundwater Sustainability Plan .....................................................................1-2
1.2. Description of the SLO Basin ........................................................................................................1-2
1.3. Basin Information ............................................................................................................................1-2
2 Agency Information (§ 354.6) .................................................................................................................2-1
2.1. Agencies Names and Mailing Addresses ...................................................................................2-2
2.2. Agencies Organization and Management Structures ..............................................................2-4
2.2.1. County of San Luis Obispo ....................................................................................................2-5
2.2.2. City of San Luis Obispo ..........................................................................................................2-5
2.2.3. Other Participating Parties in the MOA ..............................................................................2-5
2.3. Authority of Agencies .....................................................................................................................2-6
2.3.1. Groundwater Sustainability Agencies .................................................................................2-6
2.3.2. Memorandum of Agreement .................................................................................................2-6
2.3.3. Coordination Agreements ......................................................................................................2-7
2.4. Contact Information for Plan Manager ......................................................................................2-7
2.5. Notices and Communications (§ 354.10) ....................................................................................2-7
3 Description of Plan Area (§ 354.8) ........................................................................................................3-1
3.1. SLO Basin Information....................................................................................................................3-2
3.2. Adjudicated Areas .........................................................................................................................3-2
3.3. Jurisdictional Areas ........................................................................................................................3-2
3.3.1. Federal Jurisdictions ................................................................................................................3-3
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3.3.2. Tribal Jurisdiction .....................................................................................................................3-3
3.3.3. State Jurisdiction ......................................................................................................................3-3
3.3.4. County Jurisdiction ...................................................................................................................3-3
3.3.5. City and Local Jurisdictions ....................................................................................................3-3
3.3.6. Special Districts ........................................................................................................................3-3
3.4. Land Use ...........................................................................................................................................3-4
3.4.1. Water Source Types ...............................................................................................................3-7
3.4.2. Water Source Sectors .............................................................................................................3-9
3.5. Density of Wells........................................................................................................................... 3-11
3.6. Existing Monitoring and Management Programs .................................................................. 3-16
3.6.1. Service Area Population ..................................................................................................... 3-16
3.6.2. Existing Management Plans ................................................................................................ 3-22
3.6.3. Existing Groundwater Regulatory Programs................................................................... 3-23
3.7. Conjunctive Use Programs .......................................................................................................... 3-25
3.8. Land Use Plans ............................................................................................................................. 3-25
3.8.1. Service Area Population ..................................................................................................... 3-26
3.8.2. County of San Luis Obispo General Plan ........................................................................ 3-28
3.8.3. Los Ranchos/Edna Village Plan ......................................................................................... 3-31
4 Basin Setting (§354.14) ............................................................................................................................4-1
4.1. Introduction ......................................................................................................................................4-2
4.2. Basin Topography and Boundaries .............................................................................................4-2
4.3. Primary Users of Groundwater ....................................................................................................4-9
4.4. Soils Infiltration Potential ...............................................................................................................4-9
4.5. Regional Geology ....................................................................................................................... 4-11
4.5.1. Regional Geologic Structures ............................................................................................. 4-11
4.5.2. Geologic Formations within the Basin ............................................................................... 4-11
4.5.3. Geologic Formations Surrounding the Basin .................................................................... 4-15
4.6. Principal Aquifers and Aquitards ............................................................................................. 4-16
4.6.1. Cross Sections ........................................................................................................................ 4-16
4.6.2. Aquifer Characteristics ........................................................................................................ 4-33
4.6.3. Aquitards ................................................................................................................................ 4-34
4.7. Surface Water Bodies ................................................................................................................ 4-38
4.8. Subsidence Potential ................................................................................................................... 4-38
5 Groundwater Conditions (§354.16) .......................................................................................................5-1
5.1. Groundwater Elevations and Interpretation ..............................................................................5-2
5.1.1. Fall 1954 Groundwater Elevations ......................................................................................5-2
5.1.2. Spring 1990 Groundwater Elevations ................................................................................5-2
5.1.3. Modeled 1990s Groundwater Elevations ..........................................................................5-3
5.1.4. Spring 1997 Groundwater Elevations ................................................................................5-6
5.1.5. Spring 2011 Groundwater Elevations ................................................................................5-8
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5.1.6. Spring 2015 Groundwater Elevations ................................................................................5-8
5.1.7. Spring 2019 Groundwater Elevations ................................................................................5-8
5.1.8. Fall 2019 Groundwater Elevations ......................................................................................5-8
5.1.9. Changes in Groundwater Elevation .................................................................................. 5-13
5.1.10. Vertical Groundwater Gradients .................................................................................... 5-17
5.2. Groundwater Elevation Hydrographs ..................................................................................... 5-17
5.3. Groundwater Recharge and Discharge Areas ...................................................................... 5-20
5.3.1. Groundwater Recharge Areas ........................................................................................... 5-20
5.3.2. Groundwater Discharge Areas .......................................................................................... 5-24
5.4. Change in Groundwater Storage ............................................................................................ 5-25
5.5. Seawater Intrusion ....................................................................................................................... 5-25
5.6. Subsidence .................................................................................................................................... 5-25
5.7. Interconnected Surface Water .................................................................................................. 5-25
5.7.1. Depletion of Interconnected Surface Water ................................................................... 5-28
5.8. Potential Groundwater Dependent Ecosystems ..................................................................... 5-29
5.8.1. Hydrology .............................................................................................................................. 5-29
5.8.2. Vegetation and Wetland Groundwater Dependent Identification ............................ 5-32
5.8.3. Identification of Special Status Species and Sensitive Natural Communities
Associates with GDEs ................................................................................................................ 5-34
5.9. Groundwater Quality Distribution and Trends....................................................................... 5-35
5.9.1. Groundwater Quality Suitability for Drinking Water ................................................... 5-35
5.9.2. Distribution and Concentrations of Point Sources of Groundwater
Constituents ................................................................................................................................. 5-35
5.9.3. Distribution and Concentrations of Diffuse or Natural Groundwater
Constituents ....................................................................................................................................5-0
6 Water Budget (§354.18) .........................................................................................................................6-1
6.1. Introduction ......................................................................................................................................6-2
6.2. Climate .......................................................................................................................................... 6-16
6.2.1. Historical Climate/Base Period .......................................................................................... 6-16
6.3. Water Budget Data Sources ..................................................................................................... 6-21
6.4. Historical Water Budget ............................................................................................................ 6-21
6.4.1. Historical Time Period .......................................................................................................... 6-22
6.4.2. Historical Land Use ............................................................................................................... 6-22
6.4.3. Historical Surface Water Budget ...................................................................................... 6-25
6.4.4. Components of Surface Water Outflow .......................................................................... 6-29
6.4.5. Historical Groundwater Budget ......................................................................................... 6-32
6.4.6. Total Groundwater in Storage .......................................................................................... 6-43
6.4.7. Change in Storage ............................................................................................................... 6-48
6.4.8. Preliminary Sustainable Yield Estimate ............................................................................ 6-51
6.4.9. Quantification of Overdraft ............................................................................................... 6-52
6.5. Current Water Budget ................................................................................................................ 6-53
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6.6. Projected Water Budget ............................................................................................................ 6-63
6.6.1. Assumptions Used in Future Water Budget Development ............................................. 6-63
6.6.2. Projected Future Water Budget ........................................................................................ 6-64
7 Monitoring Networks (§354.32 & §354.34) .........................................................................................7-1
7.1. Introduction ......................................................................................................................................7-2
7.2. Monitoring Objectives ....................................................................................................................7-2
7.2.1. Management Areas.................................................................................................................7-2
7.2.2. Representative Monitoring Sites ...........................................................................................7-2
7.2.3. Scientific Rationale ..................................................................................................................7-3
7.2.4. Existing Monitoring Programs ................................................................................................7-4
7.2.5. Groundwater Level Monitoring Network ............................................................................7-4
7.3. Monitoring Networks ......................................................................................................................7-4
7.3.1. Groundwater Level Monitoring Network ............................................................................7-5
7.3.2. Groundwater Quality Monitoring Network ........................................................................7-8
7.3.3. Surface Water Flow Monitoring Network ....................................................................... 7-11
7.4. Sustainability Indicator Monitoring ........................................................................................... 7-14
7.4.1. Chronic Lowering of Groundwater Levels ........................................................................ 7-14
7.4.2. Reduction of Groundwater Storage ................................................................................. 7-14
7.4.3. Seawater Intrusion ................................................................................................................ 7-15
7.4.4. Degraded Groundwater Quality ...................................................................................... 7-15
7.4.5. Land Subsidence ................................................................................................................... 7-16
7.4.6. Depletion of Interconnected Surface Water ................................................................... 7-16
7.5. Monitoring Technical and Reporting Standards .................................................................... 7-18
7.5.1. Groundwater Levels ............................................................................................................. 7-18
7.5.2. Groundwater Quality .......................................................................................................... 7-18
7.5.3. Surface Water Flow ............................................................................................................ 7-18
7.5.4. Monitoring Frequency .......................................................................................................... 7-19
7.6. Data Management System ........................................................................................................ 7-19
7.7. Assessment and Improvement of Monitoring Network .......................................................... 7-19
7.8. Annual Reports and Periodic Evaluation by the GSAs ......................................................... 7-20
8 Sustainable Management Criteria (§354.22) ......................................................................................8-1
8.1. Introduction ......................................................................................................................................8-2
8.2. Definitions (§351) ...........................................................................................................................8-3
8.3. Sustainability Goal (§354.24) .....................................................................................................8-4
8.3.1. Description of Sustainability Goal .......................................................................................8-5
8.3.2. Sustainability Strategy ...........................................................................................................8-5
8.4. Generalized Process for Establishing Sustainable Management Criteria
(§354.22-30) .............................................................................................................................8-5
8.5. Chronic Lowering of Groundwater Levels Sustainability Indicator .................................... 8-12
8.5.1. Undesirable Results (§354.26) ........................................................................................... 8-12
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8.5.2. Minimum Thresholds (§354.28(c)(1)) ................................................................................. 8-13
8.5.3. Measurable Objectives §354.30(a)(g)............................................................................. 8-19
8.6. Reduction of Groundwater Storage Sustainability Indicator §354.28(c)(2) .................... 8-21
8.6.1. Undesirable Results .............................................................................................................. 8-21
8.6.2. Minimum Thresholds §354.28(c)(2) .................................................................................... 8-22
8.6.3. Measurable Objectives §354.30(a)(g)............................................................................. 8-25
8.7. Seawater Intrusion Sustainability Indicator §354.28(c)(3) ................................................... 8-25
8.8. Degradation of Groundwater Quality Sustainability Indicator §354.28(c)(4) ............... 8-25
8.8.1. Undesirable Results §354.26(a)(d).................................................................................... 8-26
8.8.2. Minimum Thresholds §354.28(c)(4) .................................................................................... 8-27
8.8.3. Measurable Objectives §354.30(a)(g)............................................................................. 8-30
8.9. Land Subsidence Sustainability Indicator §354.28(c)(5) ...................................................... 8-30
8.9.1. Undesirable Results §354.26(a)(d).................................................................................... 8-30
8.9.2. Minimum Thresholds §354.28(c)(5) .................................................................................... 8-31
8.9.3. Measurable Objectives §354.30(a)(g)............................................................................. 8-33
8.10. Depletion of Interconnected Surface Water Sustainability Indicator
§354.28(c)(6) .......................................................................................................................... 8-34
8.10.1. Undesirable Results §354.26(a)(d) ................................................................................. 8-37
8.10.2. Minimum Thresholds §354.28(c)(6) ................................................................................. 8-38
8.10.3. Measurable Objectives ..................................................................................................... 8-41
8.11. Management Areas .................................................................................................................. 8-42
9 Projects and Management Actions (§354.44) ......................................................................................9-1
9.1. Introduction ......................................................................................................................................9-2
9.2. Overview of Potential Projects and Management Actions .....................................................9-3
9.2.1. Project and Management Actions Development ................................................................9-3
9.2.2. Addressing Sustainability Indicators (§ 354.44 (1))..........................................................9-9
9.2.3. Overdraft Mitigation (§ 354.44 (2)) ...................................................................................9-9
9.3. Integrated Surface Water and Groundwater Modeling ..................................................... 9-10
9.4. Projects .......................................................................................................................................... 9-12
9.4.1. State Water Project for Agricultural Irrigation .............................................................. 9-12
9.4.2. City of SLO Recycled Water for Agricultural Irrigation ............................................... 9-16
9.4.3. State Water Project Recharge Basin ................................................................................ 9-19
9.4.4. State Water Project to Golden State Water Company .............................................. 9-23
9.4.5. Varian Ranch Mutual Water Company Arroyo Grande Subbasin Wells ................. 9-26
9.4.6. State Water Project to the Mutual Water Companies ................................................. 9-27
9.4.7. Price Canyon Discharge Relocation .................................................................................. 9-29
9.4.8. Modeling of Multiple Projects ............................................................................................ 9-33
9.5. Management Actions ................................................................................................................... 9-35
9.5.1. Expand Monitoring Network .............................................................................................. 9-35
9.5.2. Groundwater Extraction Metering and Reporting Plan ................................................ 9-35
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9.5.3. Demand Management Plan ................................................................................................ 9-36
9.6. Adaptive Management (§ 354.44A) ....................................................................................... 9-40
10 Implementation Plan ............................................................................................................................. 10-1
10.1. GSP Implementation, Administration, and Management ................................................... 10-2
10.1.1. Administrative Approach/Governance Structure ......................................................... 10-2
10.1.2. Implementation Schedule .................................................................................................. 10-2
10.1.3. Implementation Costs ......................................................................................................... 10-2
10.1.4. Outreach and Communication.......................................................................................... 10-5
10.2. Funding ........................................................................................................................................ 10-5
10.2.1. GSP Implementation Funds ............................................................................................... 10-5
10.2.2. Fee Study............................................................................................................................. 10-5
10.2.3. Grant/Low Interest Financing .......................................................................................... 10-5
10.3. Reporting .................................................................................................................................... 10-6
10.3.1. Annual Reports .................................................................................................................... 10-6
10.3.2. Five-Year Evaluation Reports .......................................................................................... 10-7
11 References ............................................................................................................................................. 11-2
Appendices
DWR Elements of the Plan Guide .................................................................................................................. A
City of San Luis Obispo Resolution to Form GSA ........................................................................................ B
County of San Luis Obispo Resolution to Form GSA .................................................................................. C
Memorandum of Agreement – Preparation of GSP .................................................................................. D
Notice and Communication .............................................................................................................................. E
Groundwater Dependent Ecosystems in the San Luis Obispo Valley Groundwater Basin .................. F
Surface Water / Groundwater Modeling Documentation ....................................................................... G
Data Management ........................................................................................................................................... H
Response to Public Comments ........................................................................................................................... I
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LIST OF FIGURES
Figure ES-1. San Luis Obispo Valley Basin GSAs and Participating Parties ......................................... 3
Figure ES-2. Stakeholder Communication and Engagement Summary .................................................... 4
Figure ES-3. Groundwater Elevation Surfaces – Fall 2019 ...................................................................... 7
Figure ES-4. Selected Hydrographs............................................................................................................... 9
Figure ES-5. Groundwater Budget – Basin Total ..................................................................................... 11
Figure ES-6. Monitoring Network ................................................................................................................ 13
Figure ES-7. Project Location Map .............................................................................................................. 17
Figure 1-1. San Luis Obispo Valley Basin and Surrounding Basins ....................................................1-4
Figure 2-1. San Luis Obispo Valley Basin GSAs and Participating Parties .......................................2-3
Figure 2-2. Groundwater Sustainability Commission (GSC) .................................................................2-4
Figure 2-3. Stakeholder Communication and Engagement Summary .................................................2-9
Figure 3-1. San Luis Obispo Historical Annual Precipitation .................................................................3-5
Figure 3-2. San Luis Obispo Valley Basin Existing Land Use Designations .......................................3-6
Figure 3-3. San Luis Obispo Valley Basin Water Supply Sources.......................................................3-8
Figure 3-4. San Luis Obispo Valley Basin Water Use Sectors .......................................................... 3-10
Figure 3-5. San Luis Obispo Valley Basin Domestic Well Density .................................................... 3-12
Figure 3-6. San Luis Obispo Valley Basin Production Well Density ................................................. 3-13
Figure 3-7. San Luis Obispo Valley Basin Public Supply Well Density ............................................ 3-14
Figure 3-8. San Luis Obispo Valley Basin Public Supply Well Density ............................................ 3-15
Figure 3-9. Monitored Wells in the San Luis Obispo Valley Basin ................................................... 3-17
Figure 3-10. San Luis Obispo Valley Basin Surface Water Features, Stream Gauges,
and Weather Stations ......................................................................................................................... 3-20
Figure 3-11. San Luis Obispo Valley Basin Historical Annual Precipitation and CDFM ............... 3-21
Figure 3-12. City Land Use Map ............................................................................................................. 3-27
Figure 3-13. County Land Use Map (San Luis Obispo Planning Area) ............................................ 3-29
Figure 3-14. County Land Use Map (South County Planning Area) ................................................. 3-30
Figure 3-15. Los Ranchos/Edna Land Use Map ................................................................................... 3-32
Figure 4-1. Topographic Map ....................................................................................................................4-4
Figure 4-2. Aerial Photograph ....................................................................................................................4-5
Figure 4-3. Annual Precipitation .................................................................................................................4-6
Figure 4-4. Bottom Elevation of Basin ........................................................................................................4-7
Figure 4-5. Thickness of Basin Sediments ..................................................................................................4-8
Figure 4-6. Soil Hydrologic Groups ........................................................................................................ 4-10
Figure 4-7. Stratigraphic Column ............................................................................................................ 4-12
Figure 4-8. Geologic Map ........................................................................................................................ 4-13
Figure 4-9. Lithologic Data Points and Cross Section Lines ................................................................ 4-20
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Figure 4-10. Cross Section A1-A2 ........................................................................................................... 4-21
Figure 4-11. Cross Section A2-A3 ........................................................................................................... 4-22
Figure 4-12. Cross Section A3-A4 ........................................................................................................... 4-23
Figure 4-13. Cross Section B-B’................................................................................................................ 4-24
Figure 4-14. Cross Section C1-C1’ .......................................................................................................... 4-25
Figure 4-15. Cross Section C2-C2’ .......................................................................................................... 4-26
Figure 4-16. Cross Section D-D’ ............................................................................................................... 4-27
Figure 4-17. Cross Section E-E’ ................................................................................................................ 4-28
Figure 4-18. Cross Section F-F’ ................................................................................................................ 4-29
Figure 4-19. Cross Section G-G’ ............................................................................................................. 4-30
Figure 4-20. Cross Section H-H’ ............................................................................................................... 4-31
Figure 4-21. Cross Section I-I’ .................................................................................................................. 4-32
Figure 4-22. Hydraulic Parameter Data Locations .............................................................................. 4-35
Figure 4-23. Subsidence Potential .......................................................................................................... 4-40
Figure 5-1. Groundwater Elevation Surface Fall 1954 .........................................................................5-4
Figure 5-2. Groundwater Elevation Surface Spring 1990 ....................................................................5-5
Figure 5-3 Groundwater Elevation Surface Spring 1997 .....................................................................5-7
Figure 5-4 Groundwater Elevation Surface Spring 2011 .....................................................................5-9
Figure 5-5 Groundwater Elevation Surface Spring 2015 .................................................................. 5-10
Figure 5-6 Groundwater Elevation Surface Spring 2019 .................................................................. 5-11
Figure 5-7 Groundwater Elevation Surface Fall 2019 ....................................................................... 5-12
Figure 5-8 Change in Groundwater Elevation Spring 1997 to Spring 2011 ................................ 5-14
Figure 5-9 Change in Groundwater Elevation Spring 2011 to Spring 2015 ................................ 5-15
Figure 5-10. Change in Groundwater Elevation Spring 2015 to Spring 2019 ............................. 5-16
Figure 5-11. Selected Hydrographs ....................................................................................................... 5-19
Figure 5-12. Stillwater Percolation Zone Study Results ...................................................................... 5-22
Figure 5-13. Soil Agricultural Groundwater Banking Index Study Results ...................................... 5-23
Figure 5-14. Groundwater Elevation Hydrograph (SLV-12) ............................................................. 5-27
Figure 5-15. Hydrographs of Wells Adjacent to West Corral de Piedras Creek ........................ 5-28
Figure 5-16. Losing and Gaining Reaches Within the Basin .............................................................. 5-31
Figure 5-17. Potential Groundwater-Dependent Ecosystems (GDEs) ............................................... 5-33
Figure 5-18. Location of Potential Point Sources of Groundwater Conditions ............................... 5-36
Figure 5-19. Distribution of TDS in Basin ..................................................................................................5-1
Figure 5-20. Distribution of Nitrate in Basin .............................................................................................5-3
Figure 5-21. Distribution of Arsenic in Basin .............................................................................................5-5
Figure 6-1. The Hydrologic Cycle. Source: Department of Water Resources (DWR,
2016) ........................................................................................................................................................6-4
Figure 6-2. Components of the Water Budget. Source: Modified from Department of
Water Resources (DWR, 2016) ...........................................................................................................6-5
Figure 6-3. Water Budget Subareas ........................................................................................................6-6
Figure 6-4. Surface Water Budget – San Luis Valley Subarea ........................................................ 6-10
Figure 6-5. Surface Water Budget – Edna Valley Subarea ............................................................. 6-11
Figure 6-6. Surface Water Budget – Basin Total ................................................................................ 6-12
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Figure 6-7. Groundwater Budget – San Luis Valley Subarea ........................................................... 6-13
Figure 6-8. Groundwater Budget – Edna Valley Subarea ................................................................ 6-14
Figure 6-9. Groundwater Budget - Basin Total .................................................................................... 6-15
Figure 6-10. 1987-2019 Historical Base Period Climate .................................................................. 6-17
Figure 6-11. Rainfall Correlation Cal Poly NOAA vs. Gas Company ............................................. 6-19
Figure 6-12. San Luis Obispo Valley Basin Irrigated Crops 2016 ................................................... 6-24
Figure 6-13. Basin Sub-watershed Areas and Isohyetals ................................................................... 6-27
Figure 6-14. Runoff vs Rainfall Correlation for Subareas .................................................................. 6-28
Figure 6-15. Rainfall vs Infiltration ......................................................................................................... 6-34
Figure 6-16. Bedrock Subsurface Inflow Reaches ................................................................................ 6-38
Figure 6-17. Consumptive Use of Applied Water ............................................................................... 6-41
Figure 6-18. Groundwater Elevation Contours Spring 1986 ............................................................ 6-45
Figure 6-19. Groundwater Elevation Contours Spring 2019 ............................................................ 6-46
Figure 6-20. Storage Volume Grids ....................................................................................................... 6-47
Figure 6-21. Groundwater Storage Estimate Comparison for Basin Subareas ............................. 6-50
Figure 6-22. Historical and Current Average Annual Surface Water Budget – San Luis
Valley Subarea .................................................................................................................................... 6-57
Figure 6-23. Historical and Current Average Annual Surface Water Budget – Edna
Valley Subarea .................................................................................................................................... 6-58
Figure 6-24. Historical and Current Average Annual Surface Water Budget – Basin
Total ....................................................................................................................................................... 6-59
Figure 6-25. Historical and Current Average Annual Groundwater Budget – San Luis
Valley Subarea .................................................................................................................................... 6-60
Figure 6-26. Historical and Current Average Annual Groundwater Budget – Edna
Valley Subarea .................................................................................................................................... 6-61
Figure 7-1. Water Level Monitoring Network .........................................................................................7-7
Figure 7-2. Water Quality Monitoring Network .................................................................................. 7-10
Figure 7-3. Surface Water Flow Monitoring Network ........................................................................ 7-13
Figure 8-1. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) SLV-19 ..........................................................................................................................................8-7
Figure 8-2. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) SLV-16 ..........................................................................................................................................8-7
Figure 8-3. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) SLV-09 ..........................................................................................................................................8-8
Figure 8-4. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) SLV-12 ..........................................................................................................................................8-8
Figure 8-5. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) EV-12 ............................................................................................................................................8-9
Figure 8-6. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
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(RMS) EV-04 ............................................................................................................................................8-9
Figure 8-7. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) EV-09 ......................................................................................................................................... 8-10
Figure 8-8. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) EV-16 ......................................................................................................................................... 8-10
Figure 8-9. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) EV-01 ......................................................................................................................................... 8-11
Figure 8-10. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE
(MO), AND INTERIM MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE
(RMS) EV-11 ......................................................................................................................................... 8-11
Figure 8-11. Comparison of Modeled Discharge to Streams in San Luis Creek
Watershed During Low Flow Months (July through September) with Baseline and
No Pumping Scenarios ........................................................................................................................ 8-36
Figure 9-1. Project Location Map ...............................................................................................................9-8
Figure 9-2. SWP with In-Lieu Agricultural Pumping Reduction - 1,000 AFY – Project
Scenario 1 ............................................................................................................................................. 9-14
Figure 9-3. SWP Recharge Basin – 500 AFY – Project Scenario 2 ................................................. 9-22
Figure 9-4. SWP Purveyor In-Lieu Pumping Reduction – GSWC = 200 AFY, VRMWC
& ERMWC = 50 AFY – Project Scenario 3 .................................................................................... 9-25
Figure 9-5. Relocation of Price Canyon Discharge Point – 500 AFY ............................................... 9-32
Figure 9-6. Model Results from the Combined Modeled Project Scenarios – Project
Scenario 5 ............................................................................................................................................. 9-34
Figure 9-7. Irrigation Efficiency Comparison......................................................................................... 9-38
Figure 10-1. SLO Basin GSP Implementation Schedule ...................................................................... 10-3
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Groundwater Sustainability Agencies xi
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
LIST OF TABLES
Table ES-1. Summary of MTs, MOs, and IMs for SLO Basin RMSs ....................................................... 15
Table ES-2. San Luis Obispo Valley Basin Groundwater Basin Water Quality Minimum
Thresholds .................................................................................................................................................. 15
Table 2-1. List of Public Meetings and Workshops ................................................................................2-8
Table 3-1. Agricultural Land use categories defined for the SLO Basin by DWR (2014)..............3-4
Table 3-2. Summary of surface water supply sources available to the SLO Basin ..........................3-7
Table 3-3. DWR and County Wells ........................................................................................................ 3-11
Table 3-4. Stream gauges and summary of records available ........................................................ 3-18
Table 3-5. Weather station Information and summary of records available................................. 3-19
Table 3-6. Average Monthly Climate Summary 1987 – 2018 at Cal Poly Weather
Station 52 .............................................................................................................................................. 3-19
Table 3-7. Los Ranchos/Edna Land Use Acreage ................................................................................ 3-31
Table 4-1. San Luis Obispo Valley Groundwater Basin Water Well Pump Test Data
Summary ................................................................................................................................................ 4-36
Table 4-2. San Luis Obispo Valley Groundwater Basin Water Well Specific Capacity
Data Summary ...................................................................................................................................... 4-37
Table 5-1. Potential Point Sources of Groundwater Contamination ................................................. 5-37
Table 6-1. Historical Water –Budget - San Luis Valley Subarea ........................................................6-7
Table 6-2. Historical Water –Budget - Edna Valley Subarea .............................................................6-8
Table 6-3. Historical Water Budget - San Luis Obispo Valley Groundwater Basin ........................6-9
Table 6-4. Historical Base Period Rainfall ............................................................................................ 6-20
Table 6-5. Irrigated Agriculture Acreages ............................................................................................ 6-23
Table 6-6. Land Cover Acreages ............................................................................................................ 6-25
Table 6-7. Stream Outflow Comparison ................................................................................................ 6-32
Table 6-8. Minimum Rainfall for Infiltration .......................................................................................... 6-35
Table 6-9. Subsurface Inflow Estimates ................................................................................................. 6-36
Table 6-10. Rural Residential Water Use ............................................................................................. 6-40
Table 6-11. Consumptive Use of Applied Water ................................................................................ 6-42
Table 6-12. Subsurface Outflow Estimates ........................................................................................... 6-43
Table 6-13. Specific Yield Averages ..................................................................................................... 6-44
Table 6-14. Spring Groundwater Storage Estimates .......................................................................... 6-48
Table 6-15. Change in Storage Comparison – Historical Base Period 1987 – 2019 ................. 6-49
Table 6-16. Preliminary Sustainable Yield Estimate (AFY) ................................................................ 6-51
Table 6-17. Estimated Overdraft (AFY) ................................................................................................ 6-52
Table 6-18. Current Water Budget - San Luis Valley Subarea ........................................................ 6-54
Table 6-19. Current Water Budget - Edna Valley Subarea ............................................................. 6-55
Table 6-20. Current Water Budget - Basin Total ................................................................................ 6-56
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Groundwater Sustainability Agencies xii
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-21 Projected Future Annual Surface Water Inflows to Basin (AFY) .................................. 6-65
Table 6-22 Future Water Budget ........................................................................................................... 6-66
Table 7-1. Groundwater Level Monitoring Network ..............................................................................7-6
Table 7-2. Recommended Groundwater Level Monitoring Network Additions ................................7-8
Table 7-3. Groundwater Quality Monitoring Network ..........................................................................7-9
Table 7-4. Existing Surface Water Flow Monitoring Network .......................................................... 7-12
Table 7-5. Recommended Surface Water Monitoring Network Additions ..................................... 7-12
Table 7-6. Proposed Interconnected Surface Water Monitoring Locations .................................... 7-18
Table 8-1. Summary of MTs, MOs, and IMs for SLO Basin RMSs ..................................................... 8-12
Table 8-2. Groundwater Levels in Domestic Wells During the 2015 Drought (Edna
Valley) ................................................................................................................................................... 8-16
Table 8-3. San Luis Obispo Valley Basin Groundwater Basin Water Quality Minimum
Thresholds .............................................................................................................................................. 8-26
Table 9-1. Initial Project Screening Evaluation Criteria .........................................................................9-5
Table 9-2. Project Evaluation Scoring Results ..........................................................................................9-6
Table 9-3. Projects and Management Actions Strategies .....................................................................9-7
Table 9-4 Summary of Project and Management Action Benefits and Impacts on
Sustainability Indicators. ........................................................................................................................9-9
Table 9-5 Description of Modeled Scenarios ....................................................................................... 9-11
Table 9-6 Summary Results of Modeled Scenarios ............................................................................. 9-33
Table 9-6. Consumptive Use of Applied Water and Total Irrigated Acreage by Land
Use/Land Cover Type......................................................................................................................... 9-39
Table 10-1. GSP Implementation Costs (2022-2027) ........................................................................ 10-8
Table of Contents
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Groundwater Sustainability Agencies xiii
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
ACRONYMS & ABBREVIATIONS
Above Mean Sea Level AMSL
Acre Feet AF
Acre Feet per Year AFY
Assembly Bill AB
Cal Poly Weather Station 52 CPWS-52
California Code of Regulations CCR
California Department of Fish and Wildlife CDFW
California Department of Public Health CDPH
California Endangered Species Act CESA
California Environmental Data Exchange Network CEDEN
California Fire and Resource Assessment Program Vegetation FVEG
California Irrigation Management Information System CIMIS
California Native Plant Protection Act CNPPA
California Polytechnic State University Cal Poly
California Public Utilities Commission CPUC
California Regional Water Quality Control Board CRWQCB
California Safe Drinking Water Information System SDWIS
California State Groundwater Elevation Monitoring program CASGEM
California State Water Resources Control Board SWRCB
California Water Code CWC
Central Coast Ambient Monitoring Program CCAMP
Central Coast Groundwater Coalition CCGC
Central Coast Regional Water Quality Control Board CCRWQCB
City of San Luis Obispo City of SLO
City of San Luis Obispo City
Communication and Engagement Plan C&E Plan
Consumptive Use Program Plus CUP+
County of San Luis Obispo County
County of San Luis Obispo Environmental Health Services EHS
Crop Evapotranspiration Etc
Cubic Feet Per Second cfs
Cumulative departure from the mean CDFM
Degrees Fahrenheit °F
Department of Toxic Substance Control DTSC
Department of Water Resources DWR
Division of Drinking Water DDW
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Groundwater Sustainability Agencies xiv
San Luis Obispo Valley Basin Groundwater
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Edna Ranch Mutual Water Company ERMWC
Edna Valley Growers Ranch Mutual Water Company EVGMWC
Endangered Species Act ESA
Environmental Protection Agency EPA
estimated applied water demand Etaw
Evapotranspiration Eto
feet/feet ft/ft
Fiscal Year FY
Gallons per day per foot gpd/ft
Gallons per Minute GPM
Golden State Water Company GSWC
GPM per foot of drawdown gpm/ft
Groundwater Ambient Monitoring and Assessment program GAMA
Groundwater Communications Portal GCP
Groundwater Management Plan GMP
Groundwater Sustainability Agency GSA
Groundwater Sustainability Commission GSC
Groundwater Sustainability Plan GSP
Groundwater/Surface water GW/SW
Integrated Regional Water Management Plan IRWMP
interconnected surface water ISW
Interim Milestones IMs
Land Use and Circulation Element LUCE
Leaky Underground Fuel Tanks LUFTs
Master Water Report MWR
Maximum Contaminant Level MCL
Measurable Objectives MOs
Memorandum of Agreement MOA
Memorandum of Understanding MOU
Milligrams per Liter Mg/L
Million Acre Feet MAF
Million Gallons MG
Million Gallons per Day MGD
Minimum Thresholds MTs
National Climate Data Center NCDC
National Land Cover Database NLCD
National Oceanic and Atmospheric Administration NOAA
National Oceanic Atmospheric Administration, National Marine Fisheries Service NOAA NMFS
National Water Information System NWIS
Natural Resources Conservation Service NRCS
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San Luis Obispo Valley Basin Groundwater
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One hundred cubic feet CCF
Operations and Maintenance O&M
Parts per billion ppb
Parts per million ppm
Polonio Pass Water Treatment Plant PPWTP
Recycled Water RW
Regional Water Quality Control Board RWQCB
Representative Monitoring Sites RMSs
Salt and Nutrient Management Plan SNMP
San Luis Obispo SLO
San Luis Obispo County Flood Control and Water Conservation District SLOCFCWCD
San Luis Obispo County Integrated Regional Water Management Plan IRWMP
San Luis Obispo Creek SLO Creek
San Luis Obispo Regional Water Management Group RWMG
San Luis Obispo Valley Groundwater Basin SLO Basin
Secondary Maximum Contaminant Level SCML
Senate Bill SB
Soil Survey Geographic Database SSURGO
Sphere of Influence SOI
State Water Project SWP
Sustainable Groundwater Management Act SGMA
Sustainable Groundwater Management Planning SGMP
Sustainable Groundwater Planning SGWP
Sustainable Management Criteria SMCs
tetrachloroethylene PCE
Total Dissolved Solids TDS
Total Maximum Daily Load TMDL
Trichloroethylene TCE
U.S. Department of Agriculture USDA
Underground Storage Tanks USTs
United States Fish and Wildlife Service USFW
United States Geological Survey USGS
Urban Water Management Plan UWMP
Varian Ranch Mutual Water Company VRMWC
Wastewater Treatment Plant WWTP
Water Code Section WCS
Water Master Plan WMP
Water Planning Areas WPA
Water Planning Areas WPA
Water Quality Control Plan for the Central Coast Basin Basin Plan
Water Resource Recovery Facility WRRF
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Water Resource Recovery Facility WRRF
Water Resources Advisory Committee WRAC
Water Supply Assessment WSA
Water Treatment Plant WTP
Well Construction Report WCR
Western Regional Climate Center WRCC
ES-1
GROUNDWATER SUSTAINABILITY PLAN
Executive
Summary
The Sustainable Groundwater Management Act (SGMA), Section 10720,
et. al., of the State Water Code, requires sustainable groundwater
management in all high and medium priority basins. The San Luis Obispo
Valley Groundwater Basin (SLO Basin) was designated as a high priority
basin.
The SLO Basin Groundwater Sustainability Plan (GSP) was
developed by two Groundwater Sustainability Agencies (GSAs)
formed by the County of San Luis Obispo (County GSA) and
the City of San Luis Obispo (City GSA). The GSAs entered into
a Memorandum of Agreement (MOA) for the purposes of
coordinating preparation of a single GSP for the SLO Basin.
The MOA also established the Groundwater Sustainability
Commission (GSC), which serves as an advisory body to the
GSAs consisting of representatives from the County GSA and
the City GSA, as well as representatives from the other
signatories to the MOA (i.e., Golden State Water Company
(GSWC), Edna Valley Growers Mutual Water Company
(EVGMWC), Edna Ranch Mutual Water Company (ERMWC),
and Varian Ranch Mutual Water Company (VRMWC).
IN THIS SECTION
• Plan Area and Basin
Overview
• Outreach
• Groundwater
Conditions
• Budget
• Monitoring Network
• Management
Criteria and Actions
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Introduction
This document fulfills the GSP development requirement for the SLO Basin. This GSP describes and
assesses the groundwater condition of the SLO Basin, develops quantifiable management objectives
that account for the interests of the SLO Basin’s beneficial groundwater uses and users, and identifies
a group of projects and management actions that will allow the SLO Basin to achieve and maintain
sustainability in the future.
Plan Area
The jurisdictional boundaries for the GSP correspond to Department of Water Resources (DWR, 2016)
Bulletin 118 basin boundary for the SLO Basin as shown in Figure ES-1. The SLO Basin is oriented in
a northwest-southeast direction and is composed of unconsolidated or loosely consolidated
sedimentary deposits. It is approximately 14 miles long and 1.5 miles wide and covers a surface area of
about 12,700 acres (19.9 square miles). The SLO Basin is bounded on the northeast by the relatively
impermeable bedrock formations of the Santa Lucia Range, and on the southwest by the formations of
the San Luis Range and the Edna fault system. The SLO Basin is commonly referenced as being
composed of two distinct valleys, with the San Luis Valley in the northwest and the Edna Valley in the
southeast. The San Luis Valley includes part of the City and California Polytechnic University (Cal Poly)
jurisdictional boundaries, while the remainder of the valley is unincorporated land. Land use in the City
is primarily municipal, residential, and industrial. The Edna Valley is entirely unincorporated and the
primary land use in the Edna Valley is agricultural. During the past two decades, wine grapes have
become the most significant crop type in the Edna Valley
The primary sources of water supply for uses in the basin include groundwater from the San Luis
Obispo Valley Basin and surface water from Whale Rock Reservoir, Salinas Reservoir, Nacimiento
Lake, and recycled water from the City’s Water Recycling Program. Water users in the basin include
municipalities, communities, rural domestic residences, and industrial, environmental, and agricultural
users.
Executive Summary
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies ES-3 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure ES-1. San Luis Obispo Valley Basin GSAs and Participating Parties
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-4
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Outreach Efforts
A Communication and Engagement Plan (C&E Plan) was executed and includes the planned activities
for engaging interested parties in SGMA implementation efforts in the San Luis Obispo Valley Basin.
The goals of the C&E plan are as follows:
• Create an inclusive and transparent participation experience that builds public trust in the GSP and
optimizes participation among all stakeholders.
• Employ outreach methods that facilitate shared understanding of the importance of sustainable
groundwater conditions and impacts on stakeholders.
• Communicate “early and often” and actively identify and eliminate barriers to participation.
• Develop a cost-effective, stakeholder-informed GSP supported by best-in-class technical data.
Outreach and communication throughout GSP development included regular presentations at GSC
meetings, meetings with community groups, meetings with individual stakeholders, and community
workshops. Comments from stakeholders were collected via the Groundwater Communications Portal
(GCP), SLOWaterBasin.com, and considered the comments from their stakeholders.
Figure ES-2 provides a summary of the engagement results regarding the stakeholder outreach
touchpoints, stakeholder lists, stakeholder participation, and statistics for the SLOWaterBasin.com
website.
Note: The Stakeholder Groups Represented is 9/10 due to the fact that Tribal interests were contacted and informed of the GSP development
process, and that they indicated that they would engage in the Implementation Phase of the GSP
Figure ES-2. Stakeholder Communication and Engagement Summary
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-5
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Basin Setting
The Basin covers approximately 20 square miles, and it is commonly referenced as being composed of
two distinct valleys, with the San Luis Valley in the northwest and the Edna Valley in the southeast.
Average annual precipitation ranges from approximately 18 inches throughout most of the Basin to
about 22 inches in relatively higher elevation areas near the City and Cal Poly. San Luis Creek and its
tributaries (Prefumo, Stenner, and Davenport Creeks) drain the San Luis Valley and its contributing
watershed. East and West Corral de Piedras Creeks drain the Edna valley and its contributing
watershed and join to form Pismo Creek immediately south of the Basin boundary. These creeks
contribute an important component of recharge to the underlying aquifers.
For the purpose of this plan, the geologic units in the Basin and vicinity may be considered as two basic
groups; 1) the Basin sediments; and 2) the consolidated bedrock formations surrounding and
underlying the Basin. From a hydrogeologic standpoint, the most important strata in the Basin are the
sedimentary basin fill deposits that define the vertical and lateral extents of the Basin. These include
recent and older deposits of terrestrial sourced sediments, underlain in the Edna Valley by older marine
sedimentary units. The sediments of the Edna Valley have significantly greater thickness (greater than
300 feet in the deepest parts) than those of the San Luis Valley (about 150 feet in the deepest parts).
The aquifers beneath the two valleys are bounded by a high point in the underlying bedrock which rises
to near the surface in the area along Hidden Springs Road; this bedrock high limits groundwater
movement between Edna Valley and San Luis Valley to the uppermost portions of the aquifer.
The three formations that comprise the Basin aquifers are summarized in the basin setting, from
youngest to oldest (or from top to bottom), are:
• Recent Alluvium. The Recent Alluvium is the mapped geologic unit composed of unconsolidated
sediments of gravel, sand, silt, and clay, deposited by fluvial processes along the courses of San
Luis Obispo Creek (SLO Creek), Davenport Creek, East and West Corral de Piedras Creeks, and
their tributaries. Alluvium is present at the surface in most of the San Luis Valley, and along the
combined riparian corridor of East and West Corral de Piedras Creeks in Edna Valley.
• Paso Robles Formation. The Paso Robles Formation underlies the Recent Alluvium throughout
most of the Basin, and overlies the Pismo Formation where present. It was deposited in a terrestrial
setting. It is composed of poorly sorted, unconsolidated to mildly consolidated sandstone, siltstone,
and claystone. The Paso Robles Formation is exposed at the surface throughout much of the Edna
Valley, except in the area around East and West Corral de Piedras Creeks, which have deposited
Recent Alluvium on top of it.
• Pismo Formation. The Pismo Formation is a Pliocene-aged sequence of marine deposited
sedimentary units composed of claystone, siltstone, sandstone, and conglomerate. It is the oldest
geologic water-bearing unit with significance to the hydrogeology of the Basin. The Pismo
Formation is extensive below the Paso Robles Formation in the Edna Valley. Thicknesses of Pismo
Formation up to 400 feet are reported or observed in Edna Valley. The Pismo Formation does not
crop out at the surface anywhere in the Basin.
All three of the geologic formations that comprise the Basin aquifer contain interbedded layers of silt,
sand, gravel, and clay. There are no significant aquitards that vertically separate the three formations in
the Basin over large areas. There may be deposits of clay and silt that are not laterally extensive that
locally separate producing zones of two formations, but there is no recognized aquitard in the Basin
that separates the aquifers over significant areas. In both the San Luis Valley and Edna Valley, wells
are commonly screened across sands of multiple formations. The three formations that comprise the
Basin aquifer essentially function as a single hydrogeologic unit. Eleven geologic cross sections are
presented in Chapter 4 (Basin Setting) that detail the lithology of the Basin sediments.
The primary bedrock formations that crop out in the contributing watersheds to the Basin are the
Monterey formation, the Obispo formation, and the Franciscan Assemblage. While fractures in
consolidated rock may yield small quantities of water locally to wells, these formations are not
considered to be aquifers for the purposes of this GSP.
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-6
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Wells screened in the Alluvium and Paso Robles Formation have transmissivities ranging from about
5,000 to 158,000 gallons per day per foot (gpd/ft), and averaging over 42,000 gpd/ft. Wells screened in
Paso Robles and Pismo Formations have transmissivities ranging from less than 1,000 to about 40,000
gpd/ft, and average about 10,000 gpd/ft.
There are several named creeks that flow across the Basin. In the San Luis Valley area of the Basin,
these include SLO Creek, Stenner Creek, Prefumo Creek, Froom Creek, and Davenport Creek, in
addition to smaller tributaries. In the Edna Valley creeks include East and West Corral de Piedras
Creeks (which join to form Pismo Creek just south of the Basin Boundary), and Canada de Verde
Creek in southeastern Edna Valley. The watersheds support important habitat for native fish and
wildlife, including the federally threatened South-Central California Coast steelhead. Groundwater
interaction with streams in the Basin is not well quantified, but it is recognized as an important
component of recharge in the water budget.
The two surface water bodies of significance to the Basin are Laguna Lake and Righetti Reservoir.
Laguna Lake is the only lake within the Basin. It is a naturally occurring lake just north of Los Osos
Valley Road and west of Highway 101. The water in the lake is partially supplied by seasonal flow in
Prefumo Creek, and partially supplied by subsurface groundwater inflow. Righetti Reservoir is a
privately-owned reservoir formed by a dam on West Corral de Piedras Creek about 1.5 miles upstream
from the Basin boundary, which impounds about 900 acre-feet of water, which is used primarily for
irrigation.
Subsidence is the gradual settling or sinking of the earth’s surface due to subsurface material
movement at depth. It is frequently associated with groundwater pumping and is one of the undesirable
results identified in SGMA. Subsidence has been historically documented in parts of the San Luis
Valley. The most severe subsidence that has occurred in the Basin was in the 1990s along the Los
Osos Valley Road corridor. The subsidence was a result of increased groundwater pumping in
response to the 1987-1991 drought and caused damage to businesses and homes within that area.
The City has discontinued significant pumping in this area, and subsidence has not been observed
since.
Groundwater Conditions
Seven groundwater elevation contour maps that cover the entire Basin are presented, ranging in time
from Spring 1954 to Fall 2019. Regional groundwater flow patterns are consistent across this period of
record, with local declines in groundwater elevations observed in Edna Valley in recent years.
In the San Luis Valley portion of the Basin, the dominant groundwater flow direction is from higher
elevations in the in the northwestern extent of the Basin southeastward toward the discharge area
where SLO Creek leaves the Basin. In the Edna Valley portion of the Basin, the dominant groundwater
flow direction is northwestward from the higher groundwater elevations in the southeastern part of the
Basin (over 280 ft AMSL) to lower elevations in the San Luis Valley. There are also local areas of
discharge coincident with the areas where SLO Creek and Pismo Creek tributaries leave the Basin.
Groundwater elevation contours for Fall 2019 are displayed in Figure ES-3.
Executive Summary
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies ES-7 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure ES-3. Groundwater Elevation Surfaces – Fall 2019
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-8
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Hydrographs of groundwater elevations in various parts of the Basin display three distinct trends from
data that extends back to the 1950s in some cases. The hydrographs for the wells in the San Luis
Valley indicate that water levels in these wells, although somewhat variable in response to seasonal
weather patterns, water use fluctuations, and longer-term dry weather periods, are essentially stable.
There are no long-term trends indicating steadily declining or increasing water levels in this area. The
wells in the vicinity of Highway 101 and Los Osos Valley Road also display water levels in relative
equilibrium, with the exception of the early 1990s, when drought-related pumping and weather patterns
resulted in noticeable declines in the water levels in this area (hydrographs 2 and 3 on Figure ES-4). A
second distinct pattern is evident in hydrographs from wells in the area immediately east of the
intersection of Biddle Ranch Road and Orcutt Road in Edna Valley, where West Corral de Piedras
Creek enters the Basin (hydrographs 5 and 6 in Figure ES-4). The hydrographs of the two wells in this
area display much greater volatility in response to seasonal and drought cycle fluctuations than the
wells in San Luis Valley, with water levels fluctuating within a range of over 40 feet, as opposed to the
range of 10 to 20 feet in the San Luis Valley wells. However, water levels appear to rebound to pre-
drought levels when each drought cycle ends. Groundwater elevations displayed in these two
hydrographs do not display a long-term decline of water levels. By contrast, several wells in the Edna
Valley display steadily declining water levels during the past 15 to 20 years. Hydrographs for four wells
(hydrographs 7, 8, 9, and 10 on Figure ES-4) in the Edna Valley display groundwater elevation declines
of about 60 to 100 feet since the year 2000. Groundwater elevations in the Edna Valley displayed the
largest historical declines in the Basin. This hydrograph pattern indicates that a reduction of
groundwater storage has occurred over this period of record in the area defined by these well locations.
It is understood that agricultural pumping has increased in Edna Valley during this time period, likely
explaining the patterns of declining groundwater elevations in these hydrographs.
The primary sources of recharge to the Basin aquifer are areal infiltration of precipitation, subsurface
inflow from surrounding bedrock, percolation of surface water from streams, and anthropogenic
recharge (including percolation of wastewater treatment plant effluent, return flow from irrigation, and
return flow from domestic septic systems). The primary sources of discharge from the Basin aquifer are
pumping from wells, evapotranspiration by phreatophytes in areas of shallow groundwater table, and
groundwater discharge to streams.
Surface water/groundwater interactions may represent a significant portion of the water budget of an
aquifer system. A desktop analysis resulted in identification of two areas of SLO Creek that may
seasonally gain water from the Alluvial Aquifer, which are the confluence of Stenner Creek and SLO
Creek, and the reach of SLO Creek downstream from the Wastewater Treatment Plant to the
confluence with Prefumo Creek. Several reaches of SLO Creek are identified that may occasionally
lose water to the Alluvial Aquifer. Groundwater levels in the San Luis Valley part of the Basin are
generally high enough that the creek is connected to the underlying aquifer. Along most of Corral de
Piedras Creeks, by contrast, surface water levels are generally greater than 30 feet above the
groundwater level, and the streams are considered disconnected from the underlying Alluvial Aquifer in
this area. These analyses will benefit from additional surface water monitoring in the Basin which is
identified within Chapter 7 (Monitoring Network). A desktop analysis is also presented that identifies
potential Groundwater Dependent Ecosystems in the Basin.
Existing groundwater quality data is presented for Total Dissolved Solids, Arsenic, and Nitrates.
Groundwater quality in the Basin aquifer is generally adequate for use as potable water supply and
irrigation. TDS has a water quality objective goal of 900 mg/l promulgated in the Basin Plan; water
quality results ranged from 180 to 3,100 mg/l with a median of 613 mg/l, and no trends of increasing
TDS with time were observed. Nitrate (as N) has federally mandated MCL of 10 mg/l; water quality
results ranged from below the detection limit to 80 mg/l ; two sampling locations are identified with
nitrate trends that have increased slightly in recent years, but most show no significant increases of
nitrates with time. Arsenic has an MCL of 10 ug/l; concentrations ranged from below the detection limit
to 28 ug/l, with an average value of 2.5 ug/l and a median value of 2 ug/l. Sampling locations with
multiple data points displayed stable or decreasing concentrations of arsenic over the data period of
record.
Executive Summary
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies ES-9 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure ES-4. Selected Hydrographs
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-10
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Water Budget
A water budget identifies and quantifies various components of the hydrologic cycle within a user-
defined area, in this case the San Luis Obispo Valley Groundwater Basin. Analytical methods are used
to generate historical and current water budgets. Analytical methods include the application of the water
budget equation and the inventory method using spreadsheets, with groundwater flow estimates based
on Darcy’s Law and change in storage calculations based on the specific yield method.
The simplified expression of the water budget equation is:
INFLOW – OUTFLOW = CHANGE IN STORAGE
Separate water budgets are presented for both surface water and groundwater systems in the Basin.
Separate water budgets were prepared for the San Luis Valley and Edna Valley, as well as a combined
water budget for the entire Basin.
All components of inflow and outflow to the groundwater system were analyzed, with annual estimates
of all water budget components generated for water years 1987 through 2019. Components of
groundwater inflow include infiltration of precipitation, infiltration of applied urban water (i.e., lawn
watering, landscaping, etc.), infiltration of applied water, percolation of streamflow, and subsurface
inflow from the Basin boundaries. Components of groundwater outflow include urban groundwater
pumping (municipal, domestic, and industrial), agricultural irrigation pumping, evapotranspiration of
shallow groundwater, groundwater discharge to streams, and subsurface outflow along the alluvial
corridors of the Basin creeks. A summary graph of the annual groundwater budgets from 1987 through
2019 are presented in Figure ES-5. A future water budget is generated from application of the
calibrated integrated groundwater-surface model prepared in conjunction with this GSP.
The three most significant findings of the water budget analysis with respect to the preparation
of the GSP are the following:
• First, it is documented that agricultural pumping in the Edna Valley has increased significantly in the
period of record of the water budget analysis, from less than 2,500 Acre-feet per year (AFY) in 1987
to over 4,000 AFY in 2015 about a 60% increase. Other components of the water budget changed
as well, but this is the single largest change of the various water budget components evaluated.
This increase in agricultural pumping corresponds to the observed decline in groundwater
elevations in monitored Edna Valley wells.
• Secondly, the sustainable yield was estimated to be 2,500 AFY for the San Luis Valley and 3,300
AFY for the Edna Valley.
• Thirdly, an estimate is made of the amount of annual groundwater overdraft for the two valleys of
the Basin. The San Luis Valley is estimated to have a surplus of 700 AFY; the “surplus” is likely
expressed as groundwater discharge to streams in the valley. The Edna Valley is estimated to have
an annual average overdraft of 1,100 AFY. Because the presence of the bedrock ridge beneath the
aquifer between Edna Valley and San Luis Valley limits flow between the subareas, the overdraft in
Edna Valley is not significantly impacted by conditions of “surplus” in San Luis Valley. The overdraft
estimate for Edna Valley may be viewed as an estimate of the gross amount of net pumping
reduction and/or supply enhancement that should be targeted to reach sustainability in the Edna
Valley.
The integrated surface water/groundwater model developed for this GSP was used to create a future
water budget and was used to assess the potential effects of climate change over the SGMA planning
horizon. Effects on the water budget due to climate change were found to be minor.
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-11
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Figure ES-5. Groundwater Budget – Basin Total
Monitoring Network
Monitoring is a fundamental component of the GSP necessary to identify impacts to beneficial uses or
Basin users, and to measure progress toward the achievement of any management goal. The
monitoring networks must be capable of capturing data on a sufficient temporal and spatial distribution
to demonstrate short-term, seasonal, and long-term trends in groundwater and related surface water
conditions, and to yield representative information about groundwater conditions for GSP
implementation.
The proposed monitoring network must be able to adequately measure changes in groundwater
conditions to accomplish the following monitoring objectives:
• Demonstrate progress toward achieving measurable objectives.
• Monitor impacts to the beneficial uses and users of groundwater.
• Monitor changes in groundwater conditions relative to measurable objectives and minimum
thresholds for sustainability indicators.
• Quantify annual changes in water budget components.
The monitoring network must provide adequate spatial resolution to properly monitor changes to
groundwater and surface water conditions relative to measurable objectives and minimum thresholds
within the Basin. The network must also provide data with sufficient temporal resolution to demonstrate
short-term, seasonal, and long-term trends in groundwater and related surface water conditions.
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-12
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
There are three monitoring networks for the Basin: a groundwater level network, a groundwater
quality network, and a surface water flow network.
• There are 40 monitoring wells in the GSP groundwater level monitoring network (Figure ES-66); 22
wells in the San Luis Valley and 18 wells in the Edna Valley. All of these wells will be used to
generate groundwater elevation maps and hydrographs during ongoing monitoring during the
SGMA planning horizon. Construction information is available for 31 of the 40 wells. Based on the
available information, 16 of the wells are interpreted to be alluvial wells, while the remaining 24
wells tap into the Paso Robles Formation, Pismo Formation, or are mixed aquifer wells that utilize
groundwater from more than one aquifer. Half of the wells are used for irrigation, seven are private
domestic wells, and 13 are dedicated monitoring wells. Data gaps are discussed, as well as
potential future improvements to the groundwater level network.
• The groundwater quality network consists of nine sites, which are all are Public Water System
supply wells. As such, they have a history of water quality data established that can be used to
compare with future data to assess trends. Water quality for these wells can be accessed using the
GAMA Groundwater Information System. Data gaps are discussed, as are potential future
improvements to the network.
• Surface water flow monitoring can provide valuable information for the Basin model and for
evaluating potential depletion of interconnected surface water, which is one of the sustainability
indicators. There are six permanent stream gages in or adjacent to the Basin, all within the San
Luis Valley. These existing gaging stations only provide stage data, and not stream flow data. It is
recommended that rating curves be established for these stream gages. In addition,
recommendations are presented for up to five new stream gages to be established in both Edna
Valley, where none currently exist, and San Luis Valley.
A subset of the monitoring network wells are defined as Representative Monitoring Sites (RMS), at
which Sustainability Management Criteria are defined for the purpose of managing the Basin. Ten wells
are identified as RMSs, and Sustainable Management Criteria (SMCs) are established for the relevant
Sustainability indicators as discussed in the following section.
Executive Summary
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies ES-13 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure ES-6. Monitoring Network
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-14
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Sustainable Management Criteria
Defining Sustainable Management Criteria (SMC) requires technical analysis of historical data, and
input from the affected stakeholders in the Basin. Data and methods used to develop the SMC are
presented, and discussion is included describing how they influence beneficial uses and users. The
SMCs presented in this GSP are based on currently available data and application of the best available
science. Data gaps exist in the hydrogeologic conceptual model, and uncertainty caused by these data
gaps was considered when developing the SMC. Due to uncertainty in the hydrogeologic conceptual
model, these SMCs are considered initial criteria and will be reevaluated and potentially modified in the
future as new data become available.
The SMCs include definition of Measurable Objectives (MOs), Minimum Thresholds (MTs), and
undesirable results. These criteria define the future sustainable conditions in the Basin and guide the
GSAs in development of policies, implementation of projects, and promulgation of management actions
that will achieve these future conditions.
SMCs are developed for the following Sustainability Indicators, which are applicable in the
Basin:
1. Chronic lowering of groundwater elevations
2. Reduction in groundwater storage
3. Degraded water quality
4. Land subsidence
5. Depletion of interconnected surface water
The sixth Sustainability Indicator, sea water intrusion, only applies to coastal basins, and is not
applicable in the Basin.
MTs for the first two Sustainability Indicators, chronic lowering of groundwater elevations and reduction
of groundwater in storage, are defined as minimum groundwater elevations as measured in the ten
wells established as Representative Monitoring Sites in the Basin; the ten RMS locations are presented
on Figure ES-6. MOs are defined as goals considered to be achievable after evaluation of historical
data in the period of record for each RMS, and Interim Milestones (IMs) are interim goals to be
assessed every 5 years when the GSPs are revised. All SMCs were developed after considerable
stakeholder input during public meetings, and public comment to published draft chapters of the GSP.
SMCs for these two Sustainability Indicators are summarized in Table ES-1.
MTs for the third Sustainability Indicator, degradation of water quality, are based on existing water
quality regulatory criteria as measured in the nine wells established as water quality Representative
Monitoring Sites (RMSs) in the Basin. (For water quality SMCs, MTs are equal to MOs). Identified
potential contaminants of concern arsenic, nitrate, and volatile organic compounds TCE and PCE have
federally mandated Maximum Contaminant Levels (MCLs) of 10 parts per billion (ppb), 10 parts per
million (ppm), and 5 ppb, respectively. The MTs for those constituents were assigned to be equal to the
MCLs. TDS has no MCL, but a water quality goal of 900 ppm is promulgated in the RWQCB Basin
Plan; the MT for the constituent TDS was set at this level. MOs are defined as goals considered to be
achievable after evaluation of historical data in the period of record for each RMS. All SMCs were
developed after considerable stakeholder input during public meetings, and public comment to
published draft chapters of the GSP. SMCs for these two Sustainability Indicators are summarized in
Table ES-2 below.
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-15
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table ES-1. Summary of MTs, MOs, and IMs for SLO Basin RMSs
RMS MT MO 2020 WL 2027 IM 2032 IM 2037 IM SUSTAINABILITY INDICATOR
SAN LUIS VALLEY
SLV-09 102 110 119 110 110 110 Subsidence/Water Levels
SLV-16 70 100 111 100 100 100 Water Levels/Storage
SLV-19 80 110 123 110 110 110 Water Levels/Storage
SLV-12 96 105 105 105 105 105 SW-GW Interaction/Water Levels
EDNA VALLEY
EV-09 82 164 146 150 155 160 Water Levels/Storage
EV-04 160 247 209 219 229 239 Water Levels/Storage
EV-13 172 248 215 223 231 238 Water Levels/Storage
EV-16 150 190 180 175 180 185 Water Levels/Storage
EV-01 263 314 290 314 314 314 SW-GW Interaction /Water levels
EV-11 177 227 219 227 227 227 SW-GW Interaction /Water levels
Note: All water level and interim milestone measurements refer to fall measurements.
Table ES-2. San Luis Obispo Valley Basin Groundwater Basin Water Quality Minimum Thresholds
ID TDS MT (PPM) NO3 MT (PPM) ARSENIC MT (PPB) TCE, PCE (PPB)
WQ-1 900 10 10 5
WQ-2 900 10 10 5
WQ-3 900 10 10 5
WQ-4 900 10 10 5
WQ-5 900 10 10 5
WQ-6 900 10 10 5
WQ-7 900 10 10 5
WQ-8 900 10 10 5
WQ-9 900 10 10 5
MTs for the fourth Sustainability Indicator, land subsidence, are based on data collected under the
California state program of InSAR data, which measures land subsidence from space using satellite
technology. There is no current measurable subsidence in the Basin. The MT is defined as no more
than 0.1 feet of subsidence due to groundwater extraction in any given year, and a cumulative
measured subsidence of 0.5 feet in any 5-year period.
MTs for the fifth Sustainability Indicator, depletion of interconnected surface water (ISW), were defined
based on the language in SGMA that allows groundwater levels to be used as a proxy in place of the
actual measurement of groundwater/surface water (GW/SW) flux, which is difficult to accurately
quantify. Three RMS wells identified in the Basin are located immediately adjacent to SLO Creek and
West Corral de Piedras Creek, and were selected as appropriate RMS wells for ISW. These three wells
have groundwater elevation data for a substantial period of record which indicate that there have been
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-16
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
no trends of declining water levels in these areas. The management goal of the GSP for these wells is
to prevent groundwater elevations from declining to levels lower than those observed in the historical
record, thereby avoiding any significant increase in depletion of ISW over recent conditions Therefore,
MTs for ISW wells were established at the observed low water level in the period of record, and MOs
were defined at the observed high water level in the period of record, thus maintaining groundwater
conditions near the creeks within the observed range of historical data, which will not induce significant
additional depletion of ISW. Additional surface water gages are proposed for the surface water
monitoring network. When installed, these gages will provide additional data to support these SMCs,
and improve the understanding of groundwater/surface water interaction during the implementation
phase.
Projects and Management Actions
The projects and management actions concepts were developed over a series of working sessions with
GSA staff, meetings with GSC members and in six public GSC meetings. Chapter 9 (Projects and
Management Actions) describes the projects and management actions information to satisfy Sections
354.42 and 354.44 of the SGMA Regulations.
A total of seven (7) projects were discussed in detail in this GSP and were centered around
supplemental water sources that could be brought into the SLO Basin to mitigate the overdraft and are
shown on Figure ES-7.
Four of the projects included the State Water Project (SWP) as a supplemental water supply to the SLO
Basin. The Coastal Branch of the SWP conveys potable water from the California Aqueduct to San Luis
Obispo and Santa Barbara Counties and transects the Edna Valley subarea and runs along Orcutt
Road as shown in Figure ES-7. The recent adoption of the Water Management Tools Amendment to
the SWP Contracts by the San Luis Obispo County Flood Control and Water Conservation District
(SLOFCWCD) and the Santa Barbara County Flood Control and Water Conservation District
(SBCWCFCD) presents new opportunities for obtaining SWP water supply and delivery capacity to
Edna Valley.
The remaining three projects utilize the City of SLO recycled water, Price Canyon discharge of treated
water to Pismo Creek, and an adjacent groundwater basin as in-lieu water supply.
The projects were further evaluated with the integrated model to quantify the benefit of the projects with
respect to the SMCs in the Edna Valley. The model results indicate that it is unlikely that any single
project presented will, by itself, maintain water levels above the defined MTs at the RMSs. Therefore,
multiple projects will likely need to be implemented.
The seven projects evaluated as part of the GSP are described in detail in Chapter 9 (Projects
and Management Actions) and included:
• State Water Project for Edna Valley Agricultural Irrigation
• Stater Water Project Recharge Basin within the Edna Valley area.
• State Water Project to the Golden State Water Company
• State Water Project to the Edna and Varian Ranch Mutual Water Companies
• City of SLO Recycled Water for Edna Valley Agriculture
• Varian Ranch Mutual Water Company Arroyo Grande Subbasin Wells
• Price Canyon Discharge Relocation
The management actions in this plan include the completion of the proposed monitoring network by
installing new monitoring sites, development and implementation of a groundwater extraction metering
and reporting plan, and the development of a demand management plan.
The proposed projects and management actions are intended to maintain groundwater levels above
minimum thresholds through in-lieu pumping reductions or increased recharge.
Executive Summary
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies ES-17 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure ES-7. Project Location Map
Executive Summary
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies ES-18
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Implementation Plan
This GSP lays out a roadmap for addressing all of the activities needed for GSP implementation
between the years 2022 and 2042, focusing mainly on the activities during the first five years of
implementation (2022 through 2027).
The implementation plan is based on current understanding of the Basin conditions and includes
consideration of the projects and management actions included in Chapter 9 (Projects and
Management Actions), as well as other actions that are needed to successfully implement the GSP
including the following:
• GSP implementation, administration, and management
• Funding
• Reporting, including annual reports and 5-year evaluations and updates
Implementation of this GSP is estimated to cost approximately $965,000 per year f or the first five years,
excluding the development of the specific projects listed in Chapter 9 (Projects and management
Actions). Estimates of future annual implementation costs (Years 6 through 20) will be developed
during future updates of the GSP, which will include the development of the various anticipated
projects. The costs of specific projects and management actions will likely vary year by year, based in
part on needed adaptive management activities.
The GSAs plan to perform a fee study to evaluate and provide recommendations for developing GSP
implementation funding mechanisms. This study will include focused public outreach and meetings to
educate and solicit input on the potential fee structures/funding mechanisms (i.e., pumping fees,
assessments, or a combination of both). It is anticipated that the fee study will cover the costs
associated with the Administration and Finance, Monitoring Network Implementation, and Reporting.
The Fee Study is not anticipated to cover the costs associated with project implementation.
As part of GSP implementation, the GSAs will develop annual reports and more detailed five-year
evaluations, which could lead to updates of the GSP. Chapter 10 (Implementation Plan) describes the
reporting requirements for both the annual reports and five-year evaluations.
1-1
1
GROUNDWATER SUSTAINABILITY PLAN
Introduction To The SLO Basin GSP
The Sustainable Groundwater Management Act (SGMA), Section 10720, et.
al., of the State Water Code, requires sustainable groundwater management
in all high and medium priority basins. The San Luis Obispo Valley
Groundwater Basin (SLO Basin) was designated as a high priority basin.
To comply with and satisfy the requirements of SGMA, the
following activities are mandated:
• Forming one or more Groundwater Sustainability Agencies
(GSAs) by June 30, 2017 to cover the entire SLO Basin. In
May 2017, both the City of San Luis Obispo (City) and the
County of San Luis Obispo (County) each formed GSAs within
their jurisdictions, resulting in full coverage of the SLO Basin.
• Developing a Groundwater Sustainability Plan (GSP) that
covers the entire SLO Basin and is adopted by the GSAs by
January 31, 2022.
• Implementing the GSP to achieve quantifiable objectives and
sustainability within 20 years (by 2042).
• Annual reporting of groundwater conditions in the basin to the
California Department of Water Resources (DWR).
• Periodic (every five years) evaluation of the GSP
implementation by the GSAs.
IN THIS CHAPTER
• Purpose of the Plan
• Basin Overview
Introduction To The SLO Basin GSP Section 1
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 1-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
1.1. Purpose of the Groundwater Sustainability Plan
This document fulfills the GSP development requirement for the SLO Basin. This GSP describes and
assesses the groundwater condition of the SLO Basin, develops quantifiable management objectives
that account for the interests of the SLO Basin’s beneficial groundwater uses and users, and identifies
a group of projects and management actions that will allow the SLO Basin to achieve and maintain
sustainability in the future. Appendix A (DWR Element of the Plan Guide) identifies the location in this
GSP where the statutory requirements of SGMA are addressed.
1.2. Description of the SLO Basin
This GSP covers the entire SLO Basin identified as Basin No. 3-009 in the DWR’s Bulletin 118 (DWR,
2016). The SLO Basin lies in the southern portion of San Luis Obispo County. The SLO Basin is
comprised of valleys of gentle flatlands and rolling hills ranging in elevation from approximately 100 to
500 feet Above Mean Sea Level (AMSL), surrounded by larger mountain ranges. A terrain map
displaying the SLO Basin boundaries is presented in Figure 1-1, which also displays the watershed
areas of the SLO Creek and Pismo Creek drainages, faults, and nearby groundwater basins
symbolized by the SGMA 2019 Basin Prioritization Phase 1. Average annual precipitation ranges from
approximately 18 inches throughout most of the SLO Basin to about 22 inches in higher elevation areas
near the City and Cal Poly. The SLO Basin is within the watershed areas of the SLO Creek and Pismo
Creek drainages, which are bounded on the northeast by the Santa Lucia Range and on the southwest
by the formations of the San Luis Range and the Edna Fault. The SLO Basin is commonly referenced
as being composed of two distinct valleys, with the San Luis Valley in the northwest and the Edna
Valley in the southeast. The San Luis Valley lies within the SLO Creek drainage and the Edna Valley
lies predominately within the Pismo Creek drainage with a smaller area within the SLO Creek drainage.
There is a bedrock high that underlies the ground surface between the San Luis Valley and Edna
Valley. The watershed divide and the bedrock high divide are not coincident. The sediments of the
Edna Valley have significantly greater thickness than those of the San Luis Valley. Precipitation that
falls west of the watershed divide ultimately flows to Davenport and SLO Creeks, and precipitation that
falls east of that divide flows to Corral de Piedras Creek or the other small tributaries, which ultimately
flow to Pismo Creek south of the SLO Basin.
San Luis Obispo and Pismo Creeks are the primary surface water features within the SLO Basin.
Significant tributaries to the SLO Creek within the Basin include Prefumo Creek, Stenner Creek, and
Davenport Creek. Significant tributaries to Pismo Creek include both the East and West branches of the
Corral de Piedras Creek. Urban areas within the SLO Basin include the City of San Luis Obispo, Cal
Poly, Edna, and Verde. Highway 101 is the most significant north-south highway in the Basin.
1.3. Basin Information
The DWR prioritized California’s groundwater basins through the California Statewide Groundwater
Elevation Monitoring (CASGEM) program and released the results in 2014. With the passage of SGMA,
DWR redefined 54 groundwater basins based on requests for basin boundary modifications and
classified the basins into four categories; high, medium, low, or very low priority. At this time the SLO
Basin was classified as a medium priority basin.
DWR later reassessed the priority of the groundwater basins following the 2016 basin boundary
modification, as required by the Water Code, and documented the results in the SGMA 2019 Basin
Prioritization ( (DWR, 2019)). DWR followed the process and methods developed for the CASGEM
2014 Basin Prioritization and incorporated new data, to the extent data was available, and then
amended the language of Water Code Section 10933(b)(8) (component 8) to include an analysis of
adverse impacts on local habitat and local streamflow.
Introduction To The SLO Basin GSP Section 1
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 1-3
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
DWR reprioritized the basins based on the following components specified in Water Code
Section 10933(b):
• The population overlying the basin or sub-basin.
• The rate of current and projected growth of the population overlying the basin or sub-basin.
• The number of public supply wells that draw from the basin or sub-basin.
• The total number of wells that draw from the basin or sub-basin.
• The irrigated acreage overlying the basin or sub-basin.
• The degree to which persons overlying the basin or sub-basin rely on groundwater as their primary
source of water.
• Any documented impacts on the groundwater within the basin or sub-basin, including overdraft,
subsidence, saline intrusion, and other water quality degradation.
• Any other information determined to be relevant by the department, including adverse impacts on
local habitat and local streamflow.
With the addition of component 8, the SLO Basin was moved from a medium priority basin to a high
priority basin not in critical overdraft and is required to submit a GSP to DWR by January 31, 2022.
The change in priority is inconsequential, as medium priority basins are also required to submit a GSP
to DWR by January 31, 2022.
Additional information about how each of these components were analyzed can be found in the 2019
SGMA Basin Prioritization Process and Results Document (DWR, 2019). DWR is required to provide
updates on basin boundaries, basin priority and critically overdrafted basins every 5 years beginning in
2020 as part of the Bulletin 118 updates.
Introduction To The SLO Basin GSP Section 1
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 1-4 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 1-1. San Luis Obispo Valley Basin and Surrounding Basins
2-1
2
GROUNDWATER SUSTAINABILITY PLAN
Agency Information (§ 354.6)
On May 16, 2017, the City formed the City of San Luis Obispo Groundwater
Sustainability Agency (City GSA) for the portion of the SLO Basin that lies
within its city boundary (Appendix B). On May 23, 2017, the County
formed the San Luis Obispo Valley Basin – County of San Luis Obispo
Groundwater Sustainability Agency (County GSA) to cover all otherwise
unrepresented areas within the SLO Basin (Appendix C).
The County, City, the Edna Valley Growers Mutual Water
Company (EVGMWC), the Varian Ranch Mutual Water Company
(VRMWC), the Edna Ranch Mutual Water Company (ERMWC)
and the Golden State Water Company (GSWC) (each referred to
individually a “Party" and collectively as he "Parties") entered into
a Memorandum of Agreement Regarding Preparation of a GSP
for the SLO Basin (MOA) effective as of January 25, 2018
(Appendix D). The MOA’s purpose is for the City and County, with
input from the Participating Parties (Parties), to coordinate
preparation of a single GSP for the entire SLO Basin pursuant to
SGMA and other applicable provisions of law. Figure 2-1 shows
the service area boundaries of each of the MOA Parties and the
GSA areas.
On October 16, 2018, the County GSA gave notice to DWR that it
intends to develop a GSP in collaboration with the City GSA for
the SLO Basin in accordance with California Water Code (CWC)
Section 10727.8 and the Title 23, Section 353.6 of the California
Code of Regulations (CCR).
IN THIS CHAPTER
• Agency Information
and Governance
Structure
• Notices and
Communication
Agency Information (§ 354.6) Section 1
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 2-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
2.1. Agencies Names and Mailing Addresses
The following contact information is provided for each
groundwater sustainability agency for the SLO Basin pursuant to
California Water Code
§10723.8.
COUNTY OF SAN LUIS O BISPO
COUNTY GOVERNMENT CENTER, ROOM 206
SAN LUIS O BISPO, CA 93408
ATTENTION : JOHN DIODATI , PUBLIC WORKS DIRECTOR
CITY OF SAN LUIS OBISPO
UTILITIES DEPARTMENT
879 MORRO STREET
SAN LUIS O BISPO, CA 93401-2710
ATTENTION : AARON FLOYD, UTILITIES DIRECTOR
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 2-3 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 2-1. San Luis Obispo Valley Basin GSAs and Participating Parties
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 2-4
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
2.2. Agencies Organization and Management Structures
The MOA establishes the Groundwater Sustainability Commission (GSC) as an advisory body to the
GSAs and the terms under which the City GSA and County GSA will jointly develop a single GSP, in
coordination with the GSC. The GSC consists of representatives of the GSAs and the Participating
Parties (i.e., EVGMWC, VRMWC, ERMWC, and GSWC). Each member of the GSC shall be entitled to
one vote on any matter under consideration by the GSC. All recommendations submitted by the GSC
to the City GSA and the County GSA shall be supported by a majority of the members, except for the
recommendation to adopt the GSP or any amendments which shall be supported by at least four of the
members.
City and County staff will collaboratively participate in developing a GSP through, among other things,
providing guidance to the GSP consultant, coordinating with the GSC, and engaging SLO Basin users
and stakeholders. Once the GSP is developed, it will be considered for adoption by the GSAs (i.e., City
Council and County Board of Supervisors) and subsequently submitted to DWR for approval. The MOA
automatically terminates upon approval of the GSP by DWR. The organization and management
structures of each of the Participating Parties are described in the following sections.
The MOA does not specify the appointment of officer positions. However, Figure 2-2 shows the names
of the appointed representative members and alternates and depicts the relationship of the GSAs and
the Participating Parties and the overall governance structure for developing the GSP:
Figure 2-2. Groundwater Sustainability Commission (GSC)
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 2-5
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
2.2.1. County of San Luis Obispo
The County is a GSA and Party of the MOA. Members of the County Board of Supervisors sit on the
GSC as a member and alternate member. The County is governed by a five-member Board of
Supervisors representing five districts in the County. Board of Supervisor members are elected to
staggered four-year terms.
2.2.2. City of San Luis Obispo
The City is a GSA and Party of the MOA. A member of the City Council and the Director of Utilities sit
on the GSC as a member and alternate member, respectively. The City is an incorporated charter city
and operates under “he "Council-Mayor-City Manager" form of municipal government. The five-member
City Council consists of the directly-elected Mayor and four City Council Members. The Mayor is
elected to a two-year term and Council Members are elected to four-year terms.
2.2.3. Other Participating Parties in the MOA
2.2.3.1. Edna Valley Growers Mutual Water Company
EVGMWC is a Party of the MOA and its representative is designated as Chair of the GSC. EVGMWC
represents the majority of the agricultural users in the unincorporated San Luis Obispo County within
the Edna Valley portion of the SLO Basin.
2.2.3.2. Varian Ranch Mutual Water Company
VRMWC is a Party of the MOA and a member of the GSC. VRMWC provides water to the residents of
unincorporated San Luis Obispo County and serves an area within the Edna Valley portion of SLO
Basin as shown in Figure 2-1. The VRMWC and ERMWC are represented by a single member on the
GSC.
2.2.3.3. Edna Ranch Mutual Water Company
ERMWC is a Party of the MOA and a member of the GSC. ERMWC provides water to the residents of
unincorporated San Luis Obispo County and serves an area within the Edna Valley portion of SLO
Basin as shown in Figure 2-1. The VRMWC and ERMWC are represented by a single member on the
GSC.
2.2.3.4. Golden State Water Company
GSWC is a Party of the MOA and its representative is designated as a Vice Chair of the GSC. GSWC
is an Investor Owned Utility regulated by the California Public Utilities Commission (CPUC) and subject
to federal Sarbanes-Oxley requirements that hold companies to the highest levels of transparency.
CPUC’s authority to regulate water, electric, natural gas, and other public utilities subject to its
jurisdiction derives from the California state constitution. GSWC provides water to the residents of
unincorporated San Luis Obispo County and serves an area within the Edna Valley portion of SLO
Basin as shown in Figure 2 1.
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 2-6
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
2.3. Authority of Agencies
The GSAs developing this coordinated GSP were formed in accordance with the requirements of
California Water Code §10723 et seq. The resolutions of formation for the GSAs and the Memorandum
of Understanding (MOA) are included in Appendices– B - D. The specific legal authorities for GSA
formation and GSP implementation are summarized below.
2.3.1. Groundwater Sustainability Agencies
“Local agency” is defined pursuant to CWC§ 10721 as a local public agency that has water supply,
water management, or land use responsibilities within a groundwater basin.
2.3.1.1. County of San Luis Obispo
The County was created as described in Government Code Section 460 which states that the state is
divided into counties, the names, boundaries, and territorial subdivisions of which are declared in Title 3
of the Government Code. The County has land use authority over the unincorporated areas of the
county, including areas overlying the SLO Basin. The County is therefore a local agency under CWC§
10721(n) with the authority to establish itself as a GSA. Upon establishing itself as a GSA, the County
retains all the rights and authorities provided to GSAs under CWC§ 10725 et seq. The City and the
County shall each be responsible for adopting the GSP and implementing the GSP within their
respective service areas.
2.3.1.2. City of San Luis Obispo
The City is incorporated under the laws of the State of California. The City provides water supply and
land use planning services to its residents. The City is therefore a local agency under CWC§ 10721(n)
with the authority to establish itself as a GSA. Upon establishing itself as a party of the GSA, San Luis
Obispo retains all the rights and authorities provided to GSAs under CWC§ 10725 et seq. The City and
the County shall each be responsible for adopting the GSP and implementing the GSP within their
respective service areas.
2.3.2. Memorandum of Agreement
The MOA Parties entered into the MOA effective as of January 25, 2018. The MOA establishes the
GSC as an advisory body to the GSAs and the terms under which the City GSA and County GSA will
jointly develop a single GSP, in coordination with the GSC pursuant to SGMA and other applicable
provisions of law. The GSC members consists of representatives of the GSAs and the Participating
Parties (i.e., EVGMWC, VRMWC, ERMWC, and GSWC). City and County staff will collaboratively
participate in developing a GSP through, among other things, providing guidance to the consultant,
coordinating with the GSC, and engaging SLO Basin users and stakeholders. Each GSC member has
one vote on the GSC. The County Board of Supervisors and the City Council may approve or reject
any advisory opinion submitted by the GSC provided that in every case that the County Board of
Supervisors or City Council rejects an advisory opinion of the GSC related to the contents or adoption
of the GSP it shall do so only after holding a public hearing, at which time the members of the GSC
shall have the right to appear and address the City Council and the County Board of Supervisors. The
MOA automatically terminates upon approval of the GSP by DWR. The Parties may then decide to
enter into a new agreement to coordinate GSP implementation. A copy of the MOA is included in
Appendix D.
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 2-7
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
2.3.3. Coordination Agreements
Only a single GSP is developed by the City and County GSAs to cover the entire SLO Basin.
Therefore, no coordination agreements with other GSAs are necessary because there is not multiple
GSPs.
2.4. Contact Information for Plan Manager
The plan manager is to be determined.
2.5. Notices and Communications (§ 354.10)
The outreach activities conducted to support GSP development are documented in Appendix E. A
Communication and Engagement Plan (C&E Plan) was executed and includes the planned activities for
engaging interested parties in SGMA implementation efforts in the San Luis Obispo Valley Basin
(Appendix E). Appendix E includes a Communications and Engagement Implementation Workplan for
SLO Basin GSP. The workplan details the target stakeholder categories, developed outreach goals
and evaluation metrics, identified communication priorities schedule, and described the outreach tools
and materials that were used throughout the GSP development.
The goals of the C&E Plan are as follows:
• Create an inclusive and transparent participation experience that builds public trust in the GSP and
optimizes participation among all stakeholders.
• Employ outreach methods that facilitate shared understanding of the importance of sustainable
groundwater conditions and impacts on stakeholders.
• Communicate “early and often,” and actively identify and eliminate barriers to participation.
• Develop a cost-effective, stakeholder-informed GSP supported by best-in-class technical data.
Outreach and communication throughout GSP development included regular presentations at GSC
meetings, meetings with community groups, meetings with individual stakeholders, and community
workshops. Comments from stakeholders were collected via the Groundwater Communications Portal
(GCP), SLOWaterBasin.com, and considered the comments from their stakeholders Table 2-1 lists the
public meetings and events that were held throughout the development of the GSP where elements of
the Plan were discussed or considered by the GSC and the GSAs. Figure 2-3 shown below provides a
summary of the engagement results regarding the stakeholder outreach touchpoints, stakeholder lists,
stakeholder participation, and statistics for the SLOWaterBasin.com website.
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 2-8
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 2-1. List of Public Meetings and Workshops
EVENT LOCATION DATE TIME
GSC Public Meeting Ludwick Community Center 4/10/2019 03:30PM
GSC Public Meeting Ludwick Community Center 6/12/2019 03:30PM
Stakeholder Workshop Library Community Room 8/14/2019 03:00PM
GSC Public Meeting Ludwick Community Center 9/11/2019 03:00PM
GSC Public Meeting Ludwick Community Center 12/11/2019 03:30PM
GSC Public Meeting Ludwick Community Center 3/11/2020 03:30PM
Stakeholder Workshop Zoom Meeting 6/10/2020 03:30PM
GSC Public Meeting Go to Meeting 7/8/2020 06:00PM
GSC Public Meeting Go to Meeting 9/9/2020 03:00PM
Stakeholder Workshop: Zoom Meeting 10/1/2020 03:30PM
GSC Public Meeting Zoom Meeting 12/9/2020 03:00PM
GSC Public Meeting Zoom Meeting 2/17/2021 03:00PM
GSC Public Meeting Zoom Meeting 3/1/2021 03:30PM
GSC Public Meeting Zoom Meeting 3/31/2021 03:30AM
GSC Public Meeting Zoom Meeting 4/7/2021 03:00PM
GSC Public Meeting Zoom Meeting 5/5/2021 03:00PM
GSC Public Meeting Zoom Meeting 5/20/2021 03:00PM
GSC Public Meeting Zoom Meeting 6/21/2021 03:30PM
GSC Public Meeting Zoom Meeting 8/18/2021 03:30PM
Agency Information (§ 354.6) Section 2
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 2-9 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Note: The Stakeholder Groups Represented is 9/10 due to the fact that Tribal interests were contacted and informed of the GSP development process, and that they indicated that they would engage in
the Implementation Phase of the GSP.
Figure 2-3. Stakeholder Communication and Engagement Summary
3-1
3
GROUNDWATER SUSTAINABILITY PLAN
Description of Plan Area (§ 354.8)
The SLO Basin is oriented in a northwest-southeast direction and is
composed of unconsolidated or loosely consolidated sedimentary deposits.
It is approximately 14 miles long and 1.5 miles wide and covers a surface
area of about 12,700 acres (19.9 square miles).
The SLO Basin is bounded on the northeast by the relatively
impermeable bedrock formations of the Santa Lucia Range, and
on the southwest by the formations of the San Luis Range and
the Edna fault system. The bottom of the SLO Basin is defined by
the contact of permeable sediments with the impermeable
bedrock Miocene-aged and Franciscan Assemblage rocks (DWR,
2003). The SLO Basin is commonly referenced as being
composed of two distinct valleys, with the San Luis Valley in the
northwest and the Edna Valley in the southeast.
IN THIS CHAPTER
• SLO Basin
Information
• Jurisdictional Areas
• Land Use
• Existing Plans
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.1. SLO Basin Information
The San Luis Valley comprises approximately the northwestern half of the SLO Basin. It is the area of
the SLO Basin drained by SLO Creek and its tributaries (Prefumo Creek and Stenner Creek west of
Highway 101, Davenport Creek and smaller tributaries east of Highway 101). Surface drainage in San
Luis Valley drains out of the SLO Basin, flowing to the south along the course of SLO Creek, toward the
coast in the Avila Beach area, approximately along the course of Highway 101. The San Luis Valley
includes part of the City and California Polytechnic State University (Cal Poly) jurisdictional boundaries,
while the remainder of the San Luis Valley is unincorporated land. Land use in the City is primarily
single- and multi-family residential, commercial, industrial, and a small amount of land in agricultural
uses. The area in the northwest part of the SLO Basin, along Los Osos Valley Road, has significant
areas of irrigated agriculture, primarily row crops.
The Edna Valley comprises approximately the southeastern half of the SLO Basin. The primary creeks
that drain the SLO Basin are the east and west branches of Corral de Piedras Creek, which join to form
Pismo Creek, draining south out of the Edna Valley into Price Canyon. In the 1960s a private reservoir
with storage capacity of 552 AF was permitted and constructed on West Corral de Piedras Creek
upstream of the Basin, which interrupted the natural runoff from the watershed upstream of the
reservoir; in 1990 this reservoir was permitted an expansion to a storage capacity of 951 AF . Smaller
unnamed tributaries drain south from the SLO Basin in the extreme southeastern part of Edna Valley,
ultimately joining Pismo Creek. Some of the unincorporated lands in Edna Valley are served by various
private water purveyors. The primary land use in the Edna Valley is agriculture. During the past two
decades wine grapes have become the most significant crop type in the Edna Valley.
The physical definition of the SLO Basin boundary is the contact between the unconsolidated or loosely
consolidated sediments and the basement rock of the Miocene-aged formations and Franciscan
Assemblage. There is a topographic high point in the underlying bedrock between the San Luis and
Edna Valley subareas. The watershed divide and the bedrock high are not coincident. The sediments of
the Edna Valley have significantly greater thickness than those of the San Luis Valley. Precipitation that
falls west of that divide ultimately flows to Davenport and SLO Creeks, and precipitation that falls east
of that divide flows to Corral de Piedras Creek or the other small tributaries, ultimately flowing to Pismo
Creek south of the SLO Basin.
The primary weather patterns for the SLO Basin derive from seasonal patterns of atmospheric
conditions that originate over the Pacific Ocean and move inland. As storm fronts move in from the
coast, rainfall in the area falls more heavily in the mountains, and the SLO Basin itself receives less
rainfall because of a muted rain shadow effect. Average annual precipitation ranges from approximately
18 inches throughout most of the SLO Basin to about 22 inches in higher elevation areas near the City
and Cal Poly. Figure 3-1 presents the time series of annual precipitation for the period of record from
1870 to 2018 at the Cal Poly weather station No. 52. The average historical rainfall at this location to
date is 21.69 inches, with a standard deviation of 8.75 inches. The historical maximum is 49.99 inches,
which occurred in 1884. The historical minimum is 4.56 inches, which occurred in 2013.
3.2. Adjudicated Areas
The SLO Basin is not an adjudicated basin.
3.3. Jurisdictional Areas
In addition to MOA Parties, there are several entities that have some degree of water management
authority in the SLO Basin. Each entity is discussed below.
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-3
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.3.1. Federal Jurisdictions
There are no federal agencies with land holdings in the SLO Basin.
3.3.2. Tribal Jurisdiction
The two prominent Native American tribes in the County are the Obispeño Chumash and Salinan
Indian Tribes. The Chumash occupied the coast between San Luis Obispo and northwestern Los
Angeles County, inland to the San Joaquin Valley. They were divided into two broad groups, of which
the Obispeño were the northern group. The Salinan were northern neighbors of the Chumash, and
although the presence of a firm boundary between the Chumash and the Salinan is uncertain,
ethnographic accounts have placed Salinan territories in the northern portion of the County. However,
these two tribes do not have any recognized tribal land in the SLO Basin.
3.3.3. State Jurisdiction
The State of California University system owns and operates land that is associated with Cal Poly
located in the northern edge of the SLO Basin off Hwy 1. Cal Poly is a significant user of local water
resources utilizing both groundwater and surface water. In addition to on-site wells which are used for
landscape irrigation and agricultural irrigation, Cal Poly has water rights to Whale Rock Reservoir which
is primarily used to meet the campus’ potable water needs. Water from Whale Rock is treated at the
City’s Water Treatment Plant and delivered through shared infrastructure from the City’s Water
Treatment Plant to the campus. The City treats the wastewater generated from Cal Poly. There are no
California State Parks or other State-owned lands or entities located within the SLO Basin.
3.3.4. County Jurisdiction
The County of San Luis Obispo and the associated San Luis Obispo County Flood Control and Water
Conservation District (SLOFCWCD) (see section under Special Districts below) have jurisdiction over
the entire County including the SLO Basin. The County owns approximately 300 acres of land in the
SLO Basin which is primarily located in the vicinity of the SLO County Airport.
3.3.5. City and Local Jurisdictions
The City is centrally located in the SLO Basin and has land and water management authority over its
incorporated area. The City has three primary water supply sources including Whale Rock Reservoir,
Salinas Reservoir, and Nacimiento Reservoir, with recycled water (for irrigation) and groundwater
serving = supplemental sources. Three major mutual water companies exist in the SLO Basin: Edna
Valley Growers, Varian Ranch, and Edna Ranch Mutual Water Companies. One investor-owned utility
exists within the SLO Basin: Golden State Water Company. GSWC provides groundwater that is
pumped from the Edna Valley Basin to residential and agricultural customers.
3.3.6. Special Districts
The San Luis Obispo County Flood Control and Water Conservation District is a dependent Special
District governed by the County Board of Supervisors. It has jurisdiction over all of the County including
the SLO Basin and was established as a resource to help individuals and communities in San Luis
Obispo County identify and address flooding problems with the purpose "to provide for control,
disposition and distribution of the flood and storm waters of the district and of streams flowing into the
district...".
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-4
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.4. Land Use
The County, City, and State have land use authority in the SLO basin within their respective
jurisdictions. Land use information for the SLO Basin was based on DWR’s land use database (DWR,
2014). The 2014 land use in the SLO Basin is shown on Figure 3-2 and is summarized by group in
Table 3-1. All land use categories except native vegetation listed in Table 3-1 are provided by DWR
(DWR, 2014). The areas of the basin that did not have a land use designation were assumed to be
native vegetation.
Table 3-1. Agricultural Land use categories defined for the SLO Basin by DWR (2014)
LAND USE CATEGORY ACRES
Citrus and subtropical 136
Deciduous fruits and nuts 21
Grain and hay crops 183
Idle 713
Pasture 179
Truck nursery and berry crops 1079
Urban 6,412
Vineyard 1,929
Young perennial 2
Native vegetation <1
TOTAL 10,656
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 3-5 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-1. San Luis Obispo Historical Annual Precipitation
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 3-6 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-2. San Luis Obispo Valley Basin Existing Land Use Designations
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-7
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.4.1. Water Source Types
Entities in the SLO Basin utilize three types of water sources to meet the demands: groundwater,
surface water, and recycled water. Excluding the City and Cal Poly, all water demand in the SLO Basin
is met with groundwater. Cal Poly has rights to 33.71% of water from Whale Rock Reservoir and the
rest of their water supply comes from local groundwater. The City has an entitlement to water from the
Nacimiento Water Project, rights to Salinas Reservoir (Santa Margarita Lake), rights to 55.05% of water
in Whale Rock Reservoir, SLO Basin groundwater, and recycled water from its Water Resource
Recovery Facility (WRRF). The City has imported supplies from Salinas Reservoir, located near the
community of Santa Margarita, since 1944, Whale Rock Reservoir, located near the community of
Cayucos, since 1961, and Nacimiento Reservoir since 2011. Table 3-2 summarizes the surface water
supply available from each source and Figure 3-3 shows the location of water supply source types
within the SLO Basin.
Table 3-2. Summary of surface water supply sources available to the SLO Basin
SUPPLY SOURCES AMOUNT AVAILABLE (AFY)
Nacimiento Reservoir- City 5,4821
Salinas Reservoir - City 4,9101 Whale Rock Reservoir - City
Recycled Water - City ~1,0001
TOTAL 11,392
1 City of San Luis Obispo, General Plan, Water and Wastewater Management Element, 2018.
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 3-8 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-3. San Luis Obispo Valley Basin Water Supply Sources
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-9
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.4.2. Water Source Sectors
Water demand in the SLO Basin is organized into the six water use sectors identified in the GSP
Emergency Regulations. These include:
• Urban. Urban water use is assigned to non-agricultural water uses in the City and census-
designated places. Domestic use outside of census-designated places is not considered urban use.
• Industrial. There is limited industrial use in the SLO Basin. The DWR land use designations in the
SLO Basin does not include industrial uses.
• Agricultural. This is the largest groundwater use sector in the SLO Basin by water demand.
• Managed wetlands. There are several managed wetlands in the SLO Basin that are managed
by both federal, state, and local agencies. In general, wetlands in the area are managed by the
following agencies: (1) City of San Luis Obispo, (2) California Department of Fish and Wildlife, (3)
California State Water Resources Control Board, (4) U.S. Fish and Wildlife Service, and (5) U.S.
Army Corps of Engineers. The wetlands and natural vegetation areas that are potentially dependent
ecosystems include Laguna Lake and reaches of the SLO Creek, Prefumo Creek, Stenner Creek,
Davenport Creek, East and West Corral De Piedra Creeks, and Pismo Creek. Water use for these
ecologically sensitive areas is addressed in Chapter 5 (Groundwater Conditions), Chapter 6 (Water
Budget), and Chapter 8 (Sustainable Management Criteria).
• Managed recharge. There is no managed recharge in the SLO Basin. Recycled water discharge
to creeks and applied irrigation is included in the urban water use sector.
• Native vegetation. This is the largest water use sector in the SLO Basin by land area. This
sector includes rural residential areas.
Figure 3-4 shows the distribution of the water use sectors and potential groundwater dependent
ecosystems in the SLO Basin.
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 3-10 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-4. San Luis Obispo Valley Basin Water Use Sectors
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-11
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.5. Density of Wells
Well types, well depth data, and well distribution data were downloaded from DWR’s well completion
report map application (DWR, 2016). DWR categorizes wells in this mapping application as either
domestic, production, or public supply. These categories are based on the well use information
submitted with the well logs to DWR. Well information was also collected from County of San Luis
Obispo Environmental Health Services (EHS). The EHS dataset was compiled from information gained
from the well construction permit application process. Table 3-3 summarizes the types of wells by use
for all well logs submitted to DWR and EHS.
Table 3-3. DWR and County Wells
WELL DATA SOURCE TYPE OF WELL TOTAL NO. OF WELLS
DWR
Domestic 75
Production 71
Public Supply 24
Total 170
County EHS
Domestic Private 355
Domestic Public 43
Irrigation 231
Total 6291
Notes:
1. The County EHS database may contain duplicates that are also included in the DWR database.
Figure 3-5, Figure 3-6, and Figure 3-7 show the density of wells in the SLO Basin by their types of use.
The DWR data used to develop these maps is not necessarily the same set of well data held EHS as
shown in Figure 3-8. DWR data was used to develop maps of well densities because they are
organized for easy mapping of well density per square mile. These maps should be considered
representative of well distributions but are not definitive. It is also important to note that both the DWR
and EHS well databases are not updated with information regarding well status and the well locations
are not verified in the field. Therefore, it is uncertain whether the wells in these databases are
currently active or have been abandoned or destroyed.
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 3-12 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-5. San Luis Obispo Valley Basin Domestic Well Density
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 3-13 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-6. San Luis Obispo Valley Basin Production Well Density
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 3-14 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-7. San Luis Obispo Valley Basin Public Supply Well Density
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 3-15 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-8. San Luis Obispo Valley Basin Public Supply Well Density
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-16
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.6. Existing Monitoring and Management Programs
3.6.1. Service Area Population
Groundwater levels and quality are currently measured in the SLO Basin by the SLOFCWCD and a
variety of other agencies as described below. Figure 3-9 shows the locations of monitored wells
identified in the Groundwater Ambient Monitoring and Assessment (GAMA) program (i.e. publicly
available data) that are monitored by several public agencies, the SLOFCWCD, and the Central Coast
Regional Water Quality Control Board (CCRWQCB) Irrigated Lands Program. The monitoring network
also includes other wells in the area designated as private that are not shown on this map (Figure 3-8).
Additional evaluation of the current monitoring program will be conducted for the GSP to establish a
representative monitoring network of public and private wells that will be used during plan
implementation to track groundwater elevations and ensure that minimum thresholds have not been
exceeded.
3.6.1.1. Groundwater Level Monitoring
The SLOFCWCD has been monitoring groundwater levels county-wide on a semi-annual basis for
more than 50 years to support general planning and for engineering purposes. Groundwater level
measurements are taken once in the spring and once in the fall. The monitoring takes place from a
voluntary network of wells. In the SLO Basin, there are 16 active wells in this program (Figure 3-9). The
voluntary monitoring network has changed over time as access to wells has been lost or new wells
have been added to the network.
3.6.1.2. Groundwater Quality Monitoring
Groundwater quality is monitored/reported under several different programs and by different agencies
including:
• Municipal and community water purveyors that collect water quality samples on a routine basis for
compliance monitoring and reporting to the California State Water Resources Control Board
(SWRCB) Division of Drinking Water (DDW).
• The USGS who collects water quality data on a routine basis under the GAMA program. These data
are stored in the State’s GeoTracker GAMA system.
• There are multiple sites that are monitoring groundwater quality as part of investigation or
compliance monitoring programs through the CCRWQCB. See Figure 3-9 for CCRWQCB well
monitoring locations through the GeoTracker GAMA system.
• The CCRWQCB under Agricultural Order No. R3-2017-0002, a Conditional Waiver of Waste
Discharge Requirements for Discharges from Irrigated Lands, requires all growers to implement
groundwater monitoring, either individually or as part of a cooperative regional monitoring program.
Growers electing to implement individual monitoring (i.e., not participating in the regional monitoring
program implemented by the Central Coast Groundwater Coalition [CCGC] within the SLO Basin)
are required to test all on-farm domestic wells and the primary irrigation supply wells for nitrate or
nitrate plus nitrite, and general minerals (including, but not limited to, TDS, sodium, chloride, and
sulfate).
• California Water Data Library contains groundwater level and water quality monitoring station
information. The data available from this resource has been used above.
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin Groundwater
Sustainability Agencies 3-17 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 3-9. Monitored Wells in the San Luis Obispo Valley Basin
Description of Plan Area (§ 354.8) Section 3
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 3-18
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
3.6.1.3. Surface Water Monitoring
The Water Resources Division of the SLO County Public Works maintains six (6) real-time data
monitoring stream gauges within the SLO Creek watershed and all except Andrews St. Bridge are
located within the SLO Basin. As summarized in Table 3-4, each stream gauge measures stage at 15-
minute intervals. Stage-discharge relationships, or rating curves, for each of the five stream gauge
stations were generated as part of the San Luis Obispo Creek Watershed Hydrology and Hydraulic
Model Calibration Study (Questa Engineering Corporation, 2007). More recently (2018/2019), Central
Coast Salmon Enhancement has approximated rating curves for the Andrews St., Elks Lane, and
Stenner Creek gauge stations based on recorded stage data and measured flows. The locations of the
five County gauges are presented in Figure 3-10.
In addition to the County gauges, the City of San Luis Obispo routinely estimates flow at four locations
(RW-4, RW-5, RW-7, RW-8) along San Luis Obispo Creek in the vicinity of the City’s WRRF outfall as
part of its National Pollutant Discharge Elimination System permitting program. RW-8 at South Higuera
Bridge is located outside of the SLO Basin. Flow at the four locations (RW-4, RW-5, RW-7, and RW-8)
is calculated weekly from April through the end of October based on the depth measurements recorded
along the creek cross-section and are located within the Basin.
Table 3-4. Stream gauges and summary of records available
STREAM GAGE SOURCE DATA RECORDED DATA INTERVAL YEAR DATA BEGINS DATUM1
Andrews St Bridge SLO County Stage 15 Minutes 2006 NAVD 88
Stenner Creek at Nipomo SLO County Stage 15 Minutes 2005 NAVD 88
Elks Ln SLO County Stage 15 Minutes 2005 NAVD 88
Madonna Rd SLO County Stage 15 Minutes 2005 NAVD 88
E. Fork at Jespersen Rd SLO County Stage 15 Minutes 2005 NAVD 88
Marsh Street Bridge SLO County Stage 15 Minutes 2019 NAVD 88
RW-4 City of SLO Depth, Flow Weekly 2005 -
RW-5 City of SLO Depth, Flow Weekly 2005 -
RW-7 City of SLO Depth, Flow Weekly 2005 -
RW-8 City of SLO Depth, Flow Weekly 2005 -
1Prior to 5/23/2017 County data was recorded on NGVD 29 datum. Conversion is 2.86 feet.
3.6.1.4. Surface Water Monitoring
Climate monitoring in the SLO Basin includes stations that collect data related to temperature,
evapotranspiration, relative humidity, atmospheric pressure, precipitation, and other climate
parameters. Four stations monitored by San Luis Obispo County Public Works collect one or more
climate parameters in the SLO Basin. The locations of these stations are shown on Figure 3-10.
The National Climatic Data Center has three stations within the County of San Luis Obispo and one
station within the SLO Basin that collect climate data. These stations do not have extensive historic
data. The station with the most precipitation data not associated with the National Climatic Data Center,
Cal Poly Weather Station 52 (CPWS-52), began recording data in 1870. The Cal Poly Weather Station
52 measures daily temperatures and other climate parameters in addition to precipitation. Daily records
are available from April 1986 to present. Table 3-5 lists the climate stations and summary of records
available.
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The long-term precipitation and cumulative departure from the mean (CDFM) measurements at CPWS-
52 are shown in Figure 3-11 from– 1870 - 2018. Average annual precipitation at this station varies from
approximately 7 to 55 inches with a mean annual average precipitation of 21.95 inches. The longest dry
period on record occurred from 1943 – 1965 and the longest wet period on record occurred from 1899
– 1916. Table 3-6 provides a summary of average monthly rainfall, temperature, and evapotranspiration
(ET0) for the SLO Basin at CPWS-52 from 1987 to 2018.
Table 3-5. Weather station Information and summary of records available
STATION SOURCE DATA RECORDED DATA INTERVAL YEAR DATA BEGINS
Cal Poly Weather Station 52 CIMIS
Precipitation,
Temperature,
Evapotranspiration
Daily 1986
SLO Reservoir SLO County Precipitation 12-Hour 2005
The Gas Company SLO County Precipitation 12-Hour 2005
South Portal SLO County Precipitation 12-Hour 2005
SLO County Farm Bureau Weather Element Precipitation,
Temperature Daily 2015
Table 3-6. Average Monthly Climate Summary 1987 – 2018 at Cal Poly Weather Station 52
MONTH AVERAGE PRECIPITATION (INCHES) AVERAGE ET0 (INCHES) AVERAGE TEMPERATURE (°F)
January 4.24 2.29 54
February 4.07 2.54 54
March 3.27 3.85 56
April 1.04 4.93 57
May 0.53 5.67 59
June 0.22 6.13 62
July 0.12 6.24 64
August 0.03 5.79 64
September 0.21 4.81 64
October 1.16 3.93 63
November 1.49 2.74 58
December 3.42 2.18 53
Description of Plan Area (§ 354.8) Section 3
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Figure 3-10. San Luis Obispo Valley Basin Surface Water Features, Stream Gauges, and Weather Stations
Description of Plan Area (§ 354.8) Section 3
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Figure 3-11. San Luis Obispo Valley Basin Historical Annual Precipitation and CDFM
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3.6.2. Existing Management Plans
There are numerous groundwater and water management plans, studies, and reports that cover either
the whole or portion of the SLO Basin. These documents are described in the following subsections,
along with brief descriptions of how they relate to the management of current water supply, projected
water supplies, and land use.
3.6.2.1. SLO Basin Characterization and Monitoring Well Installation
The SLO Basin Characterization and Monitoring Well Installation (GSI Water Solutions, 2018)
documents the available published reports, private well reports, well completion reports, geologic logs,
and other data that were reviewed to generate a comprehensive compilation of the current
understanding of the hydrogeologic setting of the SLO Basin. This information is intended to provide the
basis of knowledge for future planning and management activities performed under the requirements of
GMA, including the development of a hydrogeologic conceptual model, construction of a numerical
groundwater model, and development of a GSP.
3.6.2.2. San Luis Obispo County Master Water Report (2012)
The County’s Master Water Report (MWR) (Carollo, 2012) is a compilation of the current and future
water resource management activities being undertaken by various entities within the County and is
organized by Water Planning Areas (WPA). The MWR explores how these activities interrelate,
analyzes current and future supplies and demands, identifies future water management strategies and
ways to optimize existing strategies, and documents the role of the MWR in supporting other water
resource planning efforts. The MWR evaluates and compares the available water supplies to the water
demands for the different water planning areas.
This was accomplished by reviewing or developing the following:
• Current water supplies and demands based on available information
• Forecast water demands and water supplies available in the future under current land use policies
and designations
• Criteria under which there is a shortfall when looking at supplies versus demands
• Criteria for analyzing potential water resource management strategies, projects, programs, or
policies
• Potential water resource management strategies, projects, programs, or policies to resolve potential
supply deficiencies
3.6.2.3. San Luis Obispo County Integrated Regional Water Management Plan (2014)
The San Luis Obispo County Integrated Regional Water Management Plan (IRWMP) was initially
developed and adopted by the SLOFCWCD in 2005 (GEI Consultants, 2005), and has been updated
several times. The SLOFCWCD, in cooperation with the SLOFCWCD’s San Luis Obispo Regional
Water Management Group (RWMG), prepared the 2019 IRWMP (San Luis Obispo County Flood
Control and Water Conservation District , 2020) to align the region’s water resources management
planning efforts with the State’s planning efforts. The IRWMP is used to support the region’s water
resource management planning and the submittal of grant applications to fund these efforts.
The IRWMP includes goals and objectives that provide the basis for decision-making and are used to
evaluate project benefits. The goals and objectives reflect input from interested stakeholders on the
region’s major water resources issues. These goals and objectives help secure and enhance the water
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supply reliability, water quality, ecosystems, groundwater, flood management and water-related
communication efforts across the entire region. In addition, the IRWMP identifies resource
management strategies, recognizes other funding opportunities, and includes a list of action items
(projects, programs, and studies) that agencies around the region are undertaking to achieve and
further these goals and objectives.
3.6.2.4. City of San Luis Obispo 2015 Urban Water Management Plan (2016)
The City’s Urban Water Management Plan (UWMP) (City of San Luis Obispo, 2016) describes the
City’s current and future water demands, identifies current water supply sources, and assesses supply
reliability for the City. The UWMP describes the City’s use of groundwater and its support for efforts to
avoid overdraft by developing additional sources. The UWMP provides a forecast of future growth,
water demand, and water sources for the City through 2035. These sources include water
conservation, the Nacimiento Water Project, Salinas Reservoir (Santa Margarita Lake), Whale Rock
Reservoir, SLO Basin groundwater, and recycled water from the WRRF.
3.6.3. Existing Groundwater Regulatory Programs
3.6.3.1. Groundwater Export Ordinance (2015)
In 2015, County of San Luis Obispo adopted an Exportation of Groundwater ordinance (County Code
Chapter 8.95) that requires a permit for the export of groundwater out of a groundwater basin or out of
the County. An export permit is only approved if the Department of Public Works Director or his/her
designee finds that moving the water would not have any adverse impacts to groundwater resources,
such as causing aquifer levels to drop, disrupting the flow of neighboring wells, or resulting in seawater
intrusion. Export permits are only valid for one year.
3.6.3.2. Countywide Water Conservation Program Resolution 2015-288 (2015)
This ordinance identifies areas of severe decline in groundwater elevation and that properties overlying
these areas would be further restricted from planting new or expanding irrigated agriculture except for
those converting irrigated agriculture on the same property into a different crop type. This resolution
applies to the Nipomo Mesa Water Conservation Area which is part of the Santa Maria Groundwater
Basin, the Los Osos Groundwater Basin, and the Paso Robles Groundwater Basin. Therefore, it is not
applicable to the SLO Basin.
3.6.3.3. Agricultural Order R3-2017-002 (2017)
In 2017 the CCRWQCB issued Agricultural Order No. R3-2017-0002, a Conditional Waiver of Waste
Discharge Requirements for Discharges from Irrigated Lands. The permit requires that growers
implement practices to reduce nitrate leaching into groundwater and improve surface receiving water
quality. Specific requirements for individual growers are structured into three tiers based on the relative
risk their operations pose to water quality.
Growers must enroll, pay fees, and meet various monitoring and reporting requirements according to
the tier to which they are assigned. All growers are required to implement groundwater monitoring,
either individually or as part of a cooperative regional monitoring program. Growers electing to
implement individual monitoring (i.e., not participating in the regional monitoring program implanted by
the Central Coast Groundwater Coalition [CCGC]) are required to test all on-farm domestic wells and
the primary irrigation supply wells for nitrate or nitrate plus nitrite, and general minerals (including, but
not limited to, TDS, sodium, chloride, and sulfate).
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3.6.3.4. Water Quality Control Plan for the Central Coast Basins (2017)
The Water Quality Control Plan for the Central Coastal Basin (Basin Plan) was most recently updated
in September 2017 by the SWRCB (Regional Water Quality Control Board, Central Coast Region,
2017). The objective of the Basin Plan is to outline how the quality of the surface water and
groundwater in the Central Coast Region should be managed to provide the highest water quality
reasonably possible.
The Basin Plan lists beneficial users, describes the water quality that must be maintained to allow those
uses, provides an implementation plan, details SWRCB and CCRWQCB plans and policies to protect
water quality, and a statewide surveillance and monitoring program as well as regional surveillance and
monitoring programs.
Present and potential future beneficial uses for inland waters in the SLO Basin are: surface water and
groundwater as municipal supply (water for community, military or individual water supplies);
agricultural; groundwater recharge; recreational water contact and non-contact; sport fishing; warm
fresh water habitat; wildlife habitat; rare threatened or endangered species; and spawning,
reproduction, and/or early development of fish.
Water Quality Objectives for both groundwater (drinking water and irrigation) and surface water are
provided in the Basin Plan.
3.6.3.5. California DWR Well Standards (1991)
Under the CWC Sections 13700 to 13806, DWR has the responsibility for developing well standards.
DWR maintains these standards to protect groundwater quality. California Well Standards, published as
DWR Bulletin 74, represent minimum standards for well construction, alteration, and destruction to
protect groundwater. Cities, counties, and water agencies in California have regulatory authority over
wells and can adopt local well ordinances that meet or exceed the statewide Well Standards. When a
well is constructed, modified or destroyed a well completion report is required to be submitted to DWR.
3.6.3.6. Requirements for New Wells (2017)
Senate Bill 252 effective on January 1, 2018. SB 252 requires well permit applicants in critically
overdrafted basins to include information about the proposed well, such as location, depth, and
pumping capacity. The bill also requires the permitting agency to make the information easily
accessible to the public and the GSA. As of 2019, these requirements are under review by DWR. This
bill is not applicable because the SLO Basin is not a critically overdrafted basin.
3.6.3.7. County of San Luis Obispo Well Construction Ordinance
The County of San Luis Obispo under County Code Chapter 8.40 incorporates standards set forth in
DWR Bulletin No. 74.
3.6.3.8. Title 22 Drinking Water Program (2018)
The 2018 SWRCB DDW regulates public water systems in the State to ensure the delivery of safe
drinking water to the public. A public water system is defined as a system for the provision of water for
human consumption through pipes or other constructed conveyances that has 15 or more service
connections or regularly serves at least 25 individuals daily at least 60 days out of the year. Private
domestic wells, wells associated with drinking water systems with less than 15 residential service
connections, and industrial and irrigation wells are not regulated by the DDW. Additional information
regarding the public water systems can be found using the following link:
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https://sdwis.waterboards.ca.gov/PDWW/JSP/WaterSystems.jsp?PointOfContactType=none&number=
&name=&county=San%20Luis%20Obispo
DDW enforces the monitoring requirements established in Title 22 of CCR for public water system
wells, and all the data collected must be reported to the DDW. Title 22 also designates the regulatory
limits (e.g., maximum contaminant levels [MCLs]) for various waterborne contaminants, inc luding
volatile organic compounds, non-volatile synthetic organic compounds, inorganic chemicals,
radionuclides, disinfection byproducts, general physical constituents, and other parameters.
3.6.3.9. Waterway Management Plan – San Luis Obispo Creek Watershed (2003)
The San Luis Obispo Creek Watershed Waterway Management Plan was created in response to
several damaging floods that occurred in 1969, 1973, and 1995 that caused widespread damage
throughout the watershed that includes out-of-bank flooding and extensive bank erosion. This plan
identifies management problems and needs of the waterways, detailed hydrologic analyses of the
watershed and its main tributaries. The plan also presents a Stream Management and Maintenance
Program for the waterways of the watershed that outlines the planning, design, and permitting required
to fully implement the program and a Drainage Design Manual that contains revised policies for
floodplain and stream corridor management and redesigned flows for stream channels within the City
boundary.
3.6.3.10. Incorporation Into GSP
Information in these various plans mentioned above has been incorporated into this GSP for
consideration in the development of Sustainability Goals, when setting Minimum Thresholds and
Measurable Objectives, and was considered during development of Projects and Management Actions
to provide consistency among the above listed plans to achieve groundwater sustainability in the SLO
Basin.
3.6.3.11. Limits to Operation Flexibility
Some of the existing management plans and ordinances will limit operational flexibility. These limits to
operational flexibility have already been incorporated into the sustainability projects and programs
included in this GSP.
Examples of limits on operational flexibility include:
• The Groundwater Export Ordinance requires county approval to export of water out of the SLO
Basin. This is likely not a significant limitation because exporting water out of the SLO Basin hinders
sustainability.
• Title 22 Drinking Water Program regulates the quality of water that can be recharged into the SLO
Basin.
3.7. Conjunctive Use Programs
There are no active conjunctive use programs currently operating within SLO Basin.
3.8. Land Use Plans
The County and City have land use authority in the SLO Basin and the other MOA Parties do not.
However, SGMA requires the GSAs to consider land use documents by the overlying governing
agencies when making decisions. Government Code Section 65350.5 and 65352 require review and
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consideration of groundwater requirements before the adoption or any substantial amendment of a
city's or county's general plan. The planning agency shall review and consider GSPs and any proposed
action should refer to the GSA and GSP. Land use is an important factor in water management as
described below. The following sections provide a general description of these land use plans and how
implementation may affect groundwater supply.
3.8.1. Service Area Population
The General Plan (City of San Luis Obispo, 2018) is the principal tool the City uses when evaluating
municipal service improvements and land use proposals. Every service the City provides to its citizens
can trace its roots back to goals and policies found in the General Plan. General Plan goals, policies,
and implementation measures are based on an assessment of current and future needs and available
resources. The land use element designates the general distribution and intensity of land uses,
including the location and type of housing, businesses, industry, open space, and education, public
buildings, and parks. Figure 3-12 shows the City’s Land Use Map.
The City manages its housing supply growth so that it does not exceed one percent per year on
average, excluding dwellings affordable to residents with extremely low, very low, or low incomes, as
defined by the Housing Element. The City decided to adopt a Water and Wastewater Management
Element addressing water resources and wastewater services because of the vital role of these
resources and the far-reaching impacts of water policies on community growth and character. This
element translates the Land Use Element's capacity for development into potential demand for water
supply and wastewater services. This element outlines how the City plans to provide adequate water
and wastewater services for its citizens, consistent with the goals and policies of other General Plan
elements. As stated in the General Plan, the City has an adequate water supply to serve the
community’s existing and future water needs. The City envisions groundwater playing an important role
in ensuring continued resiliency in its water supply portfolio.
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Figure 3-12. City Land Use Map
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3.8.2. County of San Luis Obispo General Plan
The 2014 County General Plan contains three pertinent elements that are related to land use and water
supply. Pertinent sections include the Land Use, Agricultural, and Inland Area Plans elements. The
County’s General Plan also contains programs that are specific, non-mandatory actions or policies
recommended by the Land Use and Circulation Element (LUCE) to achieve community or area wide
objectives. Implementing each LUCE program is the responsibility of the County or other public agency
that is identified in the program. Programs are recommended actions rather than mandatory
requirements. Implementation of any program by the County should be based on consideration of
community needs and substantial community support for the program and its related cost.
The SLO Basin is within the San Luis Obispo Planning Area and South County Planning Area. The
planning areas do not conform to the SLO Basin boundaries but do provide a general representation of
the land use in the areas. Figure 3-13 and Figure 3-14 shows the planning areas and land uses.
The General Plan Framework for Planning does not provide tabular assessment of land use types and
acres, or population projection estimates within the San Luis Obispo Planning Area and South County
Planning Area. Therefore, projected demands and supplies based on land use aren’t identified for the
SLO Basin in the Land Use element.
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Figure 3-13. County Land Use Map (San Luis Obispo Planning Area)
Description of Plan Area (§ 354.8) Section 3
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Figure 3-14. County Land Use Map (South County Planning Area)
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3.8.3. Los Ranchos/Edna Village Plan
More specifically, the Los Ranchos/Edna Village Plan establishes a vision for the future that will guide
land use and transportation over the next 20 years. This village plan is part of Part III of the LUCE of
the County General Plan within the San Luis Obispo Planning Area. The Framework for Planning
(LUCE Part I) is the central policy document, while this plan contains programs more specifically
applicable to the Los Ranchos/Edna village area. In accordance with the Framework for Planning,
allowable densities (intensity of land use) are established (Figure 3-15). The San Luis Obispo Area
Plan contains regional land use and circulation goals, policies, and programs that also apply to Los
Ranchos/Edna. Table 3-7 and summarize the acreage and distribution of each land use category in Los
Ranchos/Edna village. Rural land use acreage is summarized in the Framework for Planning.
Table 3-7. Los Ranchos/Edna Land Use Acreage
LAND USE CATEGORIES ACREAGE
Agriculture 0
Rural Lands 0
Recreation 235
Open Space 0
Residential Rural 394
Residential Suburban 259
Residential Single Family 59
Residential Multi-Family 0
Office and Professional 0
Commercial Retail 0
Commercial Services 0
Industrial 0
Public Facilities 10
Dalidio Ranch 0
TOTAL 957
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Figure 3-15. Los Ranchos/Edna Land Use Map
4-1
4
GROUNDWATER SUSTAINABILITY PLAN
Basin Setting (§354.14)
This chapter describes the geologic setting of the San Luis Obispo Valley
Groundwater Basin (SLO Basin), including the Basin boundaries, geologic
formations and structures, principal aquifer units, geologic cross sections,
and hydraulic parameter data.
The information presented in this chapter, when considered with
the information presented in Chapter 5 (Groundwater Conditions)
and Chapter 6 (Water Budget), comprises the basis of the
Hydrogeologic Conceptual Model (HCM) of the Basin.
IN THIS CHAPTER
• Basin Information
• Regional Geology
• Aquifer Description
This section draws upon previously published studies, primarily a hydrogeologic and
geologic investigation prepared by GSI for the SLOCFCWCD in 2018, as well as a 1997
draft report, “San Luis-Edna Groundwater Basin Study, Draft Report” (DWR, 1997),
which was prepared but never finalized for official publication, and a 1991 report by
Boyle Engineering (Ground Water Basin Evaluation) that was prepared for the City of
San Luis Obispo. The data and information presented in this section is not intended to
be exhaustive but is a summary of the relevant and important aspects of the Basin
geology that influence groundwater sustainability. More detailed information can be
found in the original reports discussed above. This section presents the framework for
subsequent sections on groundwater conditions and water budgets.
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4.1. Introduction
As part of the GSP process, a numerical groundwater model was developed for the Basin to use as a
tool in the planning process (Appendix G). Much of the information comprising the HCM presented in
Chapters 4, 5, and 6 of the GSP is applied directly to the development of the groundwater model.
Physical data on the geology and hydrogeologic parameters of the Basin presented in Chapter 4 (Basin
Setting) are used to develop the model structure and parameterization while data on presented in
Chapter 5 (Groundwater Conditions) and Chapter 6 (Water Budget) are used in model calibration.
Multiple sources and types of data are presented in Chapters 4, 5, and 6. Some of this data, such as
rainfall amounts, depth to groundwater, and depth to bedrock, is directly measurable and involves a low
degree of uncertainty. Other data, such as aquifer transmissivity, is based on calculations and
interpretations of observed data, but is not directly measurable, and therefore involves a greater
amount of uncertainty than direct measurements. And finally, values presented in the water budget are
primarily derived from analysis of related data; almost none of the water budget components are
directly measurable, and as a result, involve more uncertainty than the previously discussed data types.
4.2. Basin Topography and Boundaries
The Basin is oriented in a northwest-southeast direction and is composed of unconsolidated or loosely
consolidated sedimentary deposits. It is approximately 14 miles long and 1.5 miles wide. It covers a
surface area of about 12,700 acres (19.9 square miles). The Basin is bounded on the northeast by the
relatively impermeable bedrock formations of the Santa Lucia Range, and on the southwest by the
formations of the San Luis Range and the Edna fault system. The bottom of the Basin is defined by the
contact of permeable sediments with the impermeable bedrock Miocene-aged and Franciscan
Assemblage rocks (DWR, 2003). A topographic map displaying the Basin boundaries is presented in
Figure 4-1, which also displays the watershed areas of the SLO Creek and Pismo Creek drainages. An
aerial photo of the Basin area is presented in Figure 4-2. Elevations within the Basin range from over
500 feet above mean seal level in the southeastern extent of Edna Valley, to under 100 feet above
mean sea level where SLO Creek flows out of the Basin.
The Basin is commonly referenced as being composed of two distinct valleys, with the San Luis Valley
in the northwest and the Edna Valley in the southeast. The San Luis Valley comprises approximately
the northwestern half of the Basin. It is the area of the Basin drained by SLO Creek and its tributaries
(Prefumo Creek and Stenner Creek west of Highway 101, Davenport Creek and smaller tributaries east
of Highway 101). Surface drainage in San Luis Valley drains out of the Basin flowing to the south along
the course of SLO Creek toward the coast in the Avila Beach area, approximately along the course of
Highway 101. The San Luis Valley includes part of the City and Cal Poly jurisdictional boundaries, while
the remainder of the valley is unincorporated land. Land use in the City is primarily municipal,
residential, and industrial. The area in the northwest part of the Basin, along Los Osos Valley Road,
has significant areas of irrigated agriculture, primarily row crops.
The Edna Valley comprises approximately the southeastern half of the Basin. The primary creeks that
drain the Basin are the east and west branches of Corral de Piedras Creek; the Corral de Piedras
Creek tributaries join to form Pismo Creek, draining south out of the Edna Valley into Price Canyon.
Canada de Verde Creek is also a significant tributary that flows south out of the Basin in the extreme
southeastern part of Edna Valley, ultimately joining Pismo Creek (Figure 4-1 and Figure 4-2). The Edna
Valley includes unincorporated lands, including lands associated with various private water purveyors.
The primary land use in the Edna Valley is agriculture. During the past two decades, wine grapes have
become the most significant crop type in the Edna Valley.
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The primary weather patterns for the Basin are derived from seasonal patterns of atmospheric
conditions that originate over the Pacific Ocean and move inland. As storm fronts move in from the
coast, rainfall in the area falls more heavily in the mountains, and the Basin itself receives less rainfall
because of a muted rain shadow effect. Average annual precipitation ranges from approximately 18
inches throughout most of the Basin to about 22 inches in relatively higher elevation areas near the City
and Cal Poly (Figure 4-3). The time series of annual precipitation for the period of record from 1871 to
2018 at the Cal Poly weather station is presented in Figure 3-11. The average rainfall at this location is
21.69 inches, with a standard deviation of 8.71 inches. The historical maximum is 49.99 inches, which
occurred in 1884. The historical minimum is 4.56 inches, which occurred in 2013.
The physical definition of the Basin boundary is the occurrence of unconsolidated or loosely
consolidated saturated sediments down to the contact with the basement rock of the Miocene-aged
formations and Franciscan Assemblage. (The geologic units will be described in more detail Section 4-
5.) Figure 4-4 presents a surface defining the bottom boundary of the Basin, based on the elevation of
bedrock surface below the Basin sediments. There is a topographic high point in the underlying
bedrock elevation between the San Luis Valley and Edna Valley sub-areas; physical details of this
bedrock feature are delineated in the technical memo describing a geophysical survey investigation in
this area performed as part of the GSP process (Cleath-Harris Geologists, 2019), included in Appendix
G. As shown, the watershed divide and the bedrock high are not coincident.
Figure 4-5 presents contours of total thickness of the Basin sediments; the inset figure displays the
thickness of sediments in a longitudinal cross section. It is apparent from Figure 4-6 that the sediments
of the Edna Valley have significantly greater thickness than those of the San Luis Valley. The
longitudinal profile of the Basin from the northwest on the left of the figure to the southeast on the right
indicates the watershed divide present in the vicinity of Biddle Ranch Road, indicated on Figure 4-4 and
Figure 4-5. Precipitation that falls west of that divide ultimately flows to Davenport and SLO Creeks,
and precipitation that falls east of that divide flows to Corral de Piedras Creek or the other small
tributaries, ultimately flowing to Pismo Creek south of the Basin.
Basin Setting (§354.14) Section 4
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Figure 4-1. Topographic Map
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Figure 4-2. Aerial Photograph
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Figure 4-3. Annual Precipitation
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Figure 4-4. Bottom Elevation of Basin
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Figure 4-5. Thickness of Basin Sediments
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4.3. Primary Users of Groundwater
The primary groundwater users in the Basin include municipal, agricultural, and domestic (i.e., rural
residential, small community water systems, and small commercial entities). These entities are
discussed in more detail in Chapter 2 (Agency Information) of this report. The City currently receives
most l of its supply from surface water sources including Whale Rock Reservoir, Santa Margarita
Reservoir, and Nacimiento Reservoir (Figure 3-3). However, it maintains its network of production wells
in standby mode for emergency supply and intends to utilize groundwater as a resource to meet future
water demand. The mutual and private water companies, domestic and agricultural users in the Edna
Valley rely almost exclusively on groundwater, although some have water rights along East and West
Corral de Piedras Creeks. No surface water points of diversion along SLO Creek are present in the
Basin.
4.4. Soils Infiltration Potential
Saturated hydraulic conductivity of surficial soils is a good indicator of the soil’s infiltration potential. Soil
data from the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS)
Soil Survey Geographic Database (SSURGO) (USDA-NRCS, 2007) is shown by the four hydrologic
groups on Figure 4-6. The soil hydrologic group is an assessment of soil infiltration rates that is
determined by the water transmitting properties of the soil, which includes hydraulic conductivity and
percentage of clays in the soil relative to sands and gravels.
The groups are defined as:
• Group A – High Infiltration Rate: water is transmitted freely through the soil; soils typically
less than 10 percent clay and more than 90 percent sand or gravel.
• Group B – Moderate Infiltration Rate: water transmission through the soil is unimpeded; soils
typically have between 10 and 20 percent clay and 50 to 90 percent sand
• Group C – Slow Infiltration Rate: water transmission through the soil is somewhat restricted;
soils typically have between 20 and 40 percent clay and less than 50 percent sand
• Group D – Very Slow Infiltration Rate: water movement through the soil is restricted or very
restricted; soils typically have greater than 40 percent clay, less than 50 percent sand
A higher soil infiltration capacity does not necessarily correlate to higher transmissivity in the underlying
aquifer, but it may correlate to greater recharge potential in localized areas. This will be discussed in
more detail in Chapter 5 (Groundwater Conditions).
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Figure 4-6. Soil Hydrologic Groups
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4.5. Regional Geology
This section provides a description of the geologic formations and structures in the Basin. These
descriptions are summarized from previously published reports. Figure 4-7 displays a stratigraphic
column presenting the significant geologic formations within the Basin. Figure 4-8 presents a surficial
geologic map of the Basin and surrounding area. Figure 4-9 displays the locations of lithologic data
used for this plan, and the section lines corresponding to cross sections in the following figures.
Geologic cross sections are presented in Figure 4-10 through Figure 4-21. The selected geologic cross
sections illustrate the relationship of the geologic formations that comprise the Basin and the geologic
formations that underlie and bound the Basin. The cross sections displayed on Figure 4-10 through
Figure 4-21 were directly adopted from the SLO Basin Characterization Report (GSI Water Solutions,
2018).
4.5.1. Regional Geologic Structures
The primary geologic structures of significance to the hydrogeology of the Basin are the Edna Fault
Zone and the adjacent Los Osos Fault Zone, which together form the southwestern boundary of the
Basin through the uplift of the Franciscan and Monterey Formation strata in the San Luis Range
southwest of the faults. The Edna and Los Osos Faults are normal faults, indicating primary
displacement motion is vertical rather than lateral (Figure 4-8). There are some disconnected and
unnamed fault splays mapped in the area south of the airport.
4.5.2. Geologic Formations within the Basin
For the purpose of this plan, the geologic units in the Basin and vicinity may be considered as two basic
groups; the Basin sediments and the consolidated bedrock formations surrounding and underlying the
Basin. The consolidated bedrock formations range in age and composition from (1) Jurassic-aged
serpentine and marine sediments to (2) Tertiary-aged marine and volcanic depositions. Compared to
the saturated sediments that comprise the Basin aquifers, the consolidated bedrock formations are not
considered to be significantly water-bearing. Although bedding plane and/or structural fractures in these
rocks may yield small amounts of water to wells, they do not represent a significant portion of the
pumping in the area. The delineation of the Basin boundaries is defined both laterally and vertically by
the contacts of the Basin sedimentary formations with the consolidated bedrock formations. From a
hydrogeologic standpoint, the most important strata in the Basin are the sedimentary basin fill deposits
that define the vertical and lateral extents of the Basin. These include recent and older deposits of
terrestrial sourced sediments, underlain in the Edna Valley by older marine sedimentary units. Figure
4-7 presents a stratigraphic column of the significant local geologic units. Figure 4-8 presents a map of
the Basin vicinity (assembled from a mosaic of the Dibblee maps from the San Luis Obispo, Pismo
Beach, Lopez Mountain, and Arroyo Grande NE quadrangles) showing where the various formations
crop out at the surface. Fault data displayed in Figure 4-8 were acquired via the USGS Earthquake
Hazards Program (USGS, 2004). The Quaternary fault and fold database from which the shapefiles are
derived was published in 2006 and cites a wide variety of published sources. Fault traces within the
shapefile represent surficial deformation caused by earthquakes during the Quaternary Period (the last
1.6 million years). Figure 4-8 also displays the Basin boundaries defined in DWR Bulletin 118.
Inspection of Figure 4-8 indicates that the Bulletin 118 Boundary lines for the Basin boundary do not
match up precisely with the most recently mapped extent of the water-bearing formations based on
(GSI Water Solutions, 2018). This is likely an artifact of previous mapping being performed at a larger
(statewide) scale. The water-bearing sedimentary formations and the non-water-bearing bedrock
formations are briefly described below.
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Figure 4-7. Stratigraphic Column
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Figure 4-8. Geologic Map
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4.5.2.1. Alluvium
The Recent Alluvium is the mapped geologic unit composed of unconsolidated sediments of gravel,
sand, silt, and clay, deposited by fluvial processes along the courses of SLO Creek, Davenport Creek,
East and West Corral de Piedras Creeks, and their tributaries. Lenses of sand and gravel are the
productive strata within the Recent Alluvium. These strata have no significant lateral continuity across
large areas of subsurface within the Basin. Thickness of Recent Alluvium may range from just a few
feet to more than 50 feet. Well pumping rates may range from less than 10 gallons per minute (gpm) to
more than 100 gpm. However, wells screened exclusively in Recent Alluvium are generally less
productive than wells that screen significant thicknesses of the Paso Robles and/or Pismo Formations.
4.5.2.2. Paso Robles Formation
The Paso Robles Formation underlies the Recent Alluvium throughout most of the Basin, and overlies
the Pismo Formation where present. It is composed of poorly sorted, unconsolidated to mildly
consolidated sandstone, siltstone, and claystone, with thin beds of volcanic tuff in some areas. The
Paso Robles Formation was deposited in a terrestrial setting on a mildly sloping floodplain that has
been faulted, uplifted, and eroded since deposition. The Paso Robles Formation is exposed at the
surface throughout much of the Edna Valley, except in areas where existing streams have deposited
Recent Alluvium on top of it. It is not readily distinguishable from alluvium in geophysical well logs.
Locally, the Paso Robles Formation is sometimes distinguished as being yellow in color, with sticky
clay. DWR Well Completion Reports with these types of descriptions generally were identified as Paso
Robles Formation for the purpose of interpreting the geology in the cross sections. However, it was
sometimes difficult to distinguish between Recent Alluvium and Paso Robles Formation in driller’s
descriptions, and professional judgment and broader context within the Basin were often used when
defining the contact between these two units. Wells that screen both the Recent Alluvium and Paso
Robles Formation have reported yields from less than 100 to over 500 gpm.
4.5.2.3. Pismo Formation
The oldest geologic water-bearing unit with significance to the hydrogeology of the Basin is the Pismo
Formation. The Pismo Formation is a Pliocene-aged sequence of marine deposited sedimentary units
composed of claystone, siltstone, sandstone, and conglomerate. There are five recognized members of
the Pismo Formation (Figure 4-7). While all members are part of the Pismo Formation, each member
reflects different depositional environments, and the variations in geology may affect the hydrogeologic
characteristics of the strata.
From the oldest to youngest, the members are
• The Edna Member, which lies unconformably atop the Monterey Formation, and is locally
bituminous (hydrocarbon-bearing)
• The Miguelito Member, primarily composed of thinly bedded grey or brown siltstones and
claystones
• The Gragg Member, usually described as a medium-grained sandstone
• The Bellview Member, composed of interbedded fine-grained sandstones and claystones
• The Squire Member, generally described as a medium- to coarse-grained fossiliferous sandstone of
white to grey sands
Previous reports have identified the significant thicknesses of sand at depth beneath the Paso Robles
Formation in the Edna Valley as the Squire Member of the Pismo Formation. However, it is not clear
whether these are accurately assigned as Squire. Other members of the Pismo Formation may be part
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of the sequence, and there is some ambiguity as to the actual member assignment. Even in the
adjacent Pismo Beach and Arroyo Grande NE quadrangle geologic (Dibblee, 2006) (Dibblee, 2006),
there is ambiguity in the geologic nomenclature. In the adjacent geologic maps these quadrangles, a
continuous exposure of this unit across the boundary between the two maps is referred to as Pismo
Formation in one map (Dibblee, 2006), and Squire Sandstone in the other (Dibblee, 2006). Therefore, it
is probably more accurate to generally refer to these units as the Pismo Formation, and not to
specifically identify the member designations. This convention will be followed for the remainder of this
report.
The Pismo Formation is extensive below the Paso Robles Formation in the Edna Valley. Thicknesses
of Pismo Formation up to 400 feet are reported or observed in well completion reports and in the cross
sections (Figure 4-5). The presence of sea shells in the lithologic descriptions of well completion reports
is clearly diagnostic of the Pismo Formation because of its marine origin. Many of the well completion
reports in the Edna Valley document the presence of water-bearing blue and green sands beneath the
Paso Robles Formation, and these are considered to be largely diagnostic of the Pismo Formation as
well. Wells that are completed in both the Paso Robles and Pismo Formations are reported to yield
from less than 100 gpm to approximately 700 gpm.
4.5.3. Geologic Formations Surrounding the Basin
Older geologic formations that underlie the Basin sediments typically have lower permeability and/or
porosity and are generally considered non-water-bearing. In some cases, these older beds may
occasionally yield flow adequate for local or domestic needs, but wells drilled into these units are also
often dry or produce groundwater less than 10 gpm. Generally, the water quality from the bedrock units
is poor in comparison to the Basin sediments. In general, the geologic units underlying the basin
include Tertiary-age consolidated sedimentary and volcanic beds (Monterey and Obispo Formations),
and Cretaceous-age sedimentary and metamorphic rocks (Franciscan Assemblage).
4.5.3.1. Monterey Formation
The Monterey Formation is a thinly bedded siliceous shale, with layers of chert in some locations. In
other areas of the County outside of the Basin, the Monterey Formation is the source of significant oil
production. While fractures in consolidated rock may yield small quantities of water to wells, the
Monterey Formation is not considered to be an aquifer for the purposes of this GSP. Regionally, the
unit thickness is as great as 2,000 feet, and the unit is often highly deformed. Water wells completed in
the Monterey Formation are occasionally productive if a sufficient thickness of highly deformed and
fractured shale is encountered. More often, however, the Monterey shale produces groundwater to
wells in very low quantities. Groundwater produced from the Monterey Formation often has high
concentrations of Total Dissolved Solids (TDS), hydrogen sulfide, total organic carbon, and
manganese.
4.5.3.2. Obispo Formation
The Obispo Formation and associated Tertiary volcanics are composed of materials associated with
volcanic activity along tectonic plate margins approximately 20 to 25 million years ago. The Obispo
Formation is composed of ash and other material expelled during volcanic eruptions. Although fractures
in consolidated volcanic rock may yield small quantities of water to wells, the Obispo Formation is not
considered to be an aquifer for the purposes of this GSP.
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4.5.3.3. Franciscan Assemblage
The Franciscan Assemblage contains the oldest rocks in the Basin area, ranging in age from late
Jurassic through Cretaceous (150 to 66 million years ago). The rocks include a heterogeneous
collection of basalts, which have been altered through high-pressure metamorphosis associated with
subduction of the oceanic crust beneath the North American Plate before the creation of the San
Andreas Fault. The current assemblage includes ophiolites, which weather to serpentinites and are
common in the San Luis and Santa Lucia Ranges. Although fractures may yield small quantities of
water to wells, the Franciscan Assemblage is not considered to be an aquifer for the purposes of this
GSP.
4.6. Principal Aquifers and Aquitards
Water-bearing sand and gravel beds that may be laterally and vertically discontinuous are generally
grouped together into zones that are referred to as aquifers. The aquifers can be vertically separated
by fine-grained zones that can impede movement of groundwater between aquifers, referred to as
aquitards.
Three aquifers exist in the Basin:
• Alluvial Aquifer – A relatively continuous aquifer comprising alluvial sediments that underlie the
SLO Creek and tributary streams, as well as East and West Corral de Piedras Creeks and tributary
streams;
• Paso Robles Formation Aquifer – An interbedded aquifer comprised of terrestrially-derived
sand and gravel lenses in the Paso Robles Formation.
• Pismo Formation Aquifer - An interbedded aquifer comprised of marine sand and gravel lenses
in the Pismo Formation.
There are no significant aquitards that vertically separate the three aquifers in the Basin over large
areas. There may be deposits of clay and silt that are not laterally extensive that locally separate two
aquifers, but there is no recognized aquitard in the Basin that separates the aquifers over significant
areas.
4.6.1. Cross Sections
Eleven cross sections (Figures 4-10 – 4-21) were prepared for this report; three (A1-A2, A2-A3, A3-A4)
are oriented along the longitudinal axis of the Basin and eight (B-B’ through I-I’) are oriented across the
Basin, perpendicular to the longitudinal axis (Figure 4-9). All lithologic data was reviewed during the
selection of the section line locations. The cross sections display lithology, interpretations of geologic
contacts based on available data, well screen intervals, and interpreted and mapped faults. If the
geologic interpretation was not clear from the points on the cross section lines, nearby data from other
locations was reviewed to provide broader geologic context. Each geologic cross section is discussed
in the following paragraphs. The longitudinal axis of the Basin is much longer than the cross basin
section lines, the longitudinal axis was divided into three separate cross sections for the sake of clarity
and presentation of detail.
As part of the work performed for the GSP, CHG performed a passive seismic geophysical plan in the
area along Buckley Road south of the airport (Appendix G). Data from this plan resulted in slight
adjustments in three of the previously developed cross sections.
These data have been incorporated into the following cross sections:
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• Cross Section A1-A2 (Figure 4-10) extends approximately 6.5 miles from the northwest extent of
the Basin at its boundary with the Los Osos Basin to about 1 mile east of Highway 101. Land
surface elevation is about 200 feet AMSL at the northwest extent, and slopes gently downward to
about 120 feet AMSL at the southeast extent. Recent Alluvium is exposed at the surface for the
entire length of this cross section, ranging in thickness from less than 50 feet near the Los Osos
Valley Basin boundary to about 80 feet near the center of the section. The Paso Robles Formation
is relatively thin in the northeast where it has been significantly eroded by the alluvium but thickens
to approximately 70 feet in the southeastern part of the section. Marine sands of the Pismo
Formation occur below the Paso Robles Formation in the southeastern part of the section, with a
maximum thickness of about 50 feet.
• Cross Section A2-A3 (Figure 4-11) extends approximately 4 miles along the longitudinal Basin
axis, starting near Tank Farm Road and cutting obliquely across Buckley Road to just past Edna
Road in the southeast. Land surface elevation ranges from approximately 120 feet AMSL in the
northwest to more than 270 feet AMSL in the southwest. Along the northwest half of the section
line, alluvium is exposed at the surface, with an approximate thickness of 40 to 50 feet. The
alluvium is primarily underlain by the Paso Robles Formation with thicknesses ranging from
approximately 40 to 80 feet. Just southeast of the airport, the Paso Robles Formation is exposed at
the surface, beginning at the point where there is a noticeable rise in land surface elevation. This is
approximately coincident with the maximum elevation of the underlying bedrock formations (the
bedrock divide that approximates the dividing line between the Edna Valley and the San Luis
Valley). A recent geophysical investigation by Cleath-Harris Geologists in the area of the high
bedrock elevation has provided greater detail on the Basin geometry in this area. The thickness of
the Paso Robles Formation in this area is up to 120 feet. Pismo Formation sediments underlie the
Paso Robles Formation in this area, with thickness of about 50 feet in the area of Davenport Creek.
The Pismo Formation thickness starts to increase significantly along this section line to the
southeast, with about 250 feet of Pismo sediments evident at the southeastern extent of the section
line. Several of the borings in this section indicate wells are partially or completely screened in
bedrock formations, indicating that the relatively thin saturated portions of the water-bearing
sediments did not yield enough water for the purposes of the wells.
• Cross section A3-A4 (Figure 4-12) extends about 6.5 miles along the Basin axis from
approximately Biddle Ranch Road to the southeast extent of the Basin. Land surface elevation rises
from about 250 feet AMSL on the northwest end of the section to over 500 feet AMSL in the
southeast. Relatively thin occurrences (40 feet or less) of Recent Alluvium associated with Corral
de Piedras Creek and its tributaries are evident in some areas on the western half of this section. In
the southeastern extent of the section, the Paso Robles Formation crops out at the surface where
the land is beginning to rise to the northern mountains and is dissected by small streams and
valleys in this area. The Pismo Formation sediments reach their maximum thickness of more than
400 feet along the northwestern extent of this section; the thickness of the Pismo gradually thins to
about 90 feet at the southwestern extent of the section.
• Cross section B-B’ (Figure 4-13) extends about 1.5 miles across the Basin perpendicular to the
Basin axis in the vicinity of Foothill Boulevard and Los Osos Valley Road. The section line has a
land surface elevation of about 180 feet AMSL on the northern end, sloping downward to about 130
feet AMSL along the Basin’s long axis, and rising again to about 230 feet AMSL on the southern
end. Recent Alluvium is exposed at the surface along this entire section, with thicknesses of about
20 to 30 feet. In the northern half of the section, alluvium is deposited directly on underlying
basement rock. In the southern half of the section, the Paso Robles Formation underlies the
alluvium with a maximum thickness of about 45 feet. The southern extent of the section crosses the
Los Osos Fault Zone.
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• Cross Section C1-C1’ (Figure 4-14) extends from the northern lobes of the Basin boundary,
which are formed from alluvium from Stenner and SLO Creeks, and trends southward
approximately 5.5 miles across the Basin from Cal Poly through the City, approximately along the
path of Highway 101. Land surface elevation is about 350 feet at the northern end of the section
line on some noticeable hilltops along the line, and slopes downward to an approximate altitude of
80 feet on the southern end. Most of the northern extent of this section has alluvium of about 20 to
40 feet of thickness deposited directly on underlying bedrock. Only in the southernmost 1½ miles of
the section line, where it crosses the main body of the Basin, do Paso Robles Formation sediments
underlie the alluvium. The Paso Robles Formation is about 90 feet thick here, and it is in turn
underlain by about 60 feet of Pismo Formation sediments.
• Cross Section C2-C2’ (Figure 4-15) extends about 1½ miles southward through the eastern lobe
of the northern part of San Luis Valley. Alluvium is deposited directly on top of basement rock along
this section. Alluvium is thin here, ranging from less than 10 feet to about 40 feet.
• Cross Section D-D’ (Figure 4-16) extends about 2.5 miles southward from a prominent
serpentine ridge in the north to the southern Basin boundary. Land surface elevation is about 160
feet on the northern end of the section, sloping down to about 110 feet in the Basin center, and
rising to about 180 feet on the southern end. Recent Alluvium is exposed at the surface along most
of this section, reaching a maximum thickness of about 80 feet. The alluvium is deposited directly
on basement rock through the northern half of the section. In the southern half of the section,
approximately 20 to 30 feet of Paso Robles Formation underlies the alluvium. Near the southern
extent of the Basin, the section line crosses into the combined Edna-Los Osos Fault Zone, at which
point the land surface elevation rises steeply and the Paso Robles Formation crops out at the
surface due to the upthrown formations south of the faults.
• Cross Section E-E’ (Figure 4-17) extends about 2½ miles across the Basin in the vicinity of the
airport and the area south of Buckley Road. Land surface elevation ranges from about 170 feet on
the northern end to 230 feet in the southern end. In the northern half of this section, Recent
Alluvium are exposed at the surface. In the southern half, the Paso Robles Formation is exposed.
Alluvial thickness in the northern half of the section ranges from about 20 to 70 feet and is underlain
by about 30 to 35 feet of Paso Robles Formation. In the southern half of the section, it crosses into
the Edna-Los Osos Fault Zone, and the Paso Robles Formation is upthrown to the point that it is
exposed at the surface. Paso Robles Formation thickness ranges from 50 feet to about 100 feet.
Sediments of the Pismo Formation underlie the Paso Robles Formation in this area and are about
25 to 70 feet thick.
• Cross Section F-F’ (Figure 4-18) extends about 2 miles north to south in the western extent of the
Edna Valley area. The Paso Robles Formation is exposed at the surface along most of this section.
One small pod of alluvium associated with Davenport Creek is evident in the center of the section.
The Paso Robles Formation has a maximum thickness of about 175 feet in this section. It is
underlain by about 50 to 60 feet of Pismo Formation sediments in the area north of the Edna Fault
Zone. To the south, the section line extends into the Edna Fault Zone. South of the fault, the
formations are upthrown, resulting in a small area of Pismo Formation sediments exposed at the
surface.
• Cross Section G-G’ (Figure 4-19) extends about 2 miles through the heart of the Edna Valley
area. Land surface elevation ranges from about 300 feet on the north end to more than 350 feet on
the south end. A thin veneer of alluvium, about 20 feet thick, that is associated with Corral de
Piedras Creek and tributaries is exposed at the surface along much of this section. The Paso
Robles Formation crops out in the north of the section and underlies the alluvium with an average
thickness of about 50 to 60 feet. The Pismo Formation displays its largest thickness along this
section, with a maximum thickness of about 450 feet near where this section intersects with cross
section A3-A4. The southern end of the section line crosses into the Edna Fault zone, and
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sediments are displaced such that the Pismo Formation sediments are exposed at the surface on
the southern slopes of the Basin in this area.
• Cross Section H-H’ (Figure 4-20) extends approximately 2½ miles through the Edna Valley. Land
surface is approximately 350 feet on the northern end, sloping downward to about 230 feet near
Corbett Canyon Road, then quickly rising to nearly 400 feet on the south end of the section on the
upthrown side of the Edna Fault. The Paso Robles Formation is exposed at the surface for nearly
the entire section. The section line crosses a small exposure of Recent Alluvium associated with
Corral de Piedras Creek. In the northern half of the section, the Paso Robles Formation sediments
are deposited directly on the basement rock formations, with a maximum thickness of about 80 feet.
In the southern half of the section, the basement rock elevation plunges and the thickness of the
Paso Robles Formation is about 150 to 230 feet. The Pismo Formation underlies the Paso Robles
Formation sediments in the southern half of the section, with a maximum thickness of about 200
feet. In the Corbett Canyon area, the section crosses the Edna Fault; south of the fault the
basement rock formations are thrust up to the surface and represent the boundary of the Basin.
• Cross Section I-I’ (Figure 4-21) crosses the southern extent of the Edna Valley. The northern part
of the section lies along the lower slopes of the Santa Lucia Range, and displays Paso Robles
Formation sediments deposited on top of bedrock formations. A small pod of Recent Alluvium
associated with Corral de Piedras Creek is displayed. Along the center of the Edna Valley, the Paso
Robles Formation thickness is about 200 feet, and is underlain by about 100 feet of Pismo
Formation sediments. The section crosses the Edna Fault Zone, which shows Pismo Formation
sediments upthrown to land surface on the south side of one fault splay, and bedrock of the
Monterey Formation upthrown to land surface elevation south of a second fault splay.
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Figure 4-9. Lithologic Data Points and Cross Section Lines
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Figure 4-10. Cross Section A1-A2
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Figure 4-11. Cross Section A2-A3
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Figure 4-12. Cross Section A3-A4
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Figure 4-13. Cross Section B-B’
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Figure 4-14. Cross Section C1-C1’
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Figure 4-15. Cross Section C2-C2’
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Figure 4-16. Cross Section D-D’
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Figure 4-17. Cross Section E-E’
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Figure 4-18. Cross Section F-F’
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Figure 4-19. Cross Section G-G’
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Figure 4-20. Cross Section H-H’
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Figure 4-21. Cross Section I-I’
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4.6.2. Aquifer Characteristics
The relative productivity of an aquifer can be expressed in terms of transmissivity, hydraulic
conductivity, or specific capacity. The most robust method is measuring transmissivity using a long-
term (frequently 24 hours or more) constant-rate pumping test. Water level drawdown data collected
during this test can be analyzed and used to calculate transmissivity. Specific capacity is a simple
measure of flow rate (gpm) divided by drawdown (feet), routinely measured by well service contractors
during well maintenance and reported in units of gpm per foot of drawdown (gpm/ft). Specific capacity
measurements may be affected by well construction details, and, therefore, are not only related to
aquifer characteristics. Nevertheless, the following commonly accepted empirical relationships allows
transmissivity to be estimated from specific capacity measurements.
T (GPD/FT) = SC (GPM/FT) * (1,500 – 2,000), where
T = Transmissivity (gpd/ft)
SC = Specific Capacity (gpm/ft)
1500 – 2000 = Empirical factor,
(1,500 used for unconfined, 2,000 for confined aquifer)
Data summarizing these parameters from water wells throughout the Basin were compiled. The data
was obtained from Previous regional studies or reports, previous pumping tests and well service
information provided by local stakeholders. All available reports and documents that were made
available through data requests, report reviews, etc., were reviewed for technical information, and
included in this summary if the data were judged to be sufficient.
DWR reports a range of irrigation well pumping rates from 300 to 600 gpm, and a range of specific
capacity values of 15 to 20 gpm/ft for the Basin, corresponding to transmissivity estimates from 22,500
to 40,000 gallons per day per foot (gpd/ft) (DWR, 1958). Boyle evaluated five constant-rate aquifer tests
for City wells, all in the San Luis Valley, and reported transmissivity values ranging from 11,200 to
71,000 gpd/ft, with an average of 41,240 gpd/ft (Boyle Engineering, 1991). DWR in 1997 discussed the
range of hydraulic conductivity values used in the preparation of its groundwater model, which
averaged about 15 ft/day in the San Luis Valley, and about 6 ft/day in the Edna area (DWR, 1997).
Figure 4-22 displays the spatial distribution of the available data locations for well tests in the Basin.
Inspection of Figure 4-22 indicates a good spatial coverage of locations, with reasonable data density
throughout the Basin.
Table 4-1 presents a compilation of all constant rate aquifer test data compiled during the preparation
of this GSP. Table 4-2 presents a compilation of the specific capacity data. This information is used in
the groundwater model development, and in the technical work supporting preparation of the GSP for
the Basin.
Table 4-1 presents a data summary for the constant rate aquifer test that was available, including
information on pumping rate, static and pumping water levels, screened intervals, total depth, and
formations screened. It was not always readily apparent which formations are screened from the
available data, and sometimes well screens may span more than one formation. If there is uncertainty
regarding this designation, it is indicated with a question mark in Table 4-1. Calculated transmissivity
values range from less than 1,000 gpd/ft to a maximum of 158,400 gpd/ft. (The highest reported
transmissivity value of 158,400 gpd/ft is an outlier and was likely influenced by recharge from a nearby
stream.
Table 4-2 presents all available information for the specific capacity well tests identified. Table
4-2includes a transmissivity estimate based on the empirical relationship discussed previously.
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Data presented in Table 4-1 and Table 4-2 indicate that wells screened in the Alluvium and Paso
Robles Formation have transmissivities ranging from about 5,000 to 158,000 gallons per day per foot
(gpd/ft), and averaging over 42,000 gpd/ft. Wells screened in Paso Robles and Pismo Formations have
transmissivities ranging from less than 1,000 to about 40,000 gpd/ft, and average about 10,000 gpd/ft.
4.6.3. Aquitards
An aquitard is a layer of low permeability, usually comprised of fine-grained materials such as clay or
silt, which vertically separates adjacent layers of higher permeability formations that may serve as
aquifers. Although there is some amount of clay present in nearly all of the boring logs reviewed for this
plan, there are no formally defined or laterally continuous clay layers that function as aquitards within
the Basin. In the San Luis Valley, wells are commonly screened across both the Recent Alluvium and
the underlying Paso Robles Formation, and these two formations essentially function as a single
hydrogeologic unit is this area. Similarly, in the Edna Valley, wells are commonly screened across both
the Paso Robles Formation and the underlying Pismo Formation, and these two formations essentially
function as a single hydrogeologic unit is this area.
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Figure 4-22. Hydraulic Parameter Data Locations
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Table 4-1. San Luis Obispo Valley Groundwater Basin Water Well Pump Test Data Summary
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Table 4-2. San Luis Obispo Valley Groundwater Basin Water Well Specific Capacity Data Summary
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4.7. Surface Water Bodies
Surface water/groundwater interactions represent a small, but significant, portion of the water budget of
an aquifer system. In the Basin, these interactions occur primarily at streams and lakes.
As previously discussed, there are several named creeks that flow across the Basin. In the San Luis
Valley area of the Basin, these include San Luis Obispo Creek, Stenner Creek, Prefumo Creek, Froom
Creek, and Davenport Creek, in addition to smaller unnamed tributaries. In the Edna Valley these
include East and West Corral de Piedras Creeks (which join to form Pismo Creek just south of the
Basin Boundary), and Canada de Verde Creek in southeastern Edna Valley. The watersheds support
important habitat for native fish and wildlife, including the federally threatened South-Central California
Coast steelhead (Oncorhynchus mykiss) (Stillwater Sciences, 2014).
Laguna Lake is the only lake in the Basin. It is a naturally occurring lake just north of Los Osos Valley
Road and west of Highway 101. The downstream outlet of the lake flows into the Prefumo Creek culvert
under Madonna Road. In the past, flashboards were used to maintain water elevation in the lake to
support recreation and maintain wildlife habitat. However, these are no longer used. The water in the
lake is partially supplied by seasonal flow in Prefumo Creek, which flows into Laguna Lake. and at least
partially supplied by subsurface groundwater inflow.
Groundwater interaction with streams in the Basin is not well quantified, but it is recognized as an
important component of recharge in the water budget. Where the water table is above the streambed
and slopes toward the stream, the stream receives groundwater flow from the aquifer; this is known as
a gaining reach (i.e., the stream gains flow as it moves through the reach). Where the water table is
beneath the streambed and slopes away from the stream, the stream loses water to the aquifer; this is
known as a losing reach. During seasonal dry flow conditions, it is clear that groundwater elevation is
deeper than the streambed. Therefore, it is generally understood that the streams in the Basin
discharge to the underlying aquifer, at least in the first part of the wet-weather flow season. If there is
constant seasonal surface water flow, it is possible that groundwater elevations may rise to the point
that they are higher than the stream elevation, and the creek may become a seasonally gaining stream
in some reaches. Groundwater modeling can help evaluate surface water groundwater interaction.
The amount of flow in surface water/groundwater interaction is difficult to quantify. Boyle assumed that
10 percent of the measured surface water flow coming into the Basin in San Luis Obispo Creek and
Stenner Creek was recharged to the aquifer and used an average rate of 430 acre-feet/yr (AFY) (Boyle
Engineering, 1991). In its draft report, DWR reports model-generated estimates ranging from streams
gaining 2,700 AFY from the aquifer to streams losing 680 AFY to the aquifer (DWR, 1997).
The County, through its coordination with Zone 9 and the City, maintains a network of five stream
gauges in the San Luis Valley Basin to record heights of flow throughout the year for flood warning
purposes (Figure 3-10). The gauges were constructed in November 2001 and have periods of record
from that year to the present. Continuous data monitoring of height of flow at the gages is recorded, but
equivalent discharge (cubic feet per second) is not recorded.
4.8. Subsidence Potential
Subsidence is the gradual settling or sinking of the earth’s surface due to material movement at depth
in a location, and may be associated with groundwater pumping, and is one of the undesirable results
identified in SGMA. Subsidence has been documented in parts of the San Luis Valley. The most severe
subsidence that has occurred in the Basin was in the 1990s along the Los Osos Valley Road corridor.
Subsidence occurred within young organic soil (i.e., peat) in response to extraction of groundwater
within a relatively shallow aquifer that resulted in significant settlement of the ground surface. The
settlement caused local damage to businesses and homes in that area as local groundwater pumping
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dewatered the soft soil units beneath buildings and the surrounding area. Subsidence of more than 1
foot of settlement of the ground surface in some locations damaged buildings and resulted in
reconstruction or retrofitting buildings.
Another area of known subsidence is along the shores of Laguna Lake. Homes located along the
shoreline have experienced settlement that has cracked foundations, patios, and window and door
openings. Many homes in that area have been retrofitted to address the settlement. While the
subsidence near Laguna Lake is not specifically related to extraction of groundwater, lowering of the
groundwater table in that area could result in further settlement and subsidence.
The historical manifestation of subsidence generally has been limited to the area along Los Osos Valley
Road and downstream, where there are compressible soil types that were particularly vulnerable to
large settlements in response to lowering of the local groundwater table. This history emphasizes the
importance of considering subsurface conditions that may be associated with subsidence. Not all soil
and rocks are vulnerable to the type of subsidence that occurred along Los Osos Valley Road. The
potential for subsidence to occur, and the severity of the subsidence, is dependent on the geology,
groundwater levels, and the properties of the soil and rock that may be dewatered in association with
groundwater pumping. The subsidence evaluation consisted of a review of published data and studies
performed by local, state, and federal agencies, as well as a familiarity of local geology and soil. The
following is a summary of the key findings.
DWR identifies the Basin as having a low subsidence potential. However, historical subsidence is
known to have occurred in specific geographic areas of the Basin because of groundwater pumping or
lowered groundwater levels due to drought. The Basin was evaluated on the basis of the extent of
known and mapped geologic units within the Basin (Yeh and Associates, 2017).
The relative potential for subsidence was divided into three categories and delineated as shown
in Figure 4-23.
• Category 1. Category 1 has the highest likelihood of future subsidence if subject to lowered
groundwater levels in the future. Based on a review of public data and consultant reports, alluvium
mapped in these areas contains young organic soil known in areas around Los Osos Valley Road,
Laguna Lake, and low-lying wetland areas near Tank Farm Road. These areas are known to have
experienced historical subsidence or to contain soft or organic soil and were identified as having a
potential for subsidence in relation to geology and groundwater pumping. These areas are identified
as Category 1 in Figure 4-23, with star symbols marking approximate areas of known historical
subsidence. Extraction of groundwater resources in these areas could cause further subsidence.
• Category 2. Low-lying topographic areas in the Basin that are mapped as young alluvial soil were
identified as potentially containing soft or organic soil layers that may have a potential for
subsidence in relation to groundwater pumping, but currently there is no historical or subsurface
information to further evaluate those areas. Those areas are mostly located along Prefumo Creek
and San Luis Obispo Creek and the main drainages through the west end of the Edna Valley near
Price Canyon. These areas are identified as Category 2 in in Figure 4-23. This screening criteria
recognizes the unconsolidated nature typical of young alluvium that has been mapped in these
areas potentially could subside because of compaction of the aquifer if groundwater levels were
lowered.
• Category 3. Geographic areas in the Basin that were mapped as bedrock or older surficial
sediments and are not known to be underlain by young organic soil or young alluvium, were
identified as Category 3 in Figure 4-23. These areas were evaluated and characterized as not
having factors known to be susceptible to subsidence in relation to groundwater pumping.
Generally, these are upland areas where bedrock is shallow or where bedrock is mapped at the
ground surface, such as in the areas around the airport and Orcutt Road (in Figure 4-23).
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Figure 4-23. Subsidence Potential
5-1
5
GROUNDWATER SUSTAINABILITY PLAN
Groundwater Conditions (§354.16)
This chapter describes the current and historical groundwater conditions in
the Alluvial Aquifer, the Paso Robles Formation Aquifer, and the Pismo
Formation Aquifer in the San Luis Obispo Valley Groundwater Basin.
In accordance with the SGMA Emergency Regulations §354.16,
current conditions are any conditions occurring after January 1,
2015. By implication, historical conditions are any conditions
occurring prior to January 1, 2015. This Chapter focuses on
information required by the GSP regulations and information that
is important for developing an effective plan to achieve
sustainability. The organization of Chapter 5 aligns with the six
sustainability indicators specified in the GSP regulations,
including:
1. Chronic lowering of groundwater elevations;
2. Groundwater storage reductions;
3. Seawater intrusion;
4. Land Subsidence;
5. Depletion of interconnected surface waters, and;
6. Degradation of groundwater quality.
IN THIS CHAPTER
• Groundwater
Elevations
• Groundwater
Recharge and
Discharge
• Interconnected
Surface Water
• Groundwater
Dependent
Ecosystems
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5.1. Groundwater Elevations and Interpretation
As discussed in Chapter 4 (Basin Setting), information from available boring logs indicates that there is
no regional or laterally extensive aquitard separating the Alluvial Aquifer, Paso Robles Formation
aquifer, and Pismo Formation aquifer in the Basin. In the San Luis Valley, a physical distinction
between Alluvium and Paso Robles Formation is often not apparent, and information from well
completion reports in the Basin indicate that wells are regularly screened across productive strata in
both formations, which effectively function as a single hydrogeologic unit. Likewise, in the Edna Valley,
information from well completion reports indicates that wells are routinely screened across productive
strata in both the Paso Robles Formation Aquifer and the Pismo Formation Aquifer, which effectively
function as a single hydrogeologic unit. Boyle states that there is no strict boundary between the
Alluvial Aquifer and the Paso Robles Formation Aquifer in the Buckley Road area (Boyle Engineering,
1991). DWR states that all the sediments in the Subbasin are in hydraulic continuity. Because there is
no available groundwater elevation data specific to the three individual aquifers, and because these
formations appear to function as combined hydrogeologic units, groundwater elevation data are
combined and presented as a single groundwater elevation map for each time period presented (DWR,
1997).
In general, the primary direction of groundwater flow in the Basin is from the area of highest
groundwater elevations in the Edna Valley northwestward toward San Luis Obispo Creek, where the
flow leaves the Basin along the stream course. Groundwater in the northwestern areas of the Basin
near the City of San Luis Obispo boundary and Los Osos Valley Road flows southeastward toward the
San Luis Obispo Creek alluvium. In the southeastern portion of the Basin there are also local areas of
flow discharging from the Basin along Pismo Creek tributaries of East and West Corral de Piedras
Creek, and alluvium of other smaller tributaries further to the south. Groundwater Elevation maps for
various recent and historical time periods are presented and discussed in the following sections.
5.1.1. Fall 1954 Groundwater Elevations
DWR published a series of maps depicting groundwater elevations for various basins in the County,
including groundwater elevations in the San Luis Obispo Valley Groundwater Basin for fall 1954 (Figure
5-1), with contours based on field measurements of over 40 control points in the Basin (DWR, 1958).
Groundwater flow direction arrows were added to Figure 5-1 to illustrate the primary direction of flow in
the Basin. This is the oldest Basin-wide groundwater elevation data available. In the Los Osos Valley
portion of the Basin, this map displays dominant groundwater flow direction from higher elevations in
the in the northwestern extent of the Basin southeastward toward the discharge area where San Luis
Obispo Creek leaves the Basin. The hydraulic gradient (the ratio of horizontal distance along the
groundwater flow path to the change in elevation) in this area is approximately 0.004 feet/feet (ft/ft). In
the Edna Valley portion of the Basin, the dominant groundwater flow direction is northwestward from
the higher groundwater elevations in the southeastern part of the Basin (over 280 ft AMSL) to lower
elevations (less than 110 feet AMSL) where San Luis Obispo Creek exits the Basin. The gradient
across this area is steeper than in Los Osos Valley, approximately 0.009 ft/ft. This map also displays
local areas of discharge coincident with the areas where San Luis Obispo Creek and Pismo Creek
tributaries leave the Basin.
5.1.2. Spring 1990 Groundwater Elevations
Boyle (1991) presents water level elevation contour maps for the spring of 1986 and 1990, based on
water level data collected from 18 control points in the field. A digitized recreation of the Boyle
groundwater elevation contours for spring of 1990 is presented in Figure 5-2 and displays patterns of
groundwater flow direction in the Basin similar to those exhibited in the DWR 1954 map, although the
flow gradient does not appear to be as steep as it is in the 1954 map. The year 1990 was in the midst
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of a significant period of drought in the Basin. The northwestward gradient across the central area of
the Basin is approximately 0.006 ft/ft. Contours for the spring of 1986 are not re-presented in this
report, but 1986 represents wetter conditions than the 1990 map, and it is noted in Boyle (1991) that
there is a difference of approximately 10 feet of elevation between the two maps, representing the
variation in water levels observed between wet and dry weather cycles in this time period. The contours
in Figure 5-2 do not display an area of discharge where Corral de Piedras Creeks leave the Basin, but
this is likely due to a lack of control points in this area.
5.1.3. Modeled 1990s Groundwater Elevations
In its draft report, DWR (1997) used a computer groundwater model to generate a series of modeled
water level maps representing wet, dry, and average weather conditions. The model results are not re-
presented in this GSP, but a review of the draft report indicates the maps display the same general flow
direction patterns as the DWR (1958) and Boyle (1991) maps, which were based on data collected in
the field. Water level elevations in the San Luis Valley in wet years were approximately 10 to 20 feet
higher than in dry years. In the Edna Valley, the difference in groundwater elevations between wet and
dry years was greater, approximately 20 to 30 feet.
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Figure 5-1. Groundwater Elevation Surface Fall 1954
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Figure 5-2. Groundwater Elevation Surface Spring 1990
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5.1.4. Spring 1997 Groundwater Elevations
More recent groundwater level data collected as a part of San Luis Obispo County’s groundwater
monitoring program were obtained and used to generate groundwater elevation maps to evaluate more
recent conditions. The following assessment of groundwater elevation conditions is based primarily on
data from the San Luis Obispo County Flood Control and Water Conservation District’s (SLOFCWCD)
groundwater monitoring program. Groundwater levels are measured through a network of public and
private wells in the Basin. Figure 5- through Figure 5-7 presents the contours generated from the data
for the Spring 1997, Spring 2011, Spring 2015, Spring 2019, and Fall 2019 monitoring events.
The set of wells used in the groundwater elevation assessment were selected based on the
following criteria:
• The wells have groundwater elevation data for the periods of record of interest;
• Groundwater elevation data were deemed representative of static conditions.
Additional information on the monitoring network is provided in Chapter 7(Monitoring Networks).
Based on available data, the following information is presented in subsequent subsections.
• Groundwater elevation contour maps for spring 1997, 2011, 2015, 2019, and Fall 2019;
• A map depicting the change in groundwater elevation between 1997 and 2011;
• A map depicting the change in groundwater elevation between 2011 and 2015;
• A map depicting the change in groundwater elevation between 2015 and 2019;
• Hydrographs for select wells with publicly available data.
Figure 5- presents a groundwater surface map for Spring 1997 based on field data collected by the
County (control points are not displayed to maintain confidentiality agreements negotiated with well
owners). The southeast (near Lopez Lake) and northwest (Los Osos Valley) areas of the Basin had no
wells monitored during these events to calculate water levels, so contours are not presented for those
areas. Several features on this map are apparent. First, a pronounced groundwater mound is evident at
the location where West Corral de Piedras Creek enters the Basin in Edna Valley, near the corner of
Biddle Ranch Road and Orcutt Road; three control points are present in this area, providing reliable
documentation for water levels in this vicinity. This indicates that this is a groundwater recharge area.
The regional northwesterly flow direction apparent in the previously discussed water level maps is still
evident here; the groundwater flow gradient is about 0.011 ft/ft, somewhat steeper than the Spring 1990
gradient presented by Boyle.
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Figure 5-3 Groundwater Elevation Surface Spring 1997
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5.1.5. Spring 2011 Groundwater Elevations
Spring 2011 represents a time period just prior to the recent drought, but after the expansion of
agricultural pumping in Edna Valley, as discussed further in Chapter 6 (Water Budget). As such, effects
of the recent drought should not yet be apparent, but reduced groundwater levels due to expanded
agricultural pumping should be evident.
Figure 5-4 displays groundwater elevation contours for Spring 2011. The groundwater mound near
Biddle Ranch Road and Orcutt Road is again evident, with a maximum groundwater elevation of over
320 feet. Groundwater flow direction appears to indicate areas of discharge from the Basin in Edna
Valley along Corral de Piedras Creeks and Canada Verde Creek, and along San Luis Obispo Creek in
San Luis Valley. The area near Edna Road and Biddle Ranch Road indicates a steep local gradient,
likely associated with local pumping. The contour near the exit of Corral de Piedras Creeks is 180 feet.
The gradient across the central Basin is almost identical to the Spring 1997 map, about 0.011 ft/ft. The
gradient is much shallower in the San Luis Valley part of the Basin.
5.1.6. Spring 2015 Groundwater Elevations
Figure 5-5 presents groundwater elevation contours for Spring 2015. Spring 2015 represents a time
period in the midst of the recent drought, and after the expansion of agricultural pumping in Edna
Valley.
The effects of the drought are apparent upon close inspection of the contours in Figure 5-5. In the Edna
Valley, the maximum contour of the recharge area near Orcutt Road and Biddle Ranch Road is 280
feet, about 40 feet lower than in the Spring 2011 map. The contours immediately west of the mound are
still steep, but flatten out significantly along Davenport Creek, resulting in a much shallower gradient in
this area than in the Spring 2011 map. Contours east of the mound along Orcutt Road are 20 to 40 feet
lower than in the Spring 2011 map. In the San Luis Valley, a 100-foot contour is evident near the exit of
San Luis Obispo Creek from the Basin, which is about 10 feet lower than the contour in the Spring 2011
map.
5.1.7. Spring 2019 Groundwater Elevations
Figure 5-6 presents a groundwater surface elevation map for Spring 2019. Spring 2019 represents a
time period at the end of seasonal winter rains, and after the end of the recent drought. Rebounds of
groundwater elevations from the drought are apparent upon inspection of the contours. In the Edna
Valley, the maximum contour of the recharge area near Orcutt Road and Biddle Ranch Road is 300
feet, about 20 feet higher than in the Spring 2015 map. Contours east of the mound are about 20 feet
higher than in the Spring 2015 map. Contours along Davenport Creek are about 20 feet higher than in
the Spring 2015 map. The elevation at Edna Road and Biddle Ranch Road is about 230 feet, over 50
feet higher than in the Spring 2015 map.
5.1.8. Fall 2019 Groundwater Elevations
Figure 5-7 presents a groundwater surface elevation map for Fall of 2019. This time period represents
recent conditions at the end of the summer dry season for comparison against the spring conditions.
Overall, the contours indicate lower groundwater levels than those displayed in the Spring 2019 map.
Groundwater contours east of the recharge mound at West Corral de Piedras are about 20 feet lower
than the Spring 2019 map. The groundwater elevation at Edna Road and Biddle Ranch Road is about
220 feet, approximately 10-20 feet lower than in the Spring 2019 map.
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Figure 5-4 Groundwater Elevation Surface Spring 2011
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Figure 5-5 Groundwater Elevation Surface Spring 2015
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Figure 5-6 Groundwater Elevation Surface Spring 2019
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Figure 5-7 Groundwater Elevation Surface Fall 2019
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5.1.9. Changes in Groundwater Elevation
In order to demonstrate how groundwater elevations have varied over the recent history of the Basin, a
series of maps were generated that display changes in groundwater elevation. These maps were
developed by comparing groundwater elevations from one year to another and calculating the
differences in elevation over the specified time period. It should be noted that the results of this analysis
are largely dependent on the density of data points, and should be viewed as indicative of general
trends, not necessarily as accurate in specific areas where little data is available.
The first time period selected compares changes in groundwater elevation from 1997 through 2011.
The year 1997 was selected as a starting point because it is assumed to represent conditions prior to
the significant expansion of agricultural groundwater pumping in the Basin. The year 2011 was selected
as the end point because it represents conditions prior the start of the recent drought. Calculated
changes in groundwater elevation over this 14-year period are presented in Figure 5-. This figure
indicates a maximum decline in groundwater elevation of over 60 feet in the Edna Valley, southeast of
East Corral de Piedras Creek between Orcutt Road and Corbett Canyon Road. The calculated
groundwater elevation shows declining groundwater levels to the northwest of this location. No
significant declines are indicated northwest of Biddle Ranch Road over this time period.
The next time period selected compares changes in groundwater elevation from 2011 through 2015.
This time period was selected to capture the start of the drought to a point four years into the drought,
thereby capturing the period of greatest groundwater elevation change. Calculated changes in
groundwater elevation over this 4-year period are presented in Figure 5-. This figure indicates a
maximum decline in groundwater elevation of over 80 feet located in the Edna Valley, near the
intersection of Edna Road and Biddle Ranch Road. The calculated reductions in groundwater elevation
decline in all directions from this location. No significant declines are indicated in the San Luis Valley
portion of the Basin over this time period.
The next time period selected compares changes in groundwater elevation from 2015 through 2019.
This time period was selected to capture the potential recovery of the Basin following the drought.
Calculated changes in groundwater elevation over this 3-year period are presented in Figure 5-10.
Groundwater elevations are shown to have rebounded throughout the entire area in which data was
available. The greatest increase in groundwater elevation is coincident with the area of greatest
declines from 2011-2015, near the intersection of Edna Road and Biddle Ranch Road.
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Figure 5-8 Change in Groundwater Elevation Spring 1997 to Spring 2011
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Figure 5-9 Change in Groundwater Elevation Spring 2011 to Spring 2015
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Figure 5-10. Change in Groundwater Elevation Spring 2015 to Spring 2019
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5.1.10. Vertical Groundwater Gradients
Vertical groundwater gradients are calculated by measuring the difference in head at a single location
between specific and distinct strata or aquifers. The characterization of vertical gradients may have
implications with respect to characterization of flow between aquifers, migration of contaminant plumes,
and other technical details describing groundwater flow in specific areas. In order to accurately
characterize vertical groundwater gradient, it is necessary to have two (or more) piezometers sited at
the same location, with each piezometer screened across a unique interval that does not overlap with
the screened interval of the other piezometers(s). If heads at one such piezometer are higher than the
other(s), the vertical flow direction can be established since groundwater flows from areas of higher
heads to areas of lower heads. However, because such a “well cluster” must be specifically designed
and installed as part of a broader investigation, limited data exists to assess vertical groundwater
gradients. Previous hydrologic studies of the Basin, (Boyle Engineering, 1991) (DWR, 1997), indicate
that groundwater elevations are generally higher in the Alluvial Aquifer than the underlying Paso Robles
Formation Aquifer, resulting in groundwater flow from the Alluvial Aquifer to the underlying Paso Robles
Formation aquifer (although this may change seasonally). The lack of nested or clustered piezometers
to assess vertical gradients in the Basin is a data gap that is discussed further in Chapter 7 (Monitoring
Network).
There are no paired wells that provide specific data comparing water levels in wells screening the
bedrock and the Basin sediments. However, from a conceptual standpoint, the Monterey Formation is
assumed to receive rainfall recharge in the surrounding mountains at higher elevations than the Basin
sediments. For this reason, it is assumed that an upward vertical flow gradient exists between the
bedrock and the overlying Basin sediments. Because the bedrock formations are significantly less
productive than the Basin sediments, the rate of this flux is not expected to be significant.
5.2. Groundwater Elevation Hydrographs
The San Luis Valley and the Edna Valley are characterized by different patterns of groundwater use. In
the San Luis Valley, groundwater use has been dominated by municipal and industrial use, with total
groundwater use decreasing since the 1990s, as the City has diversified its surface water supplies, and
placed most of its wells on standby status. During this time several in-City agricultural operations have
also been developed into housing and commercial districts and now rely on the City’s surface water
supplies in place of groundwater pumping. In the Edna Valley, groundwater use is dominated by
agricultural use, with total use increasing since the 1990s. During the past 15 to 20 years, wine grapes
have supplanted other crop types (such as pasture grass and row crops) as the dominant agricultural
use within the Edna Valley. Available water level data was reviewed, and data from wells with the
longest period of record are presented in Figure 5-11 and discussed in this section. Most of the data
was obtained from the County’s groundwater monitoring network database.
Figure 5-11 presents groundwater elevation hydrographs for the ten wells throughout the Basin with the
longest period of record. State well identification numbers are not displayed for reasons of owner
confidentiality. Three distinct patterns are evident in different areas of the Basin and are discussed
below.
The hydrographs for the wells in the San Luis Valley indicate that water levels in these wells, although
somewhat variable in response to seasonal weather patterns, water use fluctuations, and longer-term
dry weather periods, are essentially stable. There are no long-term trends indicating steadily declining
or increasing water levels in this area. The wells along Los Osos Valley Road (hydrographs 1 and 2 on
Figure 5-11) display fluctuations within a range of less than 20 feet over a period of record from the late
1950s to the mid-1990s. This period includes the drought of the late 1980s to early 1990s. The well just
west of the intersection of Tank Farm Road and Orcutt Road (hydrograph 4 in Figure 5-11) displays a
similar pattern, with water level variations within a range of about 10 feet from 1965 to 2013. The wells
in the vicinity of Highway 101 and Los Osos Valley Road (hydrograph 3 in Figure 5-11) also display
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water levels in relative equilibrium, with the exception of the early 1990s, when drought-related
pumping and weather patterns resulted in noticeable declines in the water level in this well. These
water levels recovered to their pre-drought levels by the mid-1990s. The long-term stability of
groundwater elevations in these hydrographs indicates that groundwater extractions and natural
discharge in the areas of these wells are in approximate equilibrium with natural recharge and
subsurface capture, and that no trends of decreasing groundwater storage are evident.
A second distinct pattern is evident in hydrographs from wells in the area immediately east of the
intersection of Biddle Ranch Road and Orcutt Road, where West Corral de Piedras Creek enters the
Basin (hydrographs 5 and 6 in Figure 5-11). The hydrographs of the two wells in this area display much
greater volatility in response to seasonal and drought cycle fluctuations than the wells in San Luis
Valley, with water levels fluctuating within a range of over 40 feet, as opposed to the range of 10 to 20
feet in the San Luis Valley wells. However, water levels appear to rebound to pre-drought levels when
each drought cycle ends. Groundwater elevations displayed in these two hydrographs do not display a
long-term decline of water levels. This pattern is likely associated with local recharge of the aquifer
derived from percolation of stream water in West Corral de Piedras Creek as it leaves the mountains
and enters the Basin.
By contrast, several wells in the Edna Valley display steadily declining water levels during the past 15
to 20 years. Hydrographs for four wells (hydrographs 7, 8, 9, and 10 on Figure 5-11) in the Edna Valley
display groundwater elevation declines of about 60 to 100 feet since the year 2000. Groundwater
elevations in the Edna Valley displayed the largest historical declines in the Basin. This hydrograph
pattern indicates that a reduction of groundwater storage has occurred over this period of record in the
area defined by these well locations. It is understood and will be discussed in greater detail in Chapter
6 (Water Budget), that agricultural pumping has increased in Edna Valley during this time period, likely
explaining the patterns of declining groundwater elevations in these hydrographs.
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Figure 5-11. Selected Hydrographs
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5.3. Groundwater Recharge and Discharge Areas
Areas of significant areal recharge and discharge within the Basin are discussed below. Quantitative
information about all natural and anthropogenic recharge and discharge is provided in Chapter 6 (Water
Budget).
5.3.1. Groundwater Recharge Areas
In general, natural areal recharge occurs via the following processes:
1. Distributed areal infiltration of precipitation,
2. Subsurface inflow from adjacent “non-water bearing bedrock”, and
3. Infiltration of surface water from streams and creeks.
4. Anthropogenic recharge
The following sections discuss each of these components.
5.3.1.1. Infiltration of Precipitation
Areal infiltration of precipitation is a significant component of recharge in the Basin. Water that does not
run off to stream or get taken up via evapotranspiration migrates vertically downward through the
unsaturated zone until it reaches the water table. By leveraging available GIS data that defines key
factors such as topography and soil type, locations with higher likelihood of recharge from precipitation
have been identified. These examinations are desktop studies and therefore are conceptual in nature,
and any recharge project would need a site-specific field characterization and feasibility study before
implementation. Still, although they differ in scope and approach, the results of these studies provide an
initial effort at identifying areas that may have the intrinsic physical characteristics to allow greater
amounts of precipitation-based recharge in the Basin.
Stillwater Sciences (Stillwater), in cooperation with the Upper Salinas-Las Tablas Resource
Conservation District (USLTRCD), published a grant funded study (Stillwater Sciences, 2015) designed
to improve data gaps in the County’s Integrated Regional Water Management (IRWM) plan. The
Percolation Zone Study of Pilot-Study Groundwater Basins in San Luis Obispo County, California
identified areas with relatively high natural percolation potential that, through management actions,
could enhance local groundwater supplies for human and ecological benefits to the aquatic
environment for steelhead habitat. The study used existing data in a GIS analysis to identify potentially
favorable areas for enhanced recharge projects in the combined San Luis Obispo Creek and Pismo
Creek Watershed. The results of the Stillwater-USLTRCD study are presented in Figure 5-12. The
analysis indicates that approximately 2,220 acres in the Basin are categorized with high potential for
intrinsic percolation, and 6,583 acres have medium potential. Conceptually, areas with higher potential
for intrinsic percolation would transmit a higher percentage of rainfall to aquifer recharge. The largest
area in the Basin that is classified with high recharge potential is the alluvium along East and West
Corral de Piedras Creeks in the Edna Valley.
The University of California (UC) at Davis and the UC Cooperative Extension published a study in 2015
that also uses existing GIS data to identify areas potentially favorable for enhanced groundwater
recharge projects (U.C. Davis Cooperative Extension, 2015). While the Stillwater study focused on local
San Luis Obispo stream corridors and emphasized fish habitat conditions, the UC study is statewide in
scope includes more than 17.5 million acres, is scientifically peer reviewed, and focuses on the
possibilities of using fallow agricultural land as temporary percolation basins during periods when
excess surface water is available. The UC study developed a methodology to determine a Soil
Agricultural Groundwater Banking Index (SAGBI) to assign an index value to agricultural lands through
the state. The SAGBI analysis incorporates deep percolation, root zone residence time, topography,
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chemical limitations (salinity), and soil surface conditions into its analysis. The results of the SAGBI
analysis in the Basin are presented in Figure 5-13. Areas with excellent recharge properties are shown
in green. Areas with poor recharge properties are shown in red. Not all land is classified, but similar to
the Stillwater map in Figure 5-12, this map provides guidance on where natural recharge likely occurs.
The two studies discussed herein yield similar results in the Basin, particularly in Edna Valley. The
Stillwater study identifies much of the drainage area of East and West Corral de Piedras Creeks in the
Basin, as well as the alluvium of smaller streams to the southeast, as having high recharge potential.
The SAGBI study identifies very similar areas in Edna Valley as having a moderately to good index
value. These two studies, with differing methodologies, study areas, and objectives, converge on the
characterization of the same portions of Edna Valley as having high natural recharge potential. By
extension, areas with high natural recharge potential would be favorable locations to investigate the
feasibility of enhanced recharge projects. If source water is available, water in these areas would have
a higher likelihood of percolating to the underlying aquifers.
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Figure 5-12. Stillwater Percolation Zone Study Results
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Figure 5-13. Soil Agricultural Groundwater Banking Index Study Results
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5.3.1.2. Subsurface Inflow
Subsurface inflow is the flow of groundwater from the surrounding bedrock into the basin sediments.
This process is sometimes referred to as mountain front recharge. Groundwater flows from areas of
high head to areas of lower head, and water levels in the mountains are at a higher elevation than the
Basin. Flow across the basin boundary is predominantly via highly conductive, but random and
discontinuous fracture systems. The rate of subsurface inflow to the Basin from the surrounding hill and
mountain area varies considerably from year to year depending upon precipitation (intensity, frequency
and duration, seasonal totals, etc.) and groundwater level gradients. There are no available published
or unpublished inflow data for the hill and mountain areas surrounding the Basin. An estimate of this
component of recharge is presented in Chapter 6 (Water Budget).
5.3.1.3. Percolation of Streamflow
Percolation of streamflow is a locally significant source of recharge in areas where the local creeks flow
through the Basin. Water levels in wells monitored by the County in the area where Corral de Piedras
Creeks flow through the Basin reflect this phenomenon, as discussed in the previous discussion of
water level elevations in the Basin. Groundwater recharge from percolation of streamflow is thought to
occur in the area along Davenport Creek, near Buckley Road as well. Most wells in this vicinity are on
the order of 100 feet deep, which is too deep to be screened only in the local alluvium; these wells are
assumed to screen the Paso Robles Formation Aquifer. During the seasonal winter rains when the
creeks are flowing, groundwater levels are at approximately the same level as the water in the creek.
During the dry season, water levels decrease to about 15 to 20 feet below land surface. Therefore, the
alluvium appears to recharge the underlying Paso Robles Formation in this area. It is likely that similar
processes contribute to recharge via percolation of streamflow along the San Luis Obispo Creek
corridor as well. Specific isolated monitoring of alluvial wells compared to the underlying aquifers’ water
levels could clarify this recharge component.
5.3.1.4. Anthropogenic Recharge
Significant anthropogenic recharge occurs via the three processes discussed below:
1. Percolation of treated wastewater treatment plant (WWTP) effluent,
2. Percolation of return flow from agricultural irrigation, and
3. Percolation of return flow from domestic septic fields.
A wastewater treatment plant serving the City of San Luis Obispo operates within the Basin on Prado
Road along San Luis Obispo Creek. Treated wastewater effluent from this plant is discharged to San
Luis Obispo Creek and used in the City’s recycled water system for irrigation and construction-related
uses. The County operates a small WWTP near the golf course in the service area of Golden State
Water Company and uses the effluent largely to irrigate the golf course. Residences in Edna Valley
beyond the City or County WWTP service area dispose of wastewater via septic tanks. Water from
septic fields can percolate into the underlying aquifers.
Irrigated agriculture is prevalent in the Basin, especially along Los Osos Valley Road and in Edna
Valley. Return flows from irrigated agriculture occur when water is supplied to the irrigated crops in
excess of the crop’s water demand. This is done to avoid excess build-up of salts in the soil and
overcome non-uniformity in the irrigation distribution system. These are all general standard practices.
5.3.2. Groundwater Discharge Areas
Natural groundwater discharge occurs as groundwater discharge from the basin into springs, seeps
and wetlands, subsurface outflows, and by evapotranspiration (ET) by phreatophytes. Figure 5-16
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includes the locations of significant active springs, seeps, and wetlands within or adjacent to the Basin
identified from previous studies or included on USGS topographic maps covering the watershed area.
There are no mapped springs or seeps located within the Basin boundaries; most are located at higher
elevations in the surrounding mountain areas.
Natural groundwater discharge can also occur as discharge from the aquifer directly to streams.
Groundwater discharge to streams and potential groundwater dependent ecosystems (GDEs) are
discussed in Section 5.8. In contrast to mapped springs and seeps, whose source water generally
comes from bedrock formations in the mountains, groundwater discharge to streams is derived from the
alluvium. Discharge to springs or streams can vary seasonally as precipitation and stream conditions
change throughout the year. Groundwater discharge to the Corral de Piedras Creeks occur seasonally
at the location where the creeks leave the basin, where relatively impermeable bedrock rises to the
surface along the Edna Fault, causing groundwater to daylight at this location, at least in the wet
season. Subsurface outflow and ET by phreatophytes are discussed in Chapter 6 (Water Budget).
5.4. Change in Groundwater Storage
Changes in groundwater storage for the Alluvial Aquifer and Paso Robles Formation Aquifer are
correlated with changes in groundwater elevation, previously discussed, and are addressed in Chapter
6 (Water Budget).
5.5. Seawater Intrusion
Seawater intrusion is not an applicable sustainability indicator for the Basin. The Basin is not adjacent
to the Pacific Ocean, a bay, or inlet.
5.6. Subsidence
Land subsidence is the lowering of the land surface. While several human-induced and natural causes
of subsidence exist, the only process applicable to the GSP is subsidence due to lowered groundwater
elevations caused by groundwater pumping. Historical incidence of subsidence within the Basin was
discussed in Chapter 4 (Basin Setting).
Direct measurements of subsidence have not been made in the Basin using extensometers or repeat
benchmark calibration; however, interferometric synthetic aperture radar (InSAR) has been used in the
County to remotely map subsidence and DWR is expected to continue to collect InSAR data. This
technology uses radar images taken from satellites that are used to map changes in land surface
elevation. One study done in the area, which evaluates the time period between spring 1997 and fall
1997 (Valentine, 1999), did not report any measurable subsidence within the Basin. Subsidence as a
sustainability indicator will be addressed further in Chapter 8 (Sustainable Management Criteria).
5.7. Interconnected Surface Water
Surface water/groundwater interactions may represent a significant, portion of the water budget of an
aquifer system. Where the water table is above the streambed and slopes toward the stream, the
stream receives groundwater from the aquifer; that is called a gaining reach (i.e., it gains flow as it
moves through the reach). Where the water table is beneath the streambed and slopes away from the
stream, the stream loses water to the aquifer; that is called a losing reach. In addition, a stream may be
disconnected from the regional aquifer system if the elevation of streamflow and alluvium is significantly
higher than the elevation of the water table in the underlying aquifer.
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The spatial extent of interconnected surface water in the Basin was evaluated using water level data
from wells screened in the Recent Alluvium and Paso Robles Formation Aquifer adjacent to the Basin
creeks and streams. In accordance with the SGMA Emergency Regulations §351 (o), “Interconnected
surface water refers to surface water that is hydraulically connected at any point by a continuous
saturated zone to the underlying aquifer and the overlying surface water is not completely depleted”.
The interconnected surface water analysis for the Basin consisted of comparing average springtime
water level elevations in wells adjacent to the San Luis Obispo Creek with the elevation of the adjacent
San Luis Obispo Creek channel. In cases where average springtime water levels were greater than the
elevation of the adjacent San Luis Obispo Creek channel, the stream reach was considered as
potentially ‘gaining’. In cases where average springtime water levels were below the adjacent channel
elevation, the stream reach was considered ‘losing’ and potentially ‘disconnected’. It is important to
recognize that the results of these analyses may reflect conditions that occur occasionally, in response
to precipitation events. They may not be representative of long-term average conditions.
The analysis outlined above resulted in identification of two areas of San Luis Obispo Creek that
occasionally ‘gain’ water from the Alluvial Aquifer; the confluence of Stenner Creek and San Luis
Obispo Creek, and the reach of San Luis Obispo Creek downstream from the Wastewater Treatment
Plant to the confluence with Prefumo Creek. These are displayed in Figure 5-14. Several reaches of
San Luis Obispo Creek are identified that occasionally ‘lose’ water to the Alluvial Aquifer. Groundwater
levels in the San Luis Valley part of the Basin are generally high enough that the creek is connected to
the underlying aquifer. Along most of Corral de Piedras Creeks, by contrast, surface water levels are
generally greater than 30 feet above the groundwater level, and the streams are considered seasonally
disconnected from the underlying Alluvial Aquifer in this area.
Evaluation of groundwater elevation hydrographs provides additional insight into the character of
interconnected surface water in San Luis Valley and Edna Valley. The differences between the surface
water regimes of the two subareas of the Basin are discussed below.
Figure 5-14 presents a hydrograph of City of SLO Well on Calle Joaquin Street, referenced as SLV-12
in Chapter 7 (Monitoring Network). This well is located near both Prefumo Creek and San Luis Obispo
Creek, the main streams draining the San Luis Valley part of the Basin. Data presented on this
hydrograph date back to 1992, the end of the drought conditions spanning the late 1980s and early
1990s. Inspection of this data indicates that groundwater elevations are very close to land surface in
this area (Future monitoring recommendations include surveying the monitoring well and channel
elevations so that this can be confirmed). This is indicative that the water level elevations are likely
correlated to surface water conditions in the adjacent stream, and that there is an interconnection
between surface water and groundwater at this location. Seasonal variations in groundwater elevations
of about 6 to 7 feet are evident in the hydrograph. But no long-term trends of declining groundwater
elevations are displayed. This indicates that the character of the surface water/groundwater interaction
at this location is likely unchanged in the 30-year period since the early 1990s.
Figure 5-15 presents hydrographs of two wells located adjacent to West Corral de Piedras Creek in the
Edna Valley subarea of the Basin. One of the wells (EV-01) is located near the location where the creek
enters the Basin, and the other well (EV-11, the CASGEM Greengate Well) is located about 1.8 miles
south, near where the creek exits the Basin. Because of their proximity to the creek, it is assumed that
the high groundwater elevations correspond to periods of surface water flow being present in the
channel, and further assumed that these groundwater elevations are close to the stream channel
elevation, although there is no streamflow data to confirm this. Data gaps and recommendations for
improved stream flow monitoring and channel surveying are discussed in Chapter 7 (Monitoring
Network). Water level data for EV-01 dates back to the late 1950s, while data for EV-11 only extends
back to 2011. The data for EV-01 indicates a pattern of seasonal variability, wherein large swings in
water level elevations of nearly 50 feet are routinely observed between spring and fall of the same year.
Extended dry periods such as those of 1988-1992, and the recent drought from 2012-2016, are evident
as prolonged periods when the groundwater elevations remain at their approximate historical lows for
years at a time. One significant feature of the EV-01 hydrograph is that the essential water level trends
have remained unchanged throughout the entire period of record. The average high-water levels and
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the average low water levels are the same now as they were in the 1960s and 1970s, even though
significant changes in land use and groundwater usage have occurred over this period. The seasonal
and drought period low groundwater elevations are over fifty feet below ground surface. This suggests
that the aquifer at these locations is at least seasonally disconnected from the stream during dry
summer and fall months. The period of record displayed in the hydrograph for EV-11 is not nearly as
extensive as that for EV-01 but displays some similar features. The 2012-2016 drought is evident as a
prolonged period wherein the water levels are over 50 feet below land surface. This suggests that
during extended dry periods (I.e., more than single season variation), the aquifer may be disconnected
from the stream. However, by 2018, when the seasonal low water level in EV-01 declines about 50 feet
to a typical seasonal low elevation, the water levels in EV-11 only decline to about 20-30 feet below the
previous high values. This suggests that EV-11 only becomes disconnected from the stream during
lengthy drought periods, and not necessarily on a seasonal basis. In this area the Basin sediments are
juxtaposed against the Edna Fault and the mountain bedrock on the south side of the Basin, which may
force groundwater flow to daylight in this area. This would explain the relative lack of range in seasonal
fluctuation in groundwater elevations in this area. Improved stream flow monitoring and channel
surveying as discussed in Chapter 7 (Monitoring Network) are recommended to better understand the
interactions.
Figure 5-14. Groundwater Elevation Hydrograph (SLV-12)
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Figure 5-15. Hydrographs of Wells Adjacent to West Corral de Piedras Creek
5.7.1. Depletion of Interconnected Surface Water
Groundwater withdrawals are balanced by a combination of reductions in groundwater storage and
changes in the rate of exchange across hydrologic boundaries. In the case of surface water depletion,
this rate change could be due to reductions in rates of groundwater discharge to surface water, and
increased rates of surface water percolation to groundwater. Seasonal variation in rates of groundwater
discharge to surface water or surface water percolation to groundwater occur naturally throughout any
given year, as driven by the natural hydrologic cycle. However, they can also be affected by
anthropogenic actions. Since, as presented in the discussion of hydrographs in Section 5.7, there have
been no long-term water level declines in San Luis Valley, it is therefore concluded that no long-term
depletion of interconnected surface water due to groundwater management has occurred in this area.
As discussed in the hydrograph analysis of alluvial wells in the Edna Valley, Corral de Piedras Creek
appears to be regularly (seasonally) disconnected from the groundwater in the underlying aquifer since
the 1950s. Additional monitoring data is proposed in Chapter 7 (Monitoring Network) and Chapter 10
(Implementation Plan), including surveying of channel elevations proximate to nearby well elevation,
establishment of additional stream gages, development of rating curves for existing flood stage gages,
and other actions.
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5.8. Potential Groundwater Dependent Ecosystems
The SGMA Emergency Regulations §351.16 require identification of groundwater dependent
ecosystems within the Basin. Several datasets were utilized to identify the spatial extent of potential
groundwater dependent ecosystems (GDEs) in the Basin, as discussed in the following sections. In
accordance with the SGMA Emergency Regulations §351 (o), “groundwater dependent ecosystems
refers to ecological communities or species that depend on groundwater emerging from aquifers or on
groundwater occurring near the ground surface”. In areas where the water table is sufficiently high,
groundwater discharge may occur as evapotranspiration (ET) from phreatophyte vegetation within
these GDEs. The overall distribution of potential GDEs within the Basin has been initially estimated in
the Natural Communities Commonly Associated with Groundwater (NCCAG) dataset (DWR, 2018).
This dataset was reviewed by Stillwater Sciences, and a Technical Memo generated (Stillwater
Sciences, 2020) that is included as Appendix F, and the resulting distribution of potential GDEs is
shown in Figure 5-17. There has been no verification that the locations shown on this map constitute
GDEs. Additional field reconnaissance is necessary to verify the existence and extent of these potential
GDEs and may be considered as part of the monitoring effort for future planning efforts.
5.8.1. Hydrology
5.8.1.1. Overview of GDE Relevant Surface and Groundwater Hydrology
Instream flows in San Luis and Pismo Creeks can be divided into wet season flows, typically occurring
from January to April, and dry season flows, typically from June to October. Short transitional periods
occur between the wet and dry seasons. Wet season instream flows originate from a range of sources
including precipitation-driven surface runoff events, water draining from surface depressions or
wetlands, shallow subsurface flows (e.g., soil), and groundwater discharge. Dry season instream flows,
however, are likely fed primarily by groundwater discharge. As groundwater levels fall over the dry
season, so do the corresponding instream flows. If groundwater elevations remain above instream
water elevations, groundwater discharges into the stream and surface flows continue through the dry
season (creating perennial streams). If groundwater elevations fall below the streambed elevation, the
stream can go dry. Streams that typically flow in the wet season and dry up in the dry season are
termed intermittent. Over time, streams can transition from historically perennial to intermittent
conditions due to climactic changes or groundwater pumping (Barlow, 2012). Dry season flows
supported by groundwater are critical for the survival of various special status species, including the
federally threatened California red-legged frog (Rana draytonii) and Steelhead (Oncorhynchus mykiss).
San Luis Obispo Creek and Pismo Creek are underlain by the Alluvial Aquifer, the Paso Robles
Formation Aquifer, and the Pismo Formation Aquifer, as previously discussed. These aquifers have
hydraulic connection to one another, and to surface waters, but the degree of connection varies
spatially. Aquifers can include confined aquifers, unconfined aquifers, and perched aquifers, as
discussed in Chapter 4 (Basin Setting). Aquifers can discharge into ponds, lakes or creeks or vice
versa. In the San Luis Obispo Valley Groundwater Basin, little data exists to characterize the
connection between surface water and groundwater.
While the groundwater in the San Luis Valley and Edna Valley is hydraulically connected, a shallow
subsurface bedrock high between the two sub-areas partially isolates the deeper portions of the two
aquifers (Figure 5-10 and Figure 5-11). Groundwater in the Edna Valley flows both towards the San
Luis Valley in the northwest portion of the basin and towards Price Canyon in the southwest portion of
the basin. Groundwater flowing towards Price Canyon rises to the surface as it approaches the bedrock
constriction of Price Canyon and the Edna fault system. The 1954 DWR groundwater elevation map
(Figure 5-1) best illustrates the pre-development groundwater flow from the Edna Valley both towards
San Luis Obispo and into Price Canyon. Observations of stream conditions indicate a perennial reach
of Pismo Creek that flows through Price Canyon and supports year-round critical habitat for threatened
steelhead just south of the Basin boundary. A conceptual explanation for this is that groundwater from
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the Edna subarea flows towards the discharge area at Price Canyon and rises to the surface (daylights)
as the groundwater flow encounters the impermeable zone of the Edna Fault and the bedrock outside
of the Basin. Piezometers in this area could confirm this interpretation of observed stream conditions.
5.8.1.2. Losing and Gaining Reaches
Streams are often subdivided into losing and gaining reaches to describe their interaction of surface
water in the stream with groundwater in the underlying aquifer. In a losing reach water flows from the
stream to the groundwater, while in a gaining reach water flows from the groundwater into the stream.
The connection between losing reaches to the regional aquifer may be unclear as water can be trapped
in perched aquifers above the regional water table. Figure 5-16 shows the likely extent of known
gaining and losing reaches in San Luis and Pismo Creeks during typical dry season conditions.
This map is compiled from various data sources, including:
• A field survey of wet and dry reaches of San Luis Obispo Creek (Bennett, 2015),
• Field surveys and flow measurements of Pismo Creek (Balance Hydrologics, 2008),
• An instream flow study of Pismo Creek (Stillwater Sciences 2012),
• A regional instream flow assessment that included San Luis and Pismo Creeks (Stillwater Sciences,
2014),
• Spring and summer low flow measurements in San Luis and Pismo Creeks (2015–2018) (Creek
Lands Conservation, 2019), and
• Consideration of the effects of local geologic features such as bedrock outcrops and faults, both of
which can force deeper groundwater to the surface.
The effect of faults and bedrock outcrops can be localized or extend for some distance downstream.
Portions of the San Luis and Pismo Creeks and their tributaries for which no data exist are left
unhighlighted in Figure 5-16. In general, the extent of losing or gaining reaches can vary by season,
water year type, or pumping conditions. East and West Corral de Piedras Creeks on the north-east side
of the basin may be dry, and disconnected from the underlying aquifer in the spring and summer during
drier years but be flowing, losing reaches in wetter years (Creek Lands Conservation, 2019). (To be
clear, a stream segment can be a losing reach even if it is not hydraulically connected to the aquifer,
since the stream will be losing surface flow to the subsurface via percolation.) In contrast, gaining
reaches shown on San Luis Obispo Creek are fairly consistent across water year types (Bennett, 2015)
(Creek Lands Conservation, 2019). Figure 5-16 is based on limited data sources. Improved surface
flow monitoring is recommended to refine and update the extent of losing and gaining reaches, as well
as to provide data for unhighlighted reaches.
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Figure 5-16. Losing and Gaining Reaches Within the Basin
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5.8.2. Vegetation and Wetland Groundwater Dependent Identification
DWR has compiled a statewide Natural Communities Commonly Associated with Groundwater
(NCCAG) database (DWR, 2018). This database identifies potentially groundwater dependent
ecosystems based on the best available vegetation and wetland data (Klausmeyer, 2018). DWR
identifies potentially groundwater dependent wetland areas using National Wetland Inventory (NWI)
wetland data (USFWS, 2018). These data were evaluated and assessed to accurately capture wetland
and riverine features. In the Basin, the best available vegetation mapping data set was from the
California Fire and Resource Assessment Program Vegetation (FVEG), (California Department of
Forestry and Fire Protection, 2015). FVEG is a remotely sensed dataset that classifies vegetation to
coarse types (i.e., the California Wildlife Habitat Relationship System). Given the limitations of this
dataset to accurately capture and identify vegetation using a precise classification system, it was
deemed inappropriate for use in determining potential GDEs. Instead, a manual assessment of
vegetation with potential groundwater dependence was conducted using National Agricultural Imagery
Program 2018 color aerial imagery (NAIP (National Agricultural Imagery Program), 2018). Vegetation
communities identified as potentially groundwater dependent included riparian trees and shrubs, and
oak woodlands. Oak woodlands were considered potentially groundwater dependent due to their deep
rooting depths (up to 70 feet (Lewis, 1964)).
Potential vegetation and wetland GDEs were retained if the underlying depth to water in 2019 was
inferred to be 30 feet or shallower based on the existing well network (Figure 5-17). Depth to
groundwater was interpolated from seventeen wells for which groundwater level data was available in
the spring of 2019 (Figure 5-6). The depth to groundwater estimated in Figure 5-17 is assumed to
represent regional groundwater levels; however, the screening depth is known for only 6 of the 17 of
the wells. Wells where the screened depth is unknown may be measuring groundwater levels for
deeper aquifers that are unconnected to the shallow groundwater system, and thus groundwater
deeper than 30 ft for a given well may not reflect the absence of shallow groundwater, but instead
reflects the absence of data. To determine the hydraulic connectivity between potential perched
aquifers to the regional aquifer, additional monitoring with nested piezometers could be utilized.
For the purposes of differentiating between potential and unlikely GDE’s, different assumptions were
made for the San Luis Valley versus Edna Valley in areas of no groundwater data. In the San Luis
Valley, underlying San Luis Obispo Creek, it was assumed that the depth to regional groundwater was
less than 30 feet because the limited available data indicate that groundwater in this sub-area is
generally relatively shallow. In the Edna Valley (underlying Pismo Creek), it was assumed that the
depth to regional groundwater was more than 30 feet because the limited available data indicate that
the groundwater in this sub-area is generally deeper; therefore, much of the area of the lower reaches
of East and West Corral de Piedras Creeks is unlikely to have GDEs. One exception to this assumption
was made on upper East Corral de Piedra, where the conditions were assumed to be similar to those
on upper West Corral de Piedra, where wet conditions have been observed to persist into late spring or
early summer (Stillwater Sciences, 2014) (Balance Hydrologics, 2008). The 30-foot depth criterion is
consistent with guidance provided by The Nature Conservancy (Rohde, 2019) for identifying GDEs.
Additionally, the area where East and West Corral de Piedras Creeks leave the Basin near Price
Canyon has groundwater elevation data within 30 feet of the streams, as a result, these areas are
presented as having potential GDEs.
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 5-33 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 5-17. Potential Groundwater-Dependent Ecosystems (GDEs)
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 5-34
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Sustainability Plan
5.8.3. Identification of Special Status Species and Sensitive Natural
Communities Associates with GDEs
For the purposes of this GSP, special-status species are defined as those:
• Listed, proposed, or under review as endangered or threatened under the federal Endangered
Species Act (ESA) or the California Endangered Species Act (CESA);
• Designated by California Department of Fish and Wildlife (CDFW) as a Species of Special Concern;
• Designated by CDFW as Fully Protected under the California Fish and Game Code (Sections 3511,
4700, 5050, and 5515);
• Protected under the Federal Bald and Golden Eagle Protection Act;
• Designated as rare under the California Native Plant Protection Act (CNPPA); and/or
• Included on CDFW’s most recent Special Vascular Plants, Bryophytes, and Lichens List (CDFW,
2019) with a California Rare Plant Rank (CRPR) of 1, 2, 3, or 4.
In addition, sensitive natural communities are defined as:
• Vegetation communities identified as critically imperiled (S1), imperiled (S2), or vulnerable (S3) on
the most recent California Sensitive Natural Communities List (CDFW, 2019).
To determine the terrestrial and aquatic special-status species that may utilize potential GDE units
overlying the Basin, Stillwater ecologists queried existing databases on regional and local occurrences
and distributions of special-status species. Databases accessed include the California Natural Diversity
Database (CNDDB) (CDFW, 2019). (eBird, 2017), and TNC freshwater species list (The Nature
Conservancy, (TNC), 2019). Spatial database queries were centered on the potential GDEs plus a 1-
mile buffer. Stillwater’s ecologists reviewed the database query results and identified special-status
species and sensitive natural communities with the potential to occur within or to be associated with the
vegetation and aquatic communities in or immediately adjacent to the potential GDEs. The table in
Appendix F lists these special-status species and sensitive natural communities, describes their habitat
preferences and potential dependence on GDEs, and identifies known nearby occurrences (Appendix F
- Table 1). Wildlife species were evaluated for potential groundwater dependence using the Critical
Species Lookbook (Rohde, 2019).
The San Luis Obispo Valley Groundwater Basin supports steelhead belonging to the South-Central
California Coast Distinct Population Segment (DPS) which is federally listed as “threatened.” Within this
DPS, the population of steelhead within the San Luis Obispo Creek, and Pismo Creek portions of the
groundwater basin have both been identified as Core 1 populations which means they have the highest
priority for recovery actions, have a known ability or potential to support viable populations, and have
the capacity to respond to recovery actions ( (National Marine Fisheries Service, (NMFS), 2013)). One
critical recovery action listed by NMFS includes the management of groundwater extractions for
protection and restoration of natural surface flow patterns to ensure surface flows allow for essential
steelhead habitat functions (National Marine Fisheries Service, (NMFS), 2013).
Based on criteria promulgated by The Nature Conservancy (TNC), the San Luis Obispo Valley
Groundwater Basin was determined to have high ecological value because: (1) the known occurrence
and presence of suitable habitat for several special-status species including the Core 1 population
status of South-Central California Coast Steelhead DPS and several special-status plants and animals
that are directly or indirectly dependent on groundwater (Appendix F - Table 1); and (2) the vulnerability
of these species and their habitat to changes in groundwater levels (Rohde, 2019).
Groundwater Conditions (§354.16) Section 5
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5.9. Groundwater Quality Distribution and Trends
Groundwater quality samples have been collected and analyzed throughout the Basin for various
studies and programs and are collected on a regular basis for compliance with regulatory programs.
Water quality data surveyed for this GSP were collected from:
• The California Safe Drinking Water Information System (SDWIS), a repository for public water
system water quality data,
• The National Water Quality Monitoring Council water quality portal (this includes data from the
recently decommissioned EPA STORET database, the USGS, and other federal and state entities
[Note: in the Basin the agencies include USGS, California Environmental Data Exchange Network
(CEDEN), and Central Coast Ambient Monitoring Program {CCAMP}]), and
• The California State Water Resources Control Board (SWRCB) GeoTracker GAMA database.
In general, the quality of groundwater in the Basin is good. Water quality trends in the Basin are stable,
with no significant trends of ongoing deterioration of water quality based on the Regional Water Quality
Control Board’s Basin Objectives, outlined in the Water Quality Control Plan for the Central Coast Basin
(Regional Water Quality Control Board, 2017). The Basin Plan takes all beneficial uses into account
and establishes measurable goals to ensure healthy aquatic habitat, sustainable land management,
and clean groundwater. The distribution, concentrations, and trends of some of the most commonly
cited major water quality constituents are presented in the following sections.
5.9.1. Groundwater Quality Suitability for Drinking Water
Groundwater in the Basin is generally suitable for drinking water purposes. Groundwater quality data
was evaluated from the SDWIS and GeoTracker GAMA datasets. The data reviewed includes 2,885
sampling events from 403 supply wells and monitoring wells in the Basin, collected between June 1953
and September 2019. Primary drinking water standards Maximum Contaminant Levels (MCLs) and
Secondary MCLs (SMCLs) are established by Federal and State agencies. MCLs are legally
enforceable standards, while SMCLs are guidelines established for nonhazardous aesthetic
considerations such as taste, odor, and color. Primary water quality standard exceedances in the Basin
include exceedance of the MCL for nitrate, which equaled or exceeded the standard in 269 samples out
of 2,605 samples (or 10% of samples, with 190 of the exceedances occurring in four wells), and
exceedance of the MCL for arsenic, which exceeded the MCL in 30 out of 771 samples (or 4% of
samples collected). The SMCL for total dissolved solids (TDS) was equaled or exceeded in 126 out of
843 samples (or 15% of total samples). In the case of public water supply systems, these water quality
exceedances are effectively mitigated with seasonal well use, treatment, or water blending practices to
reduce the constituent concentrations to below their respective water quality standard. In general, these
statistics meet the Central Coast Water Board Basin Plan measurable goals that by 2025, 80% of
groundwater will be clean, and the remaining 20% will exhibit positive trends in key parameters.
5.9.2. Distribution and Concentrations of Point Sources of Groundwater
Constituents
Potential point sources of groundwater quality degradation due to release of anthropogenic
contaminants were identified using the State Water Resources Control Board (SWRCB) Geotracker
website. Waste Discharge permits were also reviewed from on-line regional SWRCB websites Table
5-1 summarizes information from these websites for open/active sites. Figure 5-18 shows the locations
of these open groundwater contaminant point source cases, and the locations of completed/case
closed sites. Based on available information there are no mapped ground-water contamination plumes
at these sites.
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 5-36 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 5-18. Location of Potential Point Sources of Groundwater Conditions
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 5-37 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Table 5-1. Potential Point Sources of Groundwater Contamination
SITE ID SITE NAME CASE TYPE STATUS CONSTITUENT(S) OF CONCERN (COCS) POTENTIALLY AFFECTED MEDIA
T0607900100 American Gas and Tire LUST Cleanup Site Open - Verification Monitoring Benzene, Gasoline, MTBE / TBA / Other Fuel Oxygenates Aquifer used for drinking water supply
SL203011375 Chevron (Former UNOCAL) - Tank Farm Road Bulk
Storage Cleanup Program Site Open - Remediation Arsenic, Lead, Asphalt, Crude Oil, Other Petroleum
Contaminated Surface / Structure, Other
Groundwater (uses other than drinking
water), Soil, Surface water
T10000002287 Conoco Phillips site # 5143 Cleanup Program Site Open - Site Assessment Crude Oil, Diesel, Gasoline Soil
SL0607944973 COP Pipeline at San Luis Drive Cleanup Program Site Open - Assessment & Interim
Remedial Action Crude Oil
Other Groundwater (uses other than
drinking water), Well used for drinking
water supply
T10000001025 KIMBALL MOTORS Cleanup Program Site Open - Verification Monitoring Other Chlorinated Hydrocarbons, Tetrachloroethylene (PCE),
Trichloroethylene (TCE), Vinyl chloride
Aquifer used for drinking water supply,
Soil
SLT3S0851312 MODEL INDUSTRIAL SUPPLY Cleanup Program Site Open - Site Assessment Aquifer used for drinking water supply
SLT3S0161285 PG&E-FORMER MANUFACTURED GAS PLANT-SAN
LUIS OBISPO Cleanup Program Site Open - Remediation Aquifer used for drinking water supply
SL0607937854 PISMO ST. AND MORRO ST. PIPELINE RELEASE Cleanup Program Site Open - Site Assessment Crude Oil Aquifer used for drinking water supply
T10000012768 SAN LUIS COUNTY RGNL Non-Case Information Pending Review Per- and Polyfluoroalkyl Substances (PFAS)
T10000002286 South Higuera St & Pismo St Pipeline (Chevron Site
351317) Cleanup Program Site Open - Site Assessment Crude Oil, Diesel, Gasoline Aquifer used for drinking water supply,
Soil
T10000010079 Thread Lane Properties, LLC Cleanup Program Site Open - Site Assessment
SL0607965995 TRACT 1259 Cleanup Program Site Open - Assessment & Interim
Remedial Action Crude Oil Aquifer used for drinking water supply
T10000000060 Union Pacific Railroad - Round House/Pond Site Cleanup Program Site Open - Inactive Waste Oil / Motor / Hydraulic / Lubricating Other Groundwater (uses other than
drinking water), Soil
T10000012125 UPRR Tie Fire Non-Case Information Pending Review
T10000010082 Volny Investment Company Cleanup Program Site Open - Site Assessment
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 5-0
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Sustainability Plan
5.9.3. Distribution and Concentrations of Diffuse or Natural Groundwater
Constituents
The distribution and concentration of several constituents of concern are discussed in the following
subsections. Groundwater quality data was evaluated from the SDWIS and GeoTracker GAMA
datasets. The data reviewed includes 2,884 sampling events from 403 wells in the Basin, collected
between June 1953 and June 2019. Each of the constituents are compared to their drinking water
standard, if applicable, or their Basin Plan Median Groundwater Quality Objective (RWQCB Objective)
(Regional Water Quality Control Board, Central Coast Region, 2017). This GSP focuses only on
constituents that might be impacted by groundwater management activities. The constituents
discussed below are chosen because they have either a drinking water standard, a known effect on
crops, or concentrations have been observed above either the drinking water standard or the level that
affects crops.
5.9.3.1. Total Dissolved Solids
TDS is defined as the total amount of mobile charged ions, including minerals, salts or metals,
dissolved in a given volume of water and is commonly expressed in terms of milligrams per liter (mg/L).
Specific ions of salts such as chloride, sulfate, and sodium may be evaluated independently, but all are
included in the TDS analysis, so TDS concentrations are correlated to concentrations of these specific
ions. Therefore, TDS is selected as a general indicator of groundwater quality in the Basin. TDS is a
constituent of concern in groundwater because it has been detected at concentrations greater than its
RWQCB Basin Objective of 900 mg/l in the Basin. The TDS Secondary MCL has been established for
color, odor and taste, rather than human health effects. This Secondary MCL includes a recommended
standard of 500 mg/L, an upper limit of 1,000 mg/L and a short-term limit of 1,500 mg/l. TDS water
quality results ranged from 180 to 3,100 mg/l with an average of 727 mg/l and a median of 613 mg/l.
The distribution and trends of TDS concentrations in the Basin groundwater are presented Figure 5-19.
TDS concentrations are color coded and represent the average result if multiple samples are
documented. Most of the samples with the highest values (dark red in the figure) are outside or on the
edge of the Basin. This is consistent with observations that groundwater from the Basin sediments
generally has better water quality than groundwater from bedrock wells. Eleven wells with the greatest
amount of data over time were selected. Graphs displaying TDS concentration with time are included
on Figure 5-19. Most of these graphs do not display any upward trends in TDS concentrations with
time. The sustainability projects and management actions implemented as part of this GSP are not
anticipated to increase groundwater TDS concentrations in wells that are currently below the SMC
.
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 5-1 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 5-19. Distribution of TDS in Basin
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 5-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
5.9.3.2. Nitrates
Nitrate (as Nitrogen) is a widespread contaminant in California groundwater. Although it does occur
naturally at low concentrations, high levels of nitrate in groundwater are associated with agricultural
activities, septic systems, confined animal facilities, landscape fertilizers and wastewater treatment
facilities. Nitrate is the primary form of nitrogen detected in groundwater. It is soluble in water and can
easily pass through soil to the groundwater table. Nitrate can persist in groundwater for decades and
accumulate to high levels as more nitrogen is applied to the land surface each year. It is a Primary
Drinking Water Standard constituent with an MCL of 10 mg/l.
Nitrate is a constituent of concern in groundwater because it has been detected at concentrations
greater than its RWQCB Basin Objectives of 5 mg/l (as N) in the Basin. The Nitrate MCL has been
established at 10 mg/l (as N). Overall, nitrate water quality results ranged from below the detection limit
to 80 mg/l (as N) with an average of 3.9 mg/l (as N) and a median value of 2.0 mg/l (as N).
Figure 5-20 presents occurrences and trends for nitrate in the Basin groundwater. Wells with the most
sampling data over time were selected for presentation. The color-coded symbols represent the
average result if multiple samples are documented. Most of the chemographs displayed on Figure 5-20
indicate concentrations of nitrate well below the MCL, and do not indicate trends of increasing
concentrations with time. Chemographs labelled number 4 and 5 on Figure 5-20 do appear to indicate a
slight upward trend in nitrate (as nitrogen) concentrations over the data period of record. Sustainability
projects and management actions implemented as part of this GSP are not anticipated to increase
nitrate concentrations in groundwater in a well that would otherwise remain below the MCL to increase
above the MCL.
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 5-3 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 5-20. Distribution of Nitrate in Basin
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 5-4
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
5.9.3.3. Arsenic
Arsenic is also a common contaminant in California groundwater. Although it does occur naturally at
low concentrations, elevated levels of arsenic in groundwater may be associated with pesticide use,
mining activities, and release of industrial effluent. Arsenic has a Primary Drinking Water Standard with
an MCL of 10 ug/l. Overall, arsenic concentrations ranged from below the detection limit to 28 ug/l, with
an average value of 2.5 ug/l and a median value of 2 ug/l.
Figure 5-21 presents occurrences and trends for arsenic in the Basin groundwater from wells with the
most arsenic analytical data over time. The color-coded symbols represent the average result if multiple
samples are documented. Wells screened in the bedrock aquifers may be expected to have higher
natural arsenic concentrations than wells screened in Basin sediments due to increased degrees of
mineralization in these waters. Most of the chemographs displayed show stable or decreasing
concentrations of arsenic over the data period of record. (Graph number 1 shows a slight increase over
time but is still below the MCL). Sustainability projects and management actions implemented as part of
this GSP are not anticipated to directly cause arsenic concentrations in groundwater in a well that
would otherwise remain below the MCL to increase above the MCL.
5.9.3.4. Boron
Boron is an unregulated constituent and therefore does not have a regulatory standard. However,
boron is a constituent of concern because elevated boron concentrations in water can damage crops
and affect plant growth. Boron has been detected at concentrations greater than its RWQCB Basin
Objective of 200 micrograms per liter (ug/l). Boron water quality results ranged from non-detect to 2,500
ug/l with an average of 0.16 ug/l and a median value of 0.12.
Boron concentrations in the Alluvial Aquifer have been relatively consistent throughout the period of
record. Boron concentrations in the Paso Robles Formation Aquifer have generally remained steady or
declined slightly over the period of record. Sustainability projects and management actions
implemented as part of this GSP are not anticipated to directly cause boron concentrations in
groundwater in a well to increase.
5.9.3.5. Other Constituents
Other constituents found in exceedance of their respective regulatory standard include arsenic, iron,
gross alpha, manganese, selenium, and sulfate. Each of these exceedances occurred in samples from
a small number of wells, indicating isolated occurrences of these elevated constituent concentrations
rather than widespread occurrences, affecting the entire Basin. Isolated concentrations of arsenic, iron,
gross alpha, and sulfate in the Basin have been relatively consistent throughout the period of record.
Selenium concentrations have generally declined since 2007. There are not enough data to determine
the trend of the elevated manganese concentrations in the Basin. Sustainability projects and
management actions implemented as part of this GSP are not anticipated to directly cause
concentrations of any of these constituents in groundwater to increase.
Groundwater Conditions (§354.16) Section 5
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 5-5 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 5-21. Distribution of Arsenic in Basin
6-1
6
GROUNDWATER SUSTAINABILITY PLAN
Water Budget (§354.18)
The purpose of a water budget is to provide an accounting and assessment
of the total annual volume of groundwater and surface water entering and
leaving the basin, including historical, current, and projected water budget
conditions, and the change in volume stored. Both numerical and analytical
methods have been used during water budget preparations for the GSP.
The analytical method refers to application of the water budget
equation and the inventory method using spreadsheets, with
groundwater flow estimates based on Darcy’s Law and change in
storage calculations based on the specific yield method.
Numerical methods refer to surface water and groundwater flow
modeling, which provide a dynamic and more rigorous analysis of
both surface-groundwater interactions and the impacts from
pumping on groundwater in storage. The historical and current
analytical groundwater budget was used as part of the basin
conceptual model to calibrate and interpret the numerical model
GSFLOW which is documented in Appendix F. This chapter
presents the analytical water budget for the historical and current
conditions and the projected water budgets were developed using
the GSFLOW model developed for this GSP (Appendix F).
IN THIS CHAPTER
• Historical Water
Budget
• Current Water
Budget
• Sustainable Yield
Estimate
• Estimated Overdraft
• Projected Water
Budget
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.1. Introduction
A water budget identifies and quantifies various components of the hydrologic cycle within a user-
defined area, in this case the San Luis Obispo Valley Groundwater Basin. Water circulates between
the atmospheric system, land surface system, surface water bodies, and the groundwater system, as
shown in Figure 6-1 (DWR, 2016) The water budget equation used for the analytical method is as
follows:
INFLOW – OUTFLOW = CHANGE IN STORAGE
Inflow is the sum of all surface water and groundwater entering the Basin and outflow is the sum of all
surface water and groundwater leaving the Basin. The difference between total inflow and total outflow
over a selected time period is equal to the change in total storage (surface water and groundwater)
within the Basin over the same period. Components of inflow and outflow represented in the water
budget are shown in Figure 6-2. Not all the components shown are needed for the San Luis Obispo
Valley Groundwater Basin GSP. A key using letters to represent components in this water budget has
been added to Figure 6-2 for reference with the main water budget tables. Some components have
been modified and renamed from the original DWR figure to better represent this specific water budget.
The water budget equation given above is simple in concept, but it is challenging to measure and
account for all the components of inflow and outflow within a Basin. Some of these components can be
measured or estimated independently, while others are calculated using the water budget equation.
The water budget for this GSP has been prepared for the two subareas that cover the Basin, the San
Luis Valley subarea and the Edna Valley subarea (Figure 6-3). Subareas are not to be confused with
subbasins and are defined for this water budget analysis. They are then combined into a single water
budget for the entire Basin. Both subarea water budgets and the Basin water budget are included
herein. Surface water (combined atmospheric, land surface, and stream systems) and groundwater
budgets have been prepared for each subarea and for the Basin. The subarea approach for water
budget calculations follows the approach used by prior investigators (Boyle Engineering, 1991) (DWR,
1997).
As presented in Chapter 4 (Basin Setting), there is a topographic high point in bedrock elevations
underlying the Basin that creates a bedrock high between the San Luis Valley and Edna Valley
subareas (Figure 4-4). This bedrock high partially isolates the deeper portions of the Basin aquifers
(Figure 4-5) and restricts underflow between the two subareas. Figure 6-3 shows the San Luis Valley
and Edna Valley subareas used for the water budget, with the subarea boundary located along Hidden
Springs Road. Note that the boundary between the subareas is shifted slightly to the west of the
bedrock high (Figure 6-3) in order to better correlate with overlying land use. Land use for 2016 (DWR,
2016) is shown on the map to help illustrate differences across the subarea boundary. Immediately
west of the subarea boundary is rural residential land and the County airport. To the east of the
subarea boundary are residential subdivisions, a golf course, and irrigated agricultural lands. The two
subareas of the Basin are hydrologically distinct, as evidenced by the differences in watershed area
(Figure 3-10), sediment thickness (Figure 4-4), and water level hydrographs (Figure 5-11). The
groundwater budgets are also very different between the subareas and separating the two is necessary
to properly characterize the Basin. The two subarea water budgets have also been combined to create
a total Basin water budget.
The San Luis Valley subarea is 6,773 acres (10.6 square miles), and the Edna Valley subarea is 5,948
acres (9.3 square miles), with a total Basin area of 12,271 acres (19.2 square miles). The San Luis
Valley subarea receives surface inflow from a watershed of 28,823 acres (45 square miles) and the
Edna Valley subarea receives surface inflow from a watershed of 10,145 acres (15.9 square miles).
The watershed divide between San Luis Obispo Creek and Pismo Creek is not coincident with the
bedrock high or subarea boundary, and watershed area draining to Davenport Creek in the Edna Valley
subarea is part of the San Luis Obispo Creek watershed (Figure 3-10).
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-3
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-1, Table 6-2, and Table 6-3 present the historical surface water and groundwater budgets for
the San Luis Valley subarea, the Edna Valley subarea, and the Basin total, respectively. Bar graphs
are included in Figure 6-4 through Figure 6-9. The three main water budget tables contain a detailed
accounting of the water budget for the Basin and will be referred to throughout this chapter. A letter key
has been added to provide a visual reference with Figure 6-3.
Note that Figure 6-3 breaks the water budget into four components (atmospheric system, land surface
system, river & stream system, and groundwater system). The atmospheric system transfers
evaporation to precipitation and overlies the other systems. The land surface system is the portion of
the water budget that includes land surface and the unsaturated zone extending to the top of the
groundwater system. The rivers & streams system is the portion of the water budget that includes
rivers, streams, conveyance facilities and diversion ditches, and lakes and reservoirs. The
atmospheric, land surface, and river & streams water budgets for this Basins have been combined into
a single surface water budget. As a result, not all the components in Figure 6-3 have corresponding
budget items listed for the Basin. For example, the runoff and return flow components of the land
surface system into the river & stream system in Figure 6-3 are part of the surface water outflow
component (Labeled “L”).
The six bar graphs are graphical representations of the water budget that allow quick comparisons of
the various budget quantities but are not individually referenced. Figure 6-4, Figure 6-5, and Figure 6-6
illustrate the surface water budget portions of Table 6-1, Table 6-2, and Table 6-3, while Figure 6-7,
Figure 6-8, and Figure 6-9 illustrate the groundwater budget portions of the tables. Water budget
climate, historical time period, methodology, sustainable yield, and overdraft interpretation are also
presented in this chapter.
Some general observations on the water budget are worth noting. First, the surface water budget for
the two subareas shows similar patterns of increasing and decreasing total flow from year to year,
which is expected given similar precipitation with somewhat proportional stream flow. The San Luis
Valley subarea surface water budget is close to double the Edna Valley surface water budget, however.
This is due to a larger watershed area for the San Luis Valley subarea and to the significant volume of
surface water imported from outside of the Basin by the City of San Luis Obispo. Secondly, the
groundwater budget for the Edna Valley subarea shows high groundwater recharge events during all
wet years, which is expected, while the San Luis Obispo shows a more attenuated response, with some
wet years (1993, 2017) providing greater recharge than others. This is because during some wet
years, the aquifers in the San Luis Valley subarea fill up to the point where there is no more available
storage volume, and therefore no additional recharge occurs (also inferred by the relatively flat water
level hydrographs in Figure 5-11). In 1993 and 2017, there was sufficient storage room following
drought to allow greater recharge than during wet years when the subarea was effectively full.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-4 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-1. The Hydrologic Cycle. Source: Department of Water Resources (DWR, 2016)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-5 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-2. Components of the Water Budget. Source: Modified from Department of Water Resources (DWR, 2016)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-6 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-3. Water Budget Subareas
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-7 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Table 6-1. Historical Water –Budget - San Luis Valley Subarea
WATER YEAR SURFACE WATER INFLOW (AF) SURFACE WATER OUTFLOW (AF) GROUNDWATER INFLOW (AF) GROUNDWATER OUTFLOW (AF) CHANGE IN GW STORAGE (AF) PRECIPITATION GW EXTRACTIONS (URBAN) GW EXTRACTIONS (AG) STREAM INFLOW WASTEWATER DISCHARGE LOCAL IMPORTED SUPPLIES TOTAL IN ET OF PRECIPITATION ET OF APPLIED WATER (URBAN) ET OF APPLIED WATER (AG) WETLAND/LAKE RIPARIAN ET SURFACE WATER DELIVERY OFFSET INFILTRATION OF PRECIPITATION INFILT. OF APPLIED WATER (URBAN) INFILT. OF APPLIED WATER (AG) GW-SW INTERACTION STREAM OUTFLOW TOTAL OUT INFILTRATION OF PRECIPITATION INFILT. OF APPLIED WATER (URBAN) INFILT. OF APPLIED WATER (AG) GW-SW INTERACTION SUBSURFACE INFLOW TOTAL IN GW EXTRACTIONS (URBAN) GW EXTRACTIONS (AG) WETLAND DIRECT ET SUBSURFACE OUTFLOW TOTAL OUT KEY A B B C D E F F F F/G H I J J K L I J J K M B B N O P
1987 7,720 410 1,300 6,410 5,520 8,490 29,850 7,450 2,850 1,050 740 5,520 220 530 260 1,090 10,150 29,860 220 530 260 1,090 340 2,440 410 1,300 1,050 120 2,880 -440
1988 10,080 430 1,750 9,660 5,320 8,180 35,420 8,540 2,780 1,410 780 5,320 1,260 520 350 1,640 12,840 35,440 1,260 520 350 1,640 340 4,110 430 1,750 1,320 120 3,620 490
1989 7,850 660 1,580 3,600 4,070 6,020 23,780 7,550 2,180 1,270 380 4,070 250 430 310 610 6,730 23,780 250 430 310 610 340 1,940 660 1,580 1,130 120 3,490 -1,550
1990 6,790 2,180 1,850 2,140 1,970 1,280 16,210 6,660 1,200 1,490 410 1,970 110 290 370 360 3,360 16,220 110 290 370 360 340 1,470 2,180 1,850 1,250 120 5,400 -3,930
1991 9,450 2,350 1,790 5,790 2,520 1,960 23,860 8,250 1,460 1,440 380 2,520 980 320 350 980 7,160 23,840 980 320 350 980 340 2,970 2,350 1,790 1,190 120 5,450 -2,480
1992 11,250 2,240 1,820 11,250 3,070 2,910 32,540 8,590 1,720 1,460 700 3,070 2,200 360 360 1,910 12,160 32,530 2,200 360 360 1,910 340 5,170 2,240 1,820 1,090 120 5,270 -100
1993 15,700 1,030 1,790 17,350 3,630 4,980 44,480 8,640 1,980 1,440 660 3,630 5,950 400 350 1,210 20,210 44,470 5,950 400 350 1,210 340 8,250 1,030 1,790 1,190 120 4,130 4,120
1994 8,620 790 1,690 7,640 3,750 5,400 27,890 7,900 2,030 1,360 740 3,750 580 410 330 1,300 9,480 27,880 580 410 330 1,300 340 2,960 790 1,690 1,090 120 3,690 -730
1995 16,930 660 1,870 26,690 3,780 5,590 55,520 8,630 2,060 1,500 540 3,780 6,070 410 370 1,870 30,300 55,530 6,070 410 370 1,870 340 9,060 660 1,870 1,110 120 3,760 5,300
1996 11,740 740 1,910 11,930 4,210 6,160 36,690 8,530 2,250 1,530 680 4,210 1,820 440 380 830 16,010 36,680 1,820 440 380 830 340 3,810 740 1,910 1,040 120 3,810 0
1997 15,930 780 2,280 17,670 4,400 6,440 47,500 8,580 2,370 1,830 690 4,400 2,690 460 450 530 25,510 47,510 2,690 460 450 530 340 4,470 780 2,280 1,290 120 4,470 0
1998 16,930 680 1,870 26,460 4,150 6,130 56,220 8,580 2,230 1,500 520 4,150 1,770 440 370 790 35,880 56,230 1,770 440 370 790 340 3,710 680 1,870 1,040 120 3,710 0
1999 8,670 660 2,510 7,720 4,350 6,470 30,380 7,870 2,340 2,020 810 4,350 650 450 500 1,310 10,100 30,400 650 450 500 1,310 340 3,250 660 2,510 1,330 120 4,620 -1,370
2000 12,620 670 1,810 13,130 4,410 6,560 39,200 8,530 2,360 1,450 670 4,410 2,950 450 360 920 17,090 39,190 2,950 450 360 920 340 5,020 670 1,810 1,040 120 3,640 1,380
2001 12,470 710 1,740 12,920 4,250 6,270 38,360 8,570 2,290 1,400 670 4,250 1,590 440 340 900 17,900 38,350 1,590 440 340 900 340 3,610 710 1,740 1,040 120 3,610 0
2002 7,510 630 1,850 6,130 4,530 6,340 26,990 7,240 2,000 1,490 770 4,530 220 440 360 1,040 8,900 26,990 220 440 360 1,040 340 2,400 630 1,850 1,140 120 3,740 -1,340
2003 11,630 610 1,470 11,780 4,610 6,300 36,400 8,640 1,860 1,180 680 4,610 2,490 440 290 820 15,390 36,400 2,490 440 290 820 340 4,380 610 1,470 1,040 120 3,240 1,140
2004 8,140 620 1,500 6,990 4,340 6,740 28,330 7,780 2,560 1,200 760 4,340 300 460 290 1,190 9,450 28,330 300 460 290 1,190 340 2,580 620 1,500 1,140 120 3,380 -800
2005 15,120 620 1,370 16,560 5,390 6,250 45,310 8,720 1,040 1,100 600 5,390 1,850 440 270 1,160 24,730 45,300 1,850 440 270 1,160 340 4,060 620 1,370 950 120 3,060 1,000
2006 13,180 610 1,280 6,500 4,950 6,280 32,800 8,710 1,500 1,030 660 4,950 1,580 440 250 450 13,220 32,790 1,580 440 250 450 340 3,060 610 1,280 1,050 120 3,060 0
2007 4,340 610 1,510 6,140 4,200 6,840 23,640 4,330 2,770 1,210 840 4,200 0 480 290 1,040 8,440 23,600 0 480 290 1,040 340 2,150 610 1,510 1,250 120 3,490 -1,340
2008 7,800 520 1,550 11,030 4,010 6,730 31,640 7,540 2,770 1,250 790 4,010 210 470 300 1,870 12,410 31,620 210 470 300 1,870 340 3,190 520 1,550 1,260 120 3,450 -260
2009 5,890 560 1,430 7,670 3,930 6,580 26,060 5,840 2,740 1,150 790 3,930 40 480 280 1,300 9,500 26,050 40 480 280 1,300 340 2,440 560 1,430 1,140 120 3,250 -810
2010 11,980 580 1,160 22,860 4,160 5,860 46,600 8,680 1,850 940 650 4,160 2,590 450 220 1,600 25,460 46,600 2,590 450 220 1,600 340 5,200 580 1,160 960 120 2,820 2,380
2011 16,930 530 1,260 21,360 4,480 5,530 50,090 8,750 1,170 1,020 610 4,480 1,400 430 240 640 31,350 50,090 1,400 430 240 640 340 3,050 530 1,260 1,150 120 3,060 -10
2012 8,470 530 1,420 5,430 3,950 5,770 25,570 7,940 1,910 1,150 770 3,950 430 450 270 920 7,770 25,560 430 450 270 920 340 2,410 530 1,420 1,200 120 3,270 -860
2013 5,290 510 1,790 3,670 4,060 6,330 21,650 5,260 2,320 1,450 430 4,060 30 470 340 620 6,670 21,650 30 470 340 620 340 1,800 510 1,790 1,350 120 3,770 -1,970
2014 5,220 540 1,560 3,270 3,660 6,190 20,440 5,190 2,620 1,260 420 3,660 20 470 300 560 5,940 20,440 20 470 300 560 340 1,690 540 1,560 1,290 120 3,510 -1,820
2015 5,960 400 1,680 1,620 3,420 5,750 18,830 5,900 2,300 1,360 410 3,420 50 440 330 270 4,340 18,820 50 440 330 270 340 1,430 400 1,680 1,270 120 3,470 -2,040
2016 10,150 400 1,690 4,850 3,550 5,490 26,130 8,490 1,920 1,360 730 3,550 1,350 430 330 820 7,130 26,110 1,350 430 330 820 340 3,270 400 1,690 1,170 120 3,380 -110
2017 16,930 400 1,550 18,450 4,400 5,370 47,100 8,730 960 1,250 590 4,400 6,910 440 300 550 22,970 47,100 6,910 440 300 550 340 8,540 400 1,550 1,260 120 3,330 5,210
2018 6,980 400 1,190 2,630 3,330 5,790 20,320 6,870 2,430 970 800 3,330 90 450 230 180 4,970 20,320 90 450 230 180 340 1,290 400 1,190 1,270 120 2,980 -1,690
2019 15,040 400 1,030 16,360 4,360 5,080 42,270 8,800 720 830 630 4,360 4,430 420 200 490 21,400 42,280 4,430 420 200 490 340 5,880 400 1,030 1,070 120 2,620 3,260
Type Year: Dry / Below Normal / Above Normal / Wet
AF = Acre-Feet; KEY = Referenced Components on Figure 6-2
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-8 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Table 6-2. Historical Water –Budget - Edna Valley Subarea
WATER YEAR SURFACE WATER INFLOW (AF) SURFACE WATER OUTFLOW (AF) GROUNDWATER INFLOW (AF) GROUNDWATER OUTFLOW (AF) In some areas it is RMS in other areas RMSs, please ensure consistent use throughout document. PRECIPITATION GW EXTRACTIONS (URBAN) GW EXTRACTIONS (AG) STREAM INFLOW TOTAL IN ET OF PRECIPITATION ET OF APPLIED WATER (URBAN) ET OF APPLIED WATER (AG) RIPARIAN ET INFILTRATION OF PRECIPITATION INFILT. OF APPLIED WATER (URBAN) INFILT. OF APPLIED WATER (AG) GW-SW INTERACTION STREAM OUTFLOW TOTAL OUT INFILTRATION OF PRECIPITATION INFILT. OF APPLIED WATER (URBAN) INFILT. OF APPLIED WATER (AG) GW-SW INTERACTION SUBSURFACE INFLOW TOTAL IN GW EXTRACTIONS (URBAN) GW EXTRACTIONS (AG) SUBSURFACE OUTFLOW TOTAL OUT KEY A B B C F F F F I J J K L I J J K M B B O P
1987 6,780 630 2,450 2,150 12,010 6,610 450 2,000 40 140 190 440 300 1,840 12,010 140 190 440 300 110 1,180 630 2,450 100 3,180 -2,000
1988 8,860 760 2,750 3,240 15,610 7,970 560 2,240 40 660 210 510 450 2,960 15,600 660 210 510 450 110 1,940 760 2,750 100 3,610 -1,670
1989 6,900 640 2,670 1,210 11,420 6,670 470 2,190 20 180 180 480 170 1,070 11,430 180 180 480 170 110 1,120 640 2,670 100 3,410 -2,290
1990 5,960 740 3,040 730 10,470 5,860 530 2,490 20 90 220 550 100 620 10,480 90 220 550 100 110 1,070 740 3,040 100 3,880 -2,810
1991 8,300 760 2,810 1,940 13,810 7,550 530 2,300 20 570 240 510 270 1,840 13,830 570 240 510 270 110 1,700 760 2,810 100 3,670 -1,970
1992 9,880 790 2,810 3,770 17,250 8,030 530 2,300 40 1,460 270 510 530 3,590 17,260 1,460 270 510 530 110 2,880 790 2,810 100 3,700 -820
1993 13,780 840 2,710 5,810 23,140 8,000 570 2,220 40 4,800 290 490 810 5,940 23,160 4,800 290 490 810 110 6,500 840 2,710 100 3,650 2,850
1994 7,570 760 2,640 2,560 13,530 7,050 500 2,170 40 400 270 470 360 2,280 13,540 400 270 470 360 110 1,610 760 2,640 100 3,500 -1,890
1995 14,870 820 2,820 8,930 27,440 7,930 550 2,320 40 5,740 280 500 1,250 8,840 27,450 5,740 280 500 1,250 110 7,880 820 2,820 100 3,740 4,140
1996 10,310 850 3,000 3,990 18,150 7,880 550 2,470 40 1,920 310 530 560 3,900 18,160 1,920 310 530 560 110 3,430 850 3,000 100 3,950 -520
1997 13,990 1,030 3,460 5,910 24,390 7,840 690 2,850 40 5,010 350 610 830 6,190 24,410 5,010 350 610 830 110 6,910 1,030 3,460 100 4,590 2,320
1998 14,870 860 3,000 9,730 28,460 7,790 570 2,480 40 5,750 300 520 1,360 9,660 28,470 5,750 300 520 1,360 110 8,040 860 3,000 100 3,960 4,080
1999 7,620 1,020 3,720 2,590 14,950 6,990 690 3,070 40 470 340 650 360 2,340 14,950 470 340 650 360 110 1,930 1,020 3,720 100 4,840 -2,910
2000 11,080 940 2,700 4,400 19,120 7,710 600 2,230 40 2,650 350 480 620 4,470 19,150 2,650 350 480 620 110 4,210 940 2,700 100 3,740 470
2001 10,950 980 3,320 4,330 19,580 7,670 630 2,750 40 2,550 360 570 610 4,400 19,580 2,550 360 570 610 110 4,200 980 3,320 100 4,400 -200
2002 6,600 960 3,220 2,060 12,840 6,400 630 2,660 40 170 340 570 290 1,760 12,860 170 340 570 290 110 1,480 960 3,220 100 4,280 -2,800
2003 10,220 870 3,030 3,950 18,070 7,600 570 2,500 40 2,000 320 520 550 3,970 18,070 2,000 320 520 550 110 3,500 870 3,030 100 4,000 -500
2004 7,150 970 3,040 2,340 13,500 6,740 630 2,520 40 320 350 530 330 2,070 13,530 320 350 530 330 110 1,640 970 3,040 100 4,110 -2,470
2005 13,280 840 2,870 5,540 22,530 7,610 550 2,370 40 4,450 300 500 780 5,930 22,530 4,450 300 500 780 110 6,140 840 2,870 100 3,810 2,330
2006 11,570 900 3,040 2,180 17,690 7,580 590 2,520 40 3,100 320 530 310 2,730 17,720 3,100 320 530 310 110 4,370 900 3,040 100 4,040 330
2007 3,810 1,180 3,830 2,160 10,980 3,800 770 3,170 40 0 430 660 300 1,820 10,990 0 430 660 300 110 1,500 1,180 3,830 100 5,110 -3,610
2008 6,850 1,210 3,750 3,750 15,560 6,580 780 3,100 40 220 440 650 520 3,230 15,560 220 440 650 520 110 1,940 1,210 3,750 100 5,060 -3,120
2009 5,170 950 3,660 2,740 12,520 5,100 650 3,040 40 50 310 620 380 2,330 12,520 50 310 620 380 110 1,470 950 3,660 100 4,710 -3,240
2010 10,520 820 3,360 7,490 22,190 7,560 550 2,790 40 2,260 270 570 1,050 7,100 22,190 2,260 270 570 1,050 110 4,260 820 3,360 100 4,280 -20
2011 14,870 840 3,330 7,840 26,880 7,550 580 2,760 40 5,760 270 570 1,100 8,260 26,890 5,760 270 570 1,100 110 7,810 840 3,330 100 4,270 3,540
2012 7,440 940 3,560 1,810 13,750 6,830 650 2,950 40 450 290 610 250 1,660 13,730 450 290 610 250 110 1,710 940 3,560 100 4,600 -2,890
2013 4,640 1,040 3,780 1,260 10,720 4,600 740 3,120 20 40 310 660 180 1,070 10,740 40 310 660 180 110 1,300 1,040 3,780 100 4,920 -3,620
2014 4,590 960 3,580 1,120 10,250 4,550 680 2,960 20 30 280 620 160 950 10,250 30 280 620 160 110 1,200 960 3,580 100 4,640 -3,440
2015 5,230 880 4,230 490 10,830 5,160 650 3,500 20 60 230 720 70 410 10,820 60 230 720 70 110 1,190 880 4,230 100 5,210 -4,020
2016 8,920 790 3,200 1,560 14,470 7,550 580 2,680 40 980 220 530 220 1,680 14,480 980 220 530 220 110 2,060 790 3,200 100 4,090 -2,030
2017 14,870 850 3,640 6,240 25,600 7,570 640 3,030 40 5,730 220 610 870 6,890 25,600 5,730 220 610 870 110 7,540 850 3,640 100 4,590 2,950
2018 6,130 880 3,550 650 11,210 6,020 650 2,960 40 90 240 590 90 540 11,220 90 240 590 90 110 1,120 880 3,550 100 4,530 -3,410
2019 13,210 770 3,350 5,480 22,810 7,630 580 2,800 40 4,370 210 550 770 5,870 22,820 4,370 210 550 770 110 6,010 770 3,350 100 4,220 1,790
Type Year: Dry / Below Normal / Above Normal / Wet
AF = Acre-Feet; KEY = Referenced Components on Figure 6-3
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-9 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Table 6-3. Historical Water Budget - San Luis Obispo Valley Groundwater Basin
WATER YEAR SURFACE WATER INFLOW (GW) SURFACE WATER OUTFLOW (GW) GROUNDWATER INFLOW (GW) GROUNDWATER OUTFLOW (GW) CHANGE IN GW STORAGE (AF) PRECIPITATION GW EXTRACTIONS (URBAN) GW EXTRACTIONS (AG) STREAM INFLOW WASTEWATER DISCHARGE LOCAL IMPORTED SUPPLIES TOTAL IN ET OF PRECIPITATION ET OF APPLIED WATER (URBAN) ET OF APPLIED WATER (AG) WETLAND/LAKE RIPARIAN ET SURFACE WATER DELIVERIES INFILTRATION OF PRECIPITATION INFILT. OF APPLIED WATER (URBAN) INFILT. OF APPLIED WATER (AG) GW-SW INTERACTION STREAM OUTFLOW TOTAL OUT INFLITRATION OF PRECIPITATION INFILT. OF APPLIED WATER (URBAN) INFILT. OF APPLIED WATER(AG) GW-SW INTERACTION SUBSURFACE INFLOW TOTAL IN GW EXTRACTIONS (URBAN) GW EXTRACTIONS (AG) WETLAND DIRECT ET SUBSURFACE OUTFLOW TOTAL OUT KEY A B B C D E F F F F/G H I J J K L I J J K M B B N O P
1987 14,500 1,040 3,750 8,560 5,520 8,490 41,860 14,060 3,300 3,050 780 5,520 360 720 700 1,390 11,990 41,870 360 720 700 1,390 450 3,620 1,040 3,750 1,050 220 6,060 -2,440
1988 18,940 1,190 4,500 12,900 5,320 8,180 51,030 16,510 3,340 3,650 820 5,320 1,920 730 860 2,090 15,800 51,040 1,920 730 860 2,090 450 6,050 1,190 4,500 1,320 220 7,230 -1,180
1989 14,750 1,300 4,250 4,810 4,070 6,020 35,200 14,220 2,650 3,460 400 4,070 430 610 790 780 7,800 35,210 430 610 790 780 450 3,060 1,300 4,250 1,130 220 6,900 -3,840
1990 12,750 2,920 4,890 2,870 1,970 1,280 26,680 12,520 1,730 3,980 430 1,970 200 510 920 460 3,980 26,700 200 510 920 460 450 2,540 2,920 4,890 1,250 220 9,280 -6,740
1991 17,750 3,110 4,600 7,730 2,520 1,960 37,670 15,800 1,990 3,740 400 2,520 1,550 560 860 1,250 9,000 37,670 1,550 560 860 1,250 450 4,670 3,110 4,600 1,190 220 9,120 -4,450
1992 21,130 3,030 4,630 15,020 3,070 2,910 49,790 16,620 2,250 3,760 740 3,070 3,660 630 870 2,440 15,750 49,790 3,660 630 870 2,440 450 8,050 3,030 4,630 1,090 220 8,970 -920
1993 29,480 1,870 4,500 23,160 3,630 4,980 67,620 16,640 2,550 3,660 700 3,630 10,750 690 840 2,020 26,150 67,630 10,750 690 840 2,020 450 14,750 1,870 4,500 1,190 220 7,780 6,970
1994 16,190 1,550 4,330 10,200 3,750 5,400 41,420 14,950 2,530 3,530 780 3,750 980 680 800 1,660 11,760 41,420 980 680 800 1,660 450 4,570 1,550 4,330 1,090 220 7,190 -2,620
1995 31,800 1,480 4,690 35,620 3,780 5,590 82,960 16,560 2,610 3,820 580 3,780 11,810 690 870 3,120 39,140 82,980 11,810 690 870 3,120 450 16,940 1,480 4,690 1,110 220 7,500 9,440
1996 22,050 1,590 4,910 15,920 4,210 6,160 54,840 16,410 2,800 4,000 720 4,210 3,740 750 910 1,390 19,910 54,840 3,740 750 910 1,390 450 7,240 1,590 4,910 1,040 220 7,760 -520
1997 29,920 1,810 5,740 23,580 4,400 6,440 71,890 16,420 3,060 4,680 730 4,400 7,700 810 1,060 1,360 31,700 71,920 7,700 810 1,060 1,360 450 11,380 1,810 5,740 1,290 220 9,060 2,320
1998 31,800 1,540 4,870 36,190 4,150 6,130 84,680 16,370 2,800 3,980 560 4,150 7,520 740 890 2,150 45,540 84,700 7,520 740 890 2,150 450 11,750 1,540 4,870 1,040 220 7,670 4,080
1999 16,290 1,680 6,230 10,310 4,350 6,470 45,330 14,860 3,030 5,090 850 4,350 1,120 790 1,150 1,670 12,440 45,350 1,120 790 1,150 1,670 450 5,180 1,680 6,230 1,330 220 9,460 -4,280
2000 23,700 1,610 4,510 17,530 4,410 6,560 58,320 16,240 2,960 3,680 710 4,410 5,600 800 840 1,540 21,560 58,340 5,600 800 840 1,540 450 9,230 1,610 4,510 1,040 220 7,380 1,850
2001 23,420 1,690 5,060 17,250 4,250 6,270 57,940 16,240 2,920 4,150 710 4,250 4,140 800 910 1,510 22,300 57,930 4,140 800 910 1,510 450 7,810 1,690 5,060 1,040 220 8,010 -200
2002 14,110 1,590 5,070 8,190 4,530 6,340 39,830 13,640 2,630 4,150 810 4,530 390 780 930 1,330 10,660 39,850 390 780 930 1,330 450 3,880 1,590 5,070 1,140 220 8,020 -4,140
2003 21,850 1,480 4,500 15,730 4,610 6,300 54,470 16,240 2,430 3,680 720 4,610 4,490 760 810 1,370 19,360 54,470 4,490 760 810 1,370 450 7,880 1,480 4,500 1,040 220 7,240 640
2004 15,290 1,590 4,540 9,330 4,340 6,740 41,830 14,520 3,190 3,720 800 4,340 620 810 820 1,520 11,520 41,860 620 810 820 1,520 450 4,220 1,590 4,540 1,140 220 7,490 -3,270
2005 28,400 1,460 4,240 22,100 5,390 6,250 67,840 16,330 1,590 3,470 640 5,390 6,300 740 770 1,940 30,660 67,830 6,300 740 770 1,940 450 10,200 1,460 4,240 950 220 6,870 3,330
2006 24,750 1,510 4,320 8,680 4,950 6,280 50,490 16,290 2,090 3,550 700 4,950 4,680 760 780 760 15,950 50,510 4,680 760 780 760 450 7,430 1,510 4,320 1,050 220 7,100 330
2007 8,150 1,790 5,340 8,300 4,200 6,840 34,620 8,130 3,540 4,380 880 4,200 0 910 950 1,340 10,260 34,590 0 910 950 1,340 450 3,650 1,790 5,340 1,250 220 8,600 -4,950
2008 14,650 1,730 5,300 14,780 4,010 6,730 47,200 14,120 3,550 4,350 830 4,010 430 910 950 2,390 15,640 47,180 430 910 950 2,390 450 5,130 1,730 5,300 1,260 220 8,510 -3,380
2009 11,060 1,510 5,090 10,410 3,930 6,580 38,580 10,940 3,390 4,190 830 3,930 90 790 900 1,680 11,830 38,570 90 790 900 1,680 450 3,910 1,510 5,090 1,140 220 7,960 -4,050
2010 22,500 1,400 4,520 30,350 4,160 5,860 68,790 16,240 2,400 3,730 690 4,160 4,850 720 790 2,650 32,560 68,790 4,850 720 790 2,650 450 9,460 1,400 4,520 960 220 7,100 2,360
2011 31,800 1,370 4,590 29,200 4,480 5,530 76,970 16,300 1,750 3,780 650 4,480 7,160 700 810 1,740 39,610 76,980 7,160 700 810 1,740 450 10,860 1,370 4,590 1,150 220 7,330 3,530
2012 15,910 1,470 4,980 7,240 3,950 5,770 39,320 14,770 2,560 4,100 810 3,950 880 740 880 1,170 9,430 39,290 880 740 880 1,170 450 4,120 1,470 4,980 1,200 220 7,870 -3,750
2013 9,930 1,550 5,570 4,930 4,060 6,330 32,370 9,860 3,060 4,570 450 4,060 70 780 1,000 800 7,740 32,390 70 780 1,000 800 450 3,100 1,550 5,570 1,350 220 8,690 -5,590
2014 9,810 1,500 5,140 4,390 3,660 6,190 30,690 9,740 3,300 4,220 440 3,660 50 750 920 720 6,890 30,690 50 750 920 720 450 2,890 1,500 5,140 1,290 220 8,150 -5,260
2015 11,190 1,280 5,910 2,110 3,420 5,750 29,660 11,060 2,950 4,860 430 3,420 110 670 1,050 340 4,750 29,640 110 670 1,050 340 450 2,620 1,280 5,910 1,270 220 8,680 -6,060
2016 19,070 1,190 4,890 6,410 3,550 5,490 40,600 16,040 2,500 4,040 770 3,550 2,330 650 860 1,040 8,810 40,590 2,330 650 860 1,040 450 5,330 1,190 4,890 1,170 220 7,470 -2,140
2017 31,800 1,250 5,190 24,690 4,400 5,370 72,700 16,300 1,600 4,280 630 4,400 12,640 660 910 1,420 29,860 72,700 12,640 660 910 1,420 450 16,080 1,250 5,190 1,260 220 7,920 8,160
2018 13,110 1,280 4,740 3,280 3,330 5,790 31,530 12,890 3,080 3,930 840 3,330 180 690 820 270 5,510 31,540 180 690 820 270 450 2,410 1,280 4,740 1,270 220 7,510 -5,100
2019 28,250 1,170 4,380 21,840 4,360 5,080 65,080 16,430 1,300 3,630 670 4,360 8,800 630 750 1,260 27,270 65,100 8,800 630 750 1,260 450 11,890 1,170 4,380 1,070 220 6,840 5,050
Type Year: Dry / Below Normal / Above Normal / Wet
AF = Acre-Feet; KEY = Referenced Components on Figure 6-3
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-10 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-4. Surface Water Budget – San Luis Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-11 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-5. Surface Water Budget – Edna Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-12 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-6. Surface Water Budget – Basin Total
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-13 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-7. Groundwater Budget – San Luis Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-14 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-8. Groundwater Budget – Edna Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-15 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-9. Groundwater Budget - Basin Total
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-16
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.2. Climate
Climate is one of the principal measures of water supply conditions and is used for hydrologic base
period definition and for developing evapotranspiration estimates. The main component of climate
monitoring in the Basin is rainfall, with records at the Cal Poly NOAA Station (formerly Cal Poly #1)
beginning in the 1870-71 rainfall year. Rainfall is used in the water budget for establishing the
hydrologic base period needed for representing long-term water supply conditions.
Another climate parameter used in the water budget is evapotranspiration. Evapotranspiration is
calculated from a combination of monitored parameters, such as air temperature, wind speed, solar
radiation, vapor pressure, and relative humidity. These parameters, along with precipitation, have been
monitored at CIMIS Station #52 (San Luis Obispo – Cal Poly) since 1986. The water budget uses crop
evapotranspiration for estimating the applied irrigation requirements for crops (Section 6.3.4.2). Cal
Poly, the San Luis Valley, and the Edna Valley are all within DWR reference evapotranspiration Zone 6,
which is one of 18 climate zones in California based on long-term monthly average reference
evapotranspiration (CIMS, 2019).
6.2.1. Historical Climate/Base Period
The historical rainfall record at the Cal Poly NOAA Station has been used to define a period of years,
referred to as a base period, which represents long-term hydrologic conditions. As described by DWR
(DWR, 2002):
The base period should be representative of long-term hydrologic conditions, encompassing
dry, wet, and average years of precipitation. It must be contained in the historical record and
should include recent cultural conditions to assist in determining projected Basin operations.
To minimize the amount of water in transit in the zone of aeration, the beginning and end of
the base period should be preceded by comparatively similar rainfall quantities.
The historical rainfall record for the Cal Poly NOAA Station, which is the longest record in the San Luis
Obispo area, was presented in Figure 3-11. The water year in San Luis Obispo County for rainfall runs
from July 1 through June 30 (also referred to as rainfall year), while other hydrologic data is reported
from October 1 through September 30 (San Luis Obispo County Department of Public Works, 2005).
These conventions are maintained for the water budget, and water years are referenced herein based
on the ending year.
The hydrologic base period selected to represent historical climatic conditions for the Basin
encompasses the years 1987 through 2019 (33 years). Average precipitation at the Cal Poly NOAA
gage over this base period was 21.76 inches, compared to the long-term average of 21.95 inches, and
included wet, average, and dry periods (Figure 6-10). These periods are visually defined by the
movement of the cumulative departure from mean precipitation curve, which declines over dry periods,
is flat through average periods, and rises over wet periods.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-17 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-10. 1987-2019 Historical Base Period Climate
-100
-50
0
50
100
150
0
10
20
30
40
50
60
1985 1995 2005 2015 Cumulative Dparture from Mean Annual RainfallAnnual Rainfall (in)Water Year
Historical Precipitation -Cal Poly Rain Gage
Rainfall (in)Overall Average Rainfall Cumulative Departure From Mean Rainfall
Type of Rainfall Year:Dry Below Normal Above Normal Wet
DRY WET AVERAGE DRY WET DRY WET
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-18
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Water year types for this water budget have been developed and classified based on annual
precipitation as a percentage of the previous 30-year average precipitation. Each July 1 through June
30 rainfall year of the historical base period was given a ranking of 1 (wettest) through 30 (driest) based
on a comparison to a 30-year (rolling) data set. The minimum precipitation threshold for wet type
years was assigned based on the average for the 10th ranked year (26.3 inches). The maximum
precipitation threshold for dry type years was assigned based on the average for the 21st ranked year
(16.8 inches). Below normal (from 16.8 to less than 20.5 inches) represents the 16th through 20th
ranked years, while above normal (from 20.5 to 26.3 inches) represents the 10th through 15th ranked
years. Note that the division between below normal and above normal rainfall (20.5 inches) is less than
the average over the base period (21.76 inches) because there are more below average rainfall years
than above average years. The water year types were developed from Cal Poly NOAA rainfall records,
with one exception. The exception is the 2006 rainfall year, which would be classified as dry based on
15.31 inches reported at Cal Poly NOAA, but which is considered above normal when reviewing other
local rain gages, including the Gas Company rain gage (23.35 inches in 2006).
The base period includes recent cultural conditions, such as expanded recycled water use by the City
and water conservation by Basin users in response to the recent drought period. Differences between
water in transit in the vadose zone (deep percolation of precipitation and stream seepage) are minimal,
based on comparing the two rainfall years leading up to the beginning and ending of the base period.
The 1985 and 1986 rainfall years leading in the base period have 14.77 inches and 29.43 inches,
respectively, compared to 14.34 and 29.48 inches of rainfall at the end of the base period in 2018 and
2019 (Figure 6-10).
There are other rainfall gages in the Basin (Table 3-5 and Figure 3-10), and an isohyetal map of
average annual rainfall is shown in Figure 4-3. The average annual precipitation across the Basin
between 1981 and 2010 was approximately 19 inches (Figure 4-3), compared to the Cal Poly NOAA
rainfall gage, which averaged 23.03 inches over that same period.
Although the water budget uses the Cal Poly NOAA gage (formerly Cal Poly #1) to identify the historical
base period and water year types due to the extensive period of record, the Gas Company rain gage is
used in water budget calculations that involve precipitation volumes to account for the difference
between rainfall at the Cal Poly NOAA gage and the Basin. A correlation between rainfall data at the
Gas Company and Cal Poly NOAA gages was performed to estimate rainfall prior to 2006 for the
historical water budget (Figure 6-11). Based on linear regression using data recorded between 2006
and 2019, rainfall at the Gas Company gage is approximately 90 percent of rainfall at the Cal Poly
NOAA gage. No precipitation data was recorded for the Gas Company rain gage prior to 2006, and the
90 percent correlation was used to estimate precipitation at the gage between 1987 and 2005 to
complete the historical base period. Climate data from CIMIS Station #52 (located within same
enclosure as the Cal Poly NOAA rain gage) has been used for evapotranspiration and applied
agricultural water estimates.
Table 6-4 presents the annual rainfall for the historical water budget. Average annual rainfall within the
Basin over the historical base period is estimated to be 19.6 inches. This average closely matches the
estimated value for average rainfall across the Basin on the 30-year isohyetal map (Figure 4-3).
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-19 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-11. Rainfall Correlation Cal Poly NOAA vs. Gas Company
y = 0.902x
R² = 0.9625
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35 40Annual Rainfall at Gas Company (inches)Annual Rainfall at Cal Poly NOAA (inches)
Rainfall Correlation 2006-2019
Cal Poly NOAA vs. Gas Company
Water Year data Linear (Water Year data)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-20
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-4. Historical Base Period Rainfall
YEAR TYPE
CAL POLY NOAA GAS COMPANY
RAINFALL (IN.) RAINFALL (IN.)
1987 Dry 15.19 13.67
1988 Below Normal 19.85 17.87
1989 Dry 15.46 13.91
1990 Dry 13.36 12.02
1991 Below Normal 18.6 16.74
1992 Above Normal 22.14 19.93
1993 Wet 30.9 27.81
1994 Below Normal 16.96 15.26
1995 Wet 44.31 39.88
1996 Above Normal 23.11 20.8
1997 Wet 31.36 28.22
1998 Wet 43.98 39.58
1999 Below Normal 17.07 15.36
2000 Above Normal 24.84 22.36
2001 Above Normal 24.54 22.09
2002 Dry 14.79 13.31
2003 Above Normal 22.9 20.61
2004 Dry 16.02 14.42
2005 Wet 29.76 26.78
2006 Above Normal* 15.31 23.35
2007 Dry 11.03 7.68
2008 Below Normal 19.88 13.82
2009 Dry 10.35 10.43
2010 Wet 31.73 21.22
2011 Wet 31.5 32.4
2012 Dry 14.62 15
2013 Dry 14.33 9.37
2014 Dry 10.61 9.25
2015 Dry 11.52 10.55
2016 Below Normal 19.47 17.99
2017 Wet 38.93 37.23
2018 Dry 14.34 12.37
2019 Wet 29.48 26.65
AVERAGE 21.8 19.6
Gas Company Estimates in blue (approximately 90% of Cal Poly)
*2006 type year based on Gas Company gage reporting
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-21
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.3. Water Budget Data Sources
The following sources and types of data have been used for the water budget:
• Hydrogeologic and geologic studies and maps
• Groundwater monitoring reports
• County stream flow gages
• County and NOAA precipitation stations
• PRISM 30-year normal dataset (1981-2010)
• CIMIS weather station data
• Aerial Imagery
• County water level monitoring program
• San Luis Obispo City, County and DWR land use data and planning documentation
• County Ag commissioner’s office data sets
• County Water Master Plan
• Geotracker Groundwater Information System
• Stakeholder supplied information
• Environmental Impact Reports
• Water rights filings
• SRWQCB Drinking Water Division Water systems
• Wastewater discharge reports
6.4. Historical Water Budget
In accordance with GSP regulations, the historical water budget shall quantify the following,
either through direct measurement or estimates based on data (reference to location of data in
Chapter 6 also listed):
1. Total surface water entering and leaving a Basin by water source type (Table 6-3).
2. Inflow to the groundwater system by water source type, including subsurface groundwater inflow
and infiltration of precipitation, applied water, and surface water systems, such as lakes, streams,
rivers, canals, springs, and conveyance systems (Table 6-3).
3. Outflows from the groundwater system by water use sector, including evapotranspiration,
groundwater extraction, groundwater discharge to surface water sources, and subsurface
groundwater outflow (Table 6-3).
4. The change in annual volume of groundwater in storage between seasonal high conditions (Table
6-3).
5. If overdraft occurs, as defined in Bulletin 118, the water budget shall include a quantification of
overdraft over a period of years during which water year and water supply conditions approximate
average conditions (Section 6.4.9).
6. The water year type associated with the annual supply, demand, and change in groundwater stored
(Table 6-3).
7. An estimate of sustainable yield for the Basin (Section 6.4.8.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-22
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.4.1. Historical Time Period
The time period over which the historical water budget is estimated is the hydrologic base period from
1987-2019 (33 years). Groundwater storage calculations using the specific yield method were
performed for 1986, 1990, 1995, 1998, 2005, 2011, 2014, and 2019. These years include the
beginning and ending years in the base period, along with sufficient intervening years to characterize
change in storage trends through the base period.
6.4.2. Historical Land Use
Land use is one of the primary data sets used in developing a water budget. Several types of land
use/land cover in the basin have been used to estimate components of the water budget. For example,
the acreages of various crops are multiplied by their respective water use factors to estimate
agricultural groundwater extractions and acreages of various land covers are multiplied by empirical
correlations to estimate their respective evapotranspiration and percolation of precipitation.
The land uses/land covers including the following:
• Irrigated Agriculture
− Citrus
− Deciduous
− Pasture
− Vegetable
− Vineyard
• Native Vegetation
− Brush, trees, native grasses
− Wetlands/open water
• Urban/Suburban
− Developed (City, subdivisions)
− Open space (parks, empty lots)
− Turf (golf courses, play fields)
Irrigated Agriculture
Irrigated crop acreage was estimated from aerial imagery of the Basin for the following years: 1987,
1994, 1999, 2003, 2005, 2007, 2009, 2010, and 2011. San Luis Obispo County land use data was
used for crop acreage from 2013 to 2018. DWR land use surveys for 1985, 1995, and 2014 were also
reviewed during the interpretation of aerial imagery. Figure 6-12 shows an example of the County
irrigated crop data set for 2016. Some of the irrigated acreage is located outside of the Basin boundary,
but it is assumed that these areas are supplied by wells located within the Basin.
Irrigated acreage for years in the historical base period without aerial imagery, surveys, or County data
were estimated from the nearest available year with data. Acreages for irrigated crops, estimated from
aerial imagery and County datasets within the historical base period are shown in Table 6-5.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-23
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-5. Irrigated Agriculture Acreages
CROP TYPE 1987 1994 1999 2003 2005 2007 2009 2010 2011 2013 2014 2015 2016 2017 2018
SAN LUIS VALLEY SUBAREA (ACRES)
Citrus 26 26 30 51 49 49 49 49 49 45 44 44 44 46 46
Deciduous 12 12 12 12 12 12 12 12 12 67 21 17 17 17 17
Pasture 33 22 27 28 28 28 28 28 28 28 37 37 53 28 28
Vegetable 594 766 880 647 592 487 526 494 495 488 490 532 593 492 363
Vineyard 0 5 6 6 8 58 58 58 58 92 86 86 86 86 86
Subtotal 665 831 955 744 689 634 673 641 642 720 678 716 793 669 540
EDNA VALLEY SUBAREA (ACRES)
Citrus 12 6 47 49 51 51 53 49 105 105 111 111 191 191 210
Deciduous 0 0 0 0 0 0 0 0 0 0 2 2 2 4 3
Pasture 138 19 19 19 19 19 19 19 19 16 19 19 15 14 13
Vegetable 533 703 685 686 646 699 663 679 647 671 670 691 394 505 453
Vineyard 1,180 1,344 1,900 2,252 2,297 2,377 2,377 2,372 2,380 2,423 2,419 2,419 2,454 2,415 2,323
Subtotal 1,863 2,072 2,651 3,006 3,013 3,146 3,112 3,119 3,151 3,215 3,221 3,242 3,056 3,129 3,002
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-24 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-12. San Luis Obispo Valley Basin Irrigated Crops 2016
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-25
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Native Vegetation and Urban Areas
Native vegetation acreages were compiled using data sets from the National Land Cover Database
(NLCD), which is derived primarily from satellite imagery. The years for which NLCD coverage is
available are 2001, 2004, 2006, 2008, 2011, 2013, and 2016. Adjustments to the acreages in the
NLCD data were performed to reconcile with the agricultural acreages and urban turf areas (golf
course, play fields) compiled using the aerial imagery and crop survey data set. Where the NLCD data
sets showed less agricultural acreage than the aerial imagery, the native vegetation (brush, trees,
grassland) acreage was reduced so the total basin acreage remained constant. The estimated
acreages for native vegetation and urban areas, along with irrigated agriculture interpolated from Table
6-5, are presented in Table 6-6 below.
Table 6-6. Land Cover Acreages
LAND COVER 2001 2004 2006 2008 2011 2013 2016
SAN LUIS VALLEY SUBAREA (ACRES)
Native - brush, trees, grassland 2,315 2,450 2,482 2,466 2,386 2,315 2,203
Native - wetlands/open water 566 566 573 571 569 569 575
Urban - Developed 2,150 2,142 2,219 2,219 2,325 2,312 2,353
Urban - Open Space 870 875 841 841 829 835 825
Urban - Turf 23 23 23 23 23 23 23
Irrigated Agriculture 849 716 636 653 642 720 793
Subarea Total 6,773 6,773 6,773 6,773 6,773 6,773 6,773
EDNA VALLEY SUBAREA (ACRES)
Native - brush, trees, grassland 2,659 2,473 2,406 2,356 2,333 2,266 2,423
Native - wetlands/open water 13 17 13 13 15 13 13
Urban - Developed 230 230 232 232 232 235 237
Urban - Open Space 77 77 77 77 77 78 79
Urban - Turf 141 141 141 141 141 141 141
Irrigated Agriculture 2,829 3,010 3,079 3,129 3,150 3,215 3,056
SUBAREA TOTAL 5,948 5,948 5,948 5,948 5,948 5,948 5,948
6.4.3. Historical Surface Water Budget
The surface water system is represented by water at the land surface within the boundaries of the
Basin. Surface water systems for the water budget include streams and Laguna Lake.
6.4.3.1. Components of Surface Water Inflow
The surface water budget includes the following sources of inflow:
• Local Supplies
− Precipitation
− Groundwater extractions
− Stream inflow at Basin boundary
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-26
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
− Groundwater-Surface Water Interactions
− Treated wastewater discharge into streams
• Local Imported Supplies
o Nacimiento Project Water
o Salinas Reservoir Water
o Whale Rock Reservoir Water
Precipitation
Precipitation occurs as rainfall. The annual volume of rainfall within the Basin has been estimated by
multiplying the rainfall year totals in Table 6-4 by each Basin subarea. Rainfall volumes falling within
the Basin boundary are shown as precipitation in the surface water inflow budget of Table 6-1, Table
6-2, and Table 6-3.
Groundwater Extractions
Groundwater extractions are included in the surface water budget as inflow because after extraction
groundwater is distributed and applied at land surface. The surface water budget includes the land
surface system and rivers & streams system (Figure 6-2). These extractions are the divided into Urban
and Agricultural water use sectors and match the groundwater extraction outflow values from the
groundwater budget. Details on data collection and groundwater pumping estimates are provided in
the Section 6.4.5 Historical Groundwater Budget.
Stream Inflow at Basin Boundary
Inflow along stream channels at the Basin boundary has been estimated based on paired watershed
methodology. The total watershed area drained by the Basin was divided into 15 sub-watershed areas,
one of which was the subarea drained by San Luis Obispo Creek upstream of the Andrews Street gage
(sub-watershed 1, Figure 6-13). Flow from 2007 through 2018 at the Andrews Street gage was
reconstructed using stage records and a stage-discharge curve. The resulting annual flows were then
processed using a watershed area factor and an isohyetal factor to estimate annual flows for each of
the other 14 subareas. The watershed area factor was the ratio of the watershed area for which flow
was being estimated to the Andrews Street gage watershed area. The isohyetal factor addressed
differences between the average annual rainfall across each of the sub-watersheds being compared
(Figure 6-13) and consisted of the ratio of average annual precipitation over 15 inches between sub-
watersheds. Correlation between rainfall and runoff for the paired watersheds is shown in Figure 6-14.
A drought period adjustment was also made for 1989-1991 inflow estimates (Figure 6-14) consisting of
3,000 AFY less inflow for the San Luis Valley subarea and 1,000 AFY less inflow for the Edna Valley
subarea. Once these factors were applied, the estimated stream flow entering the respective SLO
subarea watershed and Edna Valley subarea watershed were totaled.
Stream inflow on the West Coral de Piedra sub-watershed 5 (Figure 6-13) was reduced to account for
surface water diversions. There is a permitted reservoir where surface water diversion is utilized mainly
for agricultural irrigation (SWRCB, 1990). The stream inflow adjustment consisted of correlating the
total reported diversions from Statements of Diversion and Use between 2010 and 2018 with annual
precipitation and applying the correlation to other years in the base period (the r-squared value of the
correlation is 0.71). Reported annual surface water diversions ranged from 14 acre-feet to 900 acre-
feet, with average annual diversion over the base period estimated at 350 acre-feet per year (AFY),
including estimated reservoir evaporation which was added to the diversion. The resulting estimated
stream inflow estimates for the historical base period are shown in the surface water budget of Table
6-1, Table 6-2, and Table 6-3.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-27 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-13. Basin Sub-watershed Areas and Isohyetals
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-28 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-14. Runoff vs Rainfall Correlation for Subareas
y = 729.88x -4367.7
R² = 0.7979
y = 258.37x -1546.1
R² = 0.7979
0
5000
10000
15000
20000
25000
30000
0 5 10 15 20 25 30 35 40 45Annual Runoff (acre-feet)Annual rainfall (inches)
Runoff vs Rainfall 2007-2019
SLO Subarea Edna Subarea Linear (SLO Subarea)Linear (Edna Subarea)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-29
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Groundwater-Surface Water Interaction (Net)
Groundwater-surface water interactions take place primarily along stream channels and lake/wetland
areas. When groundwater is rising into streams (gaining reaches of a stream), the interaction is a
surface water budget inflow and a groundwater budget outflow. Conversely, when stream flow is
percolating to groundwater (losing reaches of a stream), the interaction is a surface water budget
outflow and groundwater budget inflow. As discussed in the hydrograph analysis presented in Section
5.7, San Luis Obispo Creek is assumed to be a gaining stream through much of the Basin, while Edna
Valley streams are typically losing reaches, or seasonally disconnected from the aquifer. The Basin-
wide water budget has combined the gaining and losing stream reaches into single (net) term, the
result of which are net losing streams in the Basin which is an outflow component of the surface water
budget and inflow component of the groundwater budget. Net groundwater-surface water interaction
was estimated by adjusting the percent of stream inflow that recharges groundwater while optimizing
the water balance. The optimization consisted of minimizing the sum of squares of the residual error
between the calculated change in storage and measured change in storage.
Treated wastewater discharge to streams
The City of San Luis Obispo discharges treated wastewater into San Luis Obispo Creek. Available
records of wastewater treatment plant discharges have been compiled by water year. Daily discharge
records provided by the City were compiled for water years 2001-2019. For water years 1987-2000,
treated wastewater discharges were estimated as a nominal 65 percent of total City water deliveries,
based on the average ratio of annual wastewater flows to water deliveries in the years 2001-2019. The
treated wastewater discharges to San Luis Obispo creek are presented in the surface water budget of
Table 6-1.
Local Imported Supplies
The City of San Luis Obispo imports water from three reservoirs. Surface water deliveries from Salinas
and Whale Rock reservoirs occurred through the historical base period, while Nacimiento reservoir
water deliveries to the City began in 2011. Surface water reservoirs have historically provided most of
the water supply used by the City. Local imported water supplies are based on City records and Boyle
(Boyle Engineering, 1991). Local imported supplies are presented in the surface water budget of Table
6-1.
Cal Poly imports surface water and also pumps groundwater for agricultural irrigation. Fields overlying
and adjacent to the Basin are typically irrigated with groundwater, while imported surface water is
generally used for irrigation outside of the Basin boundary. Therefore, only the local imported supplies
used for potable water deliveries by the City have been accounted for in the GSP water budgets.
6.4.4. Components of Surface Water Outflow
The surface water budget includes the following sources of outflow:
• Evapotranspiration of Precipitation
• Evapotranspiration of Applied Water
• Infiltration of Precipitation
• Infiltration of Applied Water
• Surface Water Deliveries Offset
• Wetland/Lake ET
• Groundwater-Surface Water Interaction
• Stream outflow (runoff)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-30
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Evapotranspiration of Precipitation
The fate of precipitation that falls within the Basin boundaries can be divided into three components:
evapotranspiration, infiltration, and runoff. Of these three, infiltration has the greatest influence on the
groundwater budget and ultimately, the Basin sustainable yield. Therefore, the approach to estimating
the fate of precipitation uses a methodology focused primarily on infiltration, but from which the other
two components may also be estimated. This methodology is based on work by Blaney (Blaney, 1933)
(Blaney, 1963), and which has been used for other analytical water budgets in major studies of central
coast Basins (DWR, 2002) and (Fugro West and Cleath & Associates, 2002).
Evapotranspiration is the evaporation of water from surfaces and the transpiration of water from plants.
The first seasonal rains falling on the Basin are mostly evaporated directly from surfaces (vegetative
canopy, soil, urban area hardscapes) and used to replenish soil moisture deficits that accumulate
during the dry season. For the Arroyo Grande – Nipomo Mesa area of the Santa Maria groundwater
Basin, DWR assumed that precipitation could begin to infiltrate to groundwater (deep percolate) only
after 11 inches of annual precipitation had fallen in urban and agricultural irrigation areas, and when 17
inches of rainfall had fallen in areas of native vegetation. In the Paso Robles groundwater Basin, an
estimated 12 inches of annual rainfall was needed for infiltration below agricultural lands, while 18
inches of rainfall was needed for infiltration beneath native ground cover and urban/suburban areas
(Fugro West and Cleath & Associates, 2002).
These threshold values for minimum annual rainfall prior to infiltration are assumed to approximate the
annual evapotranspiration of precipitation. Once these thresholds are exceeded, infiltration to
groundwater and runoff would become dominant. It is recognized that a portion of the initial annual
rainfall may result in runoff, depending on rain intensity, but this is assumed to be offset by the portion
of the late season rainfall that is evapotranspired. Since infiltration is the critical component of
precipitation with respect to the Basin safe yield, offsetting of early wet season runoff with late wet
season evapotranspiration in the water budget is considered a reasonable approach.
The specific thresholds for annual rainfall that is estimated to evapotranspire prior to infiltration and
runoff have been developed from Blaney’s field studies. Evapotranspiration of precipitation has been
estimated by multiplying land use/land cover acreages by the infiltration threshold values. Results of
these estimates are shown in the surface water budget of Table 6-1, Table 6-2, and Table 6-3.
Additional details of the methodology are provided in section 6.4.5.1 (Components of Groundwater
Inflow).
Evapotranspiration of Applied Water
The evapotranspiration of applied irrigation water has been divided into urban and agricultural sectors.
Urban applied water includes residential outdoor irrigation, urban recycled water use, and golf
course/play field irrigation. Much of the urban applied water is accounted for by City of San Luis
Obispo or other water purveyor records. Estimation of applied water for urban and agricultural
irrigation not supplied by purveyors involves a soil-moisture balance approach discussed in section
6.4.5.2 (Components of Groundwater Outflow).
Most water applied for irrigation is taken up by plants and transpired. Some water, however, is lost to
evaporation or infiltrates to groundwater as return flow. The evapotranspiration of applied irrigation
water has been calculated by subtracting the estimated return flow from the applied water estimates.
Both applied water and return flow estimates are presented under the historical groundwater budget
section. Results of the calculations of evapotranspiration of applied water are shown in the surface
water budget of Table 6-1, Table 6-2, and Table 6-3.
Riparian Corridor Evapotranspiration
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-31
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Riparian plant communities present along the creeks can access surface flows and creek underflow.
Riparian areas are included within the native brush, trees, and grasses acreage for the subareas (Table
6-6). Besides evapotranspiration of precipitation, however, an additional 0.8 acre-feet per acre of
consumptive water use is estimated for riparian corridors (Fugro West and Cleath & Associates, 2002);
(Robinson, 1958) that lie within potential Groundwater Dependent Ecosystems, which cover
approximately 200 acres in the San Luis Valley subarea and 50 acres in the Edna Valley subarea
(Figure 5-15). Riparian corridor water use during severe drought is reduced a nominal 50 percent to
reflect lack of creek underflow. Riparian evapotranspiration is included in Table 6-1, Table 6-2, and
Table 6-3.
Infiltration of Precipitation and Applied Water
Infiltration of precipitation and applied water are both outflow components from the surface water
budget and inflow components to the groundwater budget. Discussion of these components is provided
in Section 6.4.5.1 (Components of Groundwater Inflow).
Surface Water Deliveries Offset
When imported surface water is brought into the Basin from local supplies (Salinas Reservoir, Whale
Rock Reservoir, and Nacimiento Reservoir), it is counted as surface water inflow. This imported water
is then provided to customers through surface water deliveries from the City’s water treatment plant.
After residential and business use, most of the delivered water is conveyed by sewer to the wastewater
treatment plant for recycling and discharge into San Luis Obispo Creek. Since wastewater discharges
to the creek are also counted as surface water inflow, an offset factor is needed to avoid double
counting that portion of imported surface water. The surface water deliveries offset is an outflow equal
to the wastewater discharges inflow and is shown in the surface water budget of Table 6-1.
Laguna Lake
Laguna Lake is an approximate 100-acre open water body within the San Luis Valley subarea (Figure
3-10). There are an additional 100 acres of adjacent wetlands connected to the lake. Evaporation from
the water surface and transpiration by phreatophytes in the wetlands are included in the water budget
as surface water outflow. Local pan evaporation is estimated at 70 inches per year (for all years), with
a reservoir coefficient of 0.7, based on a review of information from nearby reservoirs (San Luis Obispo
County Department of Public Works, 2005). The resulting estimated annual evaporation rate for this
water budget component is 4.1 feet (not including offset from direct precipitation). Evapotranspiration
by phreatophytes were estimated to use lake water at a rate equal to irrigated pasture applied water
demand. Results for Wetland/Lake ET outflow from the surface water budget are shown in Figure 6-1.
As with riparian water use, during severe drought the lake and wetland evapotranspiration is reduced
by 50 percent.
Groundwater-Surface Water Interaction (Net)
Groundwater-surface water interaction involves both surface water and groundwater budgets. For
losing stream, the net interaction may be an outflow component for the surface water budget and an
inflow component for the groundwater budget. Details of the methodology used to develop the
groundwater-surface water interaction are presented in Section 6.4.5.1
Stream Outflow from Basin
Stream outflow from each subarea was estimated using the water balance method and compared to
available flow records. No significant changes to surface water in storage are assumed in the water
budget from year to year. Storm water runoff exits the Basin annually, and Laguna Lake storage
fluctuations are considered minor compared to the total surface water budget. Surface water supply
reservoirs are outside of the Basin boundary.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-32
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Using the water budget equation, stream outflow is estimated as the difference between total surface
water inflow and all other components of surface water outflow. Results of stream outflow calculations
are presented in the main water budget Tables.
There are limited annual stream flow records available for comparison to the estimates in the historical
surface water budget. For the San Luis Valley subarea, the only applicable published records for
stream outflow from the San Luis Valley subarea are two years of data recorded on Lower San Luis
Obispo Creek at San Luis Bay Drive. In the 1971 water year, 20.46 inches of rainfall was recorded at
Cal Poly and approximately 14,000 acre-feet of stream flow was reported at the San Luis Bay Drive
gage (records missing in October). In the 1972 water year, 12.42 inches of rainfall was recorded at Cal
Poly with 4,260 acre-feet of stream flow at the San Luis Bay Drive gage (San Luis Obispo County
Engineering Department, 1974). These two years are outside of the historical water budget base
period, and a comparison of flow for water years with similar precipitation suggests that the estimated
Basin outflows are reasonable.
Measured annual flows on Pismo Creek downstream of the Basin boundary are also available for only
two water years, 1991 and 1992 (Balance Hydrologics, 2008). These are years within the historical
base period, although the flows were measured at Highway 101, where Pismo Creek has a watershed
of 38 square miles, compared to 25 square miles upstream of the Basin boundary. Estimated outflow
in the water budget from the Edna Valley subarea for 1991 and 1992 are lower than the flows
measured at Highway 101, as would be expected. Table 6-7 shows the stream outflow comparisons.
Table 6-7. Stream Outflow Comparison
LOCATION WATER YEAR
PRECIPITATION AT CAL
POLY (IN.)
FLOW
(ACRE-FEET)
San Luis Obispo Creek at San Luis Bay Drive gage 1971 20.46 13,705*
San Luis Valley subarea stream outflow estimate 2003 22.9 15,390
San Luis Obispo Creek at San Luis Bay Drive gage 1972 12.42 4,260
San Luis Valley subarea stream outflow estimate 1990 13.36 3,360
Pismo Creek at Highway 101 gage 1991 18.6 2,033
Edna Valley subarea stream outflow estimate 1,840
Pismo Creek at Highway 101 gage 1992 22.14 4,640
Edna Valley subarea stream outflow estimate 3,590
*October 1970 missing – estimate 300 acre-feet = approx. 14,000 acre-feet for year
6.4.5. Historical Groundwater Budget
The groundwater budget includes the following sources of inflow:
• Infiltration of Precipitation
• Groundwater-Surface Water Interaction
• Subsurface Inflow
• Infiltration of Applied Water
The groundwater budget includes the following sources of outflow:
• Groundwater Extractions
• Subsurface Outflow
• Groundwater-Surface Water Interaction
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-33
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.4.5.1. Components of Groundwater Inflow
Infiltration of Precipitation
Infiltration of precipitation refers to the amount of rainfall that directly recharges groundwater after
moving through the soil and unsaturated zone (Figure 6-2). Direct measurement of infiltration has not
been performed in the Basin, and estimates have been prepared based on prior work by Blaney (1933)
in Ventura County Basins and Blaney et al. (1963) in the Lompoc Area. These studies involved soil
moisture measurements at rainfall penetration test plots with various types of land cover, and the
resulting deep percolation versus rainfall correlations have been considered applicable to central coast
Basins (DWR, 2002) (Fugro West and Cleath & Associates, 2002). The work by Blaney is several
decades old, however, modeling efforts have shown the generalizations are relatively accurate for
semi-arid climates (Rosenberg, 2001). The main advantage of Blaney’s approach is that it is based on
direct measurements of infiltration of precipitation.
Criteria based on Blaney et al. (1963) were used for analytical water budgets in the Santa Maria Valley
and Tri-Cities Mesa areas, where it was assumed that precipitation could infiltrate only in urban and
agricultural areas when 11 inches of precipitation had fallen annually, and on areas of native vegetation
when 17 inches of precipitation had fallen annually. Any amount of rainfall above 30 inches annually
was not considered to contribute to deep percolation of precipitation, regardless of the land use
classification (DWR, 2002). Correlations between infiltration and annual rainfall based on Blaney
(1933) were also used for the 2002 Paso Robles groundwater Basin analytical water budget (Fugro
West and Cleath & Associates, 2002).
Estimates for infiltration of precipitation for the SLO Basin have been developed by applying Blaney
correlations that restrict deep percolation to precipitation in agricultural areas that occurs after 11-12
inches of rainfall, and in native vegetation areas after approximately 18 inches of rainfall. Native
vegetation was the most restrictive land cover for infiltration when tested by Blaney due to high initial
soil moisture deficiencies.
Urban areas were not part of the original studies by Blaney. The low permeability of hardscape
(buildings and paving) limits infiltration and increases surface evaporation, compared to other types of
land cover, but hardscape also increases runoff, which can lead to greater infiltration in adjacent areas
receiving the runoff. Therefore, the infiltration threshold was set higher than irrigated agricultural land,
but not as high as native grasslands. The Blaney correlation that produces infiltration between irrigated
agriculture and native grassland is the curve for non-irrigated grain, with an infiltration threshold of
approximately 14 inches of rainfall. Figure 6-15 plots the data collected by Blaney (1933).
As with prior work by the DWR in northern Santa Barbara and southern San Luis Obispo Counties,
rainfall above 30 inches was not considered to contribute to deep percolation in the Basin (DWR,
2002). Infiltration of precipitation results are shown in the water budget tables and graphs.
The land use classifications for which infiltration thresholds have been developed for this GSP include
citrus, deciduous, pasture, vegetable, vineyard, native brush/grassland (includes riparian corridors),
wetland, urban developed/open space, and Urban turf. The minimum rainfall needed before infiltration
of precipitation can occur for various land uses and covers are summarized in Table 6-8.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-34 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-15. Rainfall vs Infiltration
y = 1.2404x + 11.006
R² = 0.8275
y = 1.4949x + 13.632
R² = 0.7941
y = 1.2348x + 11.632
R² = 0.8123
y = 1.228x + 14.397
R² = 0.9371y = 1.084x + 18.378
R² = 0.9049
6
10
14
18
22
26
30
0 2 4 6 8 10 12 14Annual Rainfall (inches)Infiltration (inches)
Blaney (1933)
Rainfall vs. Infiltration (Blaney Correlations)
Citrus
Deciduous
Truck, Alfalfa, Misc
Non-irrigated Grain
Grass & Weeds
Linear (Citrus)
Linear (Deciduous)
Linear (Truck, Alfalfa, Misc)
Linear (Non-irrigated Grain)
Linear (Grass & Weeds)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-35
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-8. Minimum Rainfall for Infiltration
Wetland soils are assumed to be close to field capacity due to shallow groundwater and the infiltration
threshold is only used for estimating ET in the surface water budget, with the remaining precipitation as
runoff (mainly into Laguna Lake).
Groundwater-Surface Water Interaction (Net)
As previously mentioned, groundwater-surface water Interaction involves both components of the
surface water and groundwater budgets. The net interaction is an outflow component of the surface
water budget and inflow component of the groundwater budget (losing streams).
The groundwater-surface water interaction component is estimated using a mass balance approach for
the Edna Valley subarea by adjusting the percent of stream inflow that percolates to groundwater (as
Basin recharge) while minimizing the sum of squares of the residual error between the calculated
change in storage and the measured change in storage (specific yield method) for multiple years. A
similar optimization was performed for the San Luis Valley subarea except a variable percentage was
used depending on the type of year (a greater percentage of stream flow percolation during lower
rainfall years). A spill mechanism was developed in the budget to allow groundwater outflow to streams
when storage reached full capacity, which was set to a nominal 37,000 acre-feet based on historical
storage estimates using the specific yield method. The groundwater-surface water interaction
estimates are in the water budget tables. Additional details of the calibration methodology used to
minimize the residual error are presented in Change in Storage (Section 6.4.7).
Subsurface inflow
Subsurface inflow from bedrock surrounding the groundwater Basin flows into both subareas.
Subsurface inflows were estimated using Darcy’s Law, which is an empirical formula describing the flow
of fluid though a porous material, and expressed as:
𝑄= −𝐾𝑑ℎ
𝑑𝑙𝐴
Where:
Q = groundwater discharge rate through a cross-sectional area of the porous material
K = hydraulic conductivity of the material
𝑑ℎ
𝑑𝑙 = hydraulic gradient at the cross-section
A = cross-sectional area
LAND USE/COVER INFILTRATION THRESHOLD (IN.)
Citrus 11.0
Deciduous 13.6
Pasture 11.6
Vegetable 11.6
Vineyard 13.6
Native brush/grassland 18.4
Wetland* 11.6
Urban developed/open space 14.4
Urban turf 11.6
* ET of precip. prior to runoff (no infiltration)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-36
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
The negative sign denotes that flow is in the direction of decreasing pressure. Since groundwater
pressures are greater within the bedrock hills surrounding the Basin than beneath the alluvial valleys,
there is subsurface inflow to the Basin from bedrock. Similarly, groundwater elevations in the Edna
Valley subarea are greater than in the San Luis Valley subarea and the direction of subsurface flow is
from the Edna Valley to the San Luis Valley. The application of Darcy’s Law to estimate subsurface
inflow from bedrock involves simplification and assumptions of uniformity in the subsurface. The Basin
boundary was divided into six reaches, each representing different boundary conditions. Cross-
sectional areas for boundary flows were based on the length of each reach times the average thickness
of adjacent saturated Basin sediments determined from cross-sections presented in Chapter 4 (Basin
Setting). Hydraulic gradients for each reach were developed by averaging topographic slopes between
a line along the Basin boundary and a line drawn at a 5,000-foot setback from the Basin boundary, and
assuming the hydraulic gradient paralleled these slopes. Hydraulic conductivity was estimated for
each reach based on the bedrock type, a review of pumping test data in the SLO Basin
Characterization Report (GSI Water Solutions, 2018), and structural features. Table 6-9 summarizes
the results of subsurface inflow estimates. Bedrock subsurface inflow reaches are shown on Figure
6-16.
Table 6-9. Subsurface Inflow Estimates
REACH BEDROCK FORMATION
BOUNDARY
DESCRIPTION
LENGTH THICKNESS
HYDRAULIC
GRADIENT
HYDRAULIC
CONDUCTIVITY INFLOW
FT FT FT/FT FT/DAY AFY
1 KJf melange w/serp. Depositional 43,900 100 0.05 0.05 90
2 Monterey/Lower Pismo Edna fault 38,100 200 0.01 0.03 30
3 KJf melange w/serp. Depositional 88,300 20 0.09 0.05 130
4 JKf metavolcanics Los Osos fault 28,600 40 0.09 0.2 220
5 KJf melange w/serp. Los Osos fault 12,200 60 0.05 0.05 20
6 Obispo/Rincon w/ serp. Depositional 9,500 60 0.06 0.05 10
Note: KJf - Fransiscan Assemblage SAN LUIS VALLEY SUBAREA 320
Serp. = serpentinite EDNA VALLEY SUBAREA 110
AFY = acre-feet per year BASIN TOTAL 430
Basin boundary types for evaluating subsurface inflow are depositional or fault-bounded. Depositional
boundaries occur where Basin sediments gradually thin toward the Basin boundary, while fault
boundaries are where Basin sediments are abruptly offset by faulting. Fault boundaries are generally
on the south side of the Basin, while depositional boundaries are on the north side. Geologic-cross
sections presented in Chapter 4 (Basin Setting) were used for reference in this analysis. Thicknesses
at the Basin boundary are estimated from Basin cross-sections presented in Chapter 4 (Basin Setting).
The hydraulic conductivity of bedrock across the Basin boundary was estimated at a nominal 0.05 feet
per day, with two exceptions (Table 6-9). The Franciscan Assemblage metavolcanics are more
permeable where fractured along the Los Osos fault zone (southwest Basin boundary; Figure 4-8) and
are assigned a greater hydraulic conductivity. The Edna fault (Figure 4-8) offsets sedimentary beds
along the Basin boundary and is interpreted to create a barrier to groundwater flow, corresponding to
lower permeability.
Subsurface inflow to the San Luis Valley subarea also takes place as Basin cross-flow from the Edna
Valley subarea. A subsurface profile of the bedrock high was developed as part of this GSP using
geophysical methods (Cleath-Harris Geologists, 2019). Darcy’s Law was used to estimate subsurface
flow based on a cross-sectional area of 140,000 square feet (approximately 3,500 feet in length and 40
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-37
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
feet saturated depth), a typical hydraulic gradient perpendicular to the boundary of 0.004 feet per foot
(average of high and low values from 1986 and 2019 water level contour maps) and an estimated
hydraulic conductivity for the sediments of 7 ft/day from local pumping tests listed in the SLO Basin
Characterization Report (GSI Water Solutions, 2018). The resulting estimated average subsurface flow
from the Edna Valley subarea to the San Luis Valley subarea is 30 AFY.
Infiltration of Applied Water (Return Flows)
Estimates for infiltration of applied water include urban return flow and agricultural return flow. Urban
return flow comes from water delivered for domestic or commercial/industrial uses that infiltrates to
groundwater, mainly through landscape/turf irrigation and septic system discharges (includes
suburban/rural residential return flow and recycled water return flow). Urban return flow does not
include City wastewater that is discharged to San Luis Obispo Creek, which is accounted for in the
surface water budget. Agricultural return flows come from applied irrigation water to crops.
The first step in estimating urban return flows was to separate all delivered water (groundwater pumped
from the Basin and imported surface water supplies) into indoor and outdoor use. An estimated 5
percent of indoor use is assumed to be consumptive use (95 percent return flow; (EPA, 2008)), while
85 percent of outdoor use is consumed (15 percent return flow) based on the typical range of estimates
for other local Basins (DWR, 2002) (Fugro West and Cleath & Associates, 2002). Almost all Indoor
water use drains to septic systems or sewer systems. Outdoor water use is generally for irrigation,
most of which evapotranspires into the atmosphere.
The distribution of indoor to outdoor water use will vary based on the user. City customers ar e
estimated to average 70 percent indoor use and 30 percent outdoor use, based on approximately 65
percent of delivered water reaching the wastewater treatment plant (with 5 percent indoor consumptive
use). Large parcel residential water users outside of City limits tend to use a greater percentage of
water for outdoor use than City residents. Businesses served by small water companies can have a
wide range of indoor and outdoor distribution and were assigned values based on the results of a local
study on business water use (City of San Luis Obispo, 2000).
The indoor and outdoor water use and associated return flows from water use by City, suburban/rural
residential, and small water systems were compiled, together with estimated return flow from recycled
water use. Infiltration of Applied Water estimates for urban and agricultural sectors are presented in the
historical water budget Table 6-1, Table 6-2, and Table 6-3.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-38 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-16. Bedrock Subsurface Inflow Reaches
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-39
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.4.5.2. Components of Groundwater Outflow
Urban Groundwater Extractions
Groundwater extraction from wells is the primary component of outflow in the groundwater budget.
Estimates for historical pumping were derived from various sources, including purveyor records, land
use data and water duty factors, and daily soil-moisture budgets.
Available purveyor records (meter records) were obtained from the following Basin users:
• City of San Luis Obispo
• Golden State Water Company
• Edna Valley East Mutual Water Company
• Varian Ranch Mutual Water Company
Production records ranged from weekly to quarterly and were compiled to reflect the water year per
GSP requirements. The City used groundwater from wells between 1989 and 2014, with the highest
use in water years 1990, 1991, and 1992, averaging 1,830 AFY. Overall City groundwater use
averaged 405 AFY between 1989 and 2014. Golden State Water Company averaged 335 AFY over
the historical base period (1987-2019), although average water use over the last 5 water years is
approximately 210 AFY. Edna Valley East MWC and Varian Ranch MWC have averaged
approximately 100 AFY combined since reaching full development in the late 1990s, with 80 AFY
combined over the last 5 years.
There are also 42 small water systems, mostly in the San Luis Valley subarea, which use groundwater
from wells. Each water system was assigned a use category, and a corresponding water use factor.
For example, groundwater use for commercial service connections were assigned water use based on
building square footage (from aerial image review), with a 0.06 acre-foot per year per square foot use
factor. Water use factors for local use categories were obtained from the results of a study conducted
by the City of San Luis Obispo utilities conservation office (City of San Luis Obispo, 2000). The water
use estimate was developed for current conditions, as almost all water companies were active
throughout the historical base period. The total amount of water used by small water systems in the
Basin is estimated at 270 AFY, with the majority of use (260 AFY) in the San Luis Valley subarea. Less
than 10 of the 42 small water systems using groundwater are connected to the City sewer.
Urban groundwater extractions have also been used for golf course irrigation (turf). Laguna Lake golf
course was served entirely by groundwater wells through 2007, with recycled water use from the City
beginning in 2008. San Luis Country Club uses a combination of recycled water use from County
Service Area 18 and groundwater. The groundwater extractions and recycled water use components of
urban turf irrigation are accounted for separately in the water budget. Estimates for turf irrigation water
demand used the same daily soil moisture balance program as crop irrigation (see Agricultural
Irrigation).
Rural Residential Groundwater Extractions
Rural residential groundwater use was estimated based on the number of residences identified on
aerial images outside of water company service areas. Each rural residence was assigned a water use
of 0.8 AFY, consistent with the San Luis Obispo County Master Water Plan (Carollo, 2012). As a
comparison, the City study reported residential use for large parcels (>0.26 acres) at 0.6 AFY (City of
San Luis Obispo, 2000), which is similar to the average estimated use per service connection in the
Golden State Water Company service area over the historical base period. Water use per connection at
Varian Ranch MWC and Edna Valley East MWC has ranged from 0.6 to 1.5 AFY, averaging
approximately 1 acre-foot per year over the historical base period defined in Section 6.1.1.
Aerial images for 1986, 1994, 2009, and 2018 were reviewed for rural residential development. The
estimated number of residences outside of water company service areas was compiled, and resulting
computed rural residential water use for these years is presented in Table 6-10.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-40
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-10. Rural Residential Water Use
YEAR SLO SUBAREA EDNA SUBAREA BASIN TOTAL
ESTIMATED NUMBER OF RESIDENCES1
1986 108 54 162
1994 119 61 180
2009 162 145 307
2018 173 158 331
ESTIMATED WATER USE (AFY)2
1986 86 43 130
1994 95 49 144
2009 130 116 246
2018 138 126 265
1outside of water company service areas
2based on 0.8 AFY per residence
Agricultural Groundwater Extractions
Groundwater use for agricultural irrigation has been estimated using the DWR Consumptive Use
Program Plus (CUP+)(DWR, 2015) which is a crop water use estimator that uses a daily soil moisture
balance. CUP+ was developed as part of the 2013 California Water Plan Update to help growers and
agencies estimate the net irrigation water needed to produce a crop.
Daily climate data from CIMIS Station #52 (San Luis Obispo) from 1986 to 2019 were used by the
CUP+ program, along with estimates for various crop and soil parameters. The climate data is used to
determine local reference evapotranspiration (ETo) on a daily basis. Crop coefficients are then
estimated for up to four growth stages (initial, rapid, mid-season, late-season) which determine the crop
evapotranspiration (ETc) values. Lastly, the CUP+ program uses variables related to the soil and crop
type to determine the estimated applied water demand (ETaw), which is equivalent to the net irrigation
requirement. Figure 6-17 shows the annual ETaw for various crops during the historical base period,
along with the reference evapotranspiration (ETo) and precipitation at CIMIS Station #52.
Crop types were grouped according to the classification used by County Agricultural Commissioner’s
Office for crops overlying the Basin. These crop types included citrus, deciduous (non-vineyard),
pasture, vegetable, and vineyard. A turf grass classification was added for estimating Urban sector
water demand served by groundwater. The CUP+ program provides monthly water demand for each
crop type during the hydrologic base period (1987-2019). Low, medium, and high consumptive use of
applied irrigation water estimates are presented in Table 6-11. Low and high consumptive use are the
respective annual minimum and maximum estimates over the base period, while medium consumptive
use is the average. The CUP+ applied water requirement for vegetables was reduced by 40 percent to
account for fallow acreage, which is not in production at any given time, based on historical aerial
image review.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-41 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-17. Consumptive Use of Applied Water
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
Applied Water Demand (Feet)Year
Consumptive Use of Applied Water (ETaw)
Vineyard Citrus Deciduous Vegetables Pasture Turfgrass CIMIS Precip (ft)CIMIS ET0 (ft)
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-42
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-11. Consumptive Use of Applied Water
CROP TYPE
ACRE-FEET PER ACRE PER YEAR
LOW MED HIGH
Citrus 1.1 1.6 2.2
Deciduous 1.8 2.2 2.5
Pasture 2.6 3.1 3.7
Vegetables* 1.4 1.6 2.0
Vineyard 0.5 0.6 0.8
Turfgrass 2 2.6 4.1
*60 percent of ETaw to account for fallow fields
As previously discussed in section 6.4.2 (Historical Land Use), the distribution of crop acreage was
determined by a review and correlation of DWR and County crop surveys with aerial imagery. Crop
acreages were interpolated between the years with data.
Applied water demand volumes were calculated by multiplying the annual acreage for each crop by the
average annual applied water demand during each year. The final applied water estimates used for the
water budget were adjusted to include efficiency (with system leakage) factors of 80 percent for
drip/micro emitter and high-efficiency sprinkler irrigation (citrus, deciduous, vineyard, and turfgrass) and
75 percent for mostly sprinkler with some drip irrigation (pasture and vegetables). The estimated
groundwater extractions for agricultural water use are shown in the main water budget Table 6-1, Table
6-2, and Table 6-3.
Wetland Direct ET
There are approximately 570 acres of wetlands and open water in the San Luis subarea (Table 6-6), of
which approximately 100 acres are open water and 100 acres are wetlands directly connected to
Laguna Lake (based on aerial image review) and part of the surface water budget. The remaining 370
acres of wetlands, most of which extend northwest of Laguna Lake into the Los Osos Valley, are
assumed to be areas with seasonally shallow groundwater where evapotranspiration by native grasses
effectively draws from the groundwater reservoir.
The water demand of wetlands through direct groundwater use is assumed to be equivalent to average
consumptive use of irrigated pasture as shown in Table 6-11. Any rainfall over 11.6 inches (Table 6-8)
also contributes to meeting wetland water demand. Wetland direct ET estimates are shown in Table
6-1.
Subsurface Outflow
Subsurface outflow from Basin sediments occurs as underflow along the main creek channels (San
Luis Obispo Creek and Pismo Creek). Outflow volumes were estimated using Darcy’s Law (Section
6.4.5.1). Table 6-12 presents the parameters used for subsurface outflow estimates.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-43
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-12. Subsurface Outflow Estimates
LOCATION
CROSS-SECTIONAL AREA
HYDRAULIC
GRADIENT
HYDRAULIC
CONDUCTIVITY OUTFLOW
FT2 FT/FT FT/DAY AFY
San Luis Obispo Creek 46,800 0.004 65 100
Pismo Creek* 20,600 0.01 20 35
*begins at confluence of West Corral and East Corral de Piedra Creeks
Cross sectional areas for outflow were based on the estimated width and saturated depth of alluvial
deposits in the vicinity of where the creeks exit the groundwater Basin. Hydraulic gradients are the
approximate grade of the stream channel, and the hydraulic conductivities are based on pumping tests
(GSI Water Solutions, 2018) (Cleath-Harris Geologists, 2018). Additional subsurface outflow from the
San Luis Valley subarea occurs along Davenport Creek and East Fork Creek but would be significantly
less than San Luis Obispo Creek due to shallower and less permeable alluvial deposits. Total average
subsurface outflow from the San Luis Valley subarea is estimated at 100 AFY from San Luis Obispo
Creek and a nominal 20 AFY from the smaller tributaries, for a total of 120 AFY. Subsurface outflow
from the Edna Valley subarea along the Canada Verde drainage and tributaries is estimated to be
similar to Pismo Creek (35 AFY), for a total subsurface outflow from that subarea of 90 AFY (35 AFY
each from Pismo Creek and Canada Verde, and 20 AFY to San Luis Valley.)
6.4.6. Total Groundwater in Storage
Groundwater is stored within the pore space of Basin sediments. The Specific yield is a ratio of the
volume of pore water that will drain under the influence of gravity to the total volume of saturated
sediments. The specific yield method for estimating groundwater in storage is the product of total
saturated Basin volume and average specific yield. Calculation of total groundwater in storage for
selected years was performed based on the specific yield method.
Estimates of specific yield for Basin sediments were obtained based on a review of 21 representative
well logs. The lithology for each well log was correlated with specific yield values reported for sediment
types in San Luis Obispo County (Johnson, 1967). A summary of the correlations is shown in Table 6-
13. Locations of well logs used for the specific yield correlations are shown in the referenced cross-
sections from the SLO Basin Characterization Report (GSI Water Solutions, 2018).
Groundwater in storage calculations were performed for the Spring conditions of 1986, 1990, 1995,
1998, 2011, 2014, and 2019 using the specific yield method. Water level contours for each year were
prepared based on available water level data from various sources, including the County water level
monitoring program, Geotracker Groundwater Information System data, groundwater monitoring
reports, Stakeholder provided information, and Environmental Impact Reports. Water level contour
maps for the Spring 1986 and Spring 2019 are shown in Figure 6-18 and Figure 6-19.
The water level contours for storage calculations extend to the Basin boundaries. Groundwater levels
in the San Luis Valley subarea may contour at, or slightly above, ground surface in areas where
wetlands are present, and there are no major differences between Spring 1986 and Spring 2019 water
levels. In the Edna Valley subarea, water level contours show some notable areas of decline between
1986 and 2019 near the intersection of Edna Road (Highway 227) and Biddle Ranch Road and at the
southeast end of the Basin. Declines in these areas are also shown for other time intervals in Figure 5-
and Figure 5- of Chapter 5 (Groundwater Conditions). Of note, however, is that Spring 2019 water
levels shown in Figure 6-18 are lower near the intersection of Edna and Biddle Ranch Road than for the
same period shown in Figure 5-6. This is because Figure 5-6 contours pressure in a shallow alluvial
aquifer in this area while Figure 6-19 contours pressure in the deeper Pismo Formation aquifer that is
the main supply aquifer for irrigation, and more appropriate for water budget storage calculations.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-44
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-13. Specific Yield Averages
WELL ID BASIN CROSS-SECTION
AQUIFER SPECIFIC YIELD (PERCENT)
QAL QTP PISMO
139405 B-B' 3.0 4.7
158599 G-G' 6.8 6.9 18.0
279128 C2-C2' 11.0
279130 A1-A2 8.2 6.5 3.0
287786 C1-C1' 7.2
319126 C1-C1' 5.5 11.7
438979 A1-A2 4.4 8.1
469906 A3-A4 12.0 10.7
529099 E-E' 8.1 11.2
68734 A2-A3 5.9 8.0
710817 G-G' 3.0 5.0 10.8
73143 A1-A2 12.7 5.8
782309 A2-A3 7.1 10.5 15.8
782656 D-D' 5.0 16.0
e026022 H-H' 7.4 18.6
e0047435 G-G' 6.6 4.5 17.6
e0115806 offset I-I' 9.1 16.2
e0161526 F-F' 5.4 15.6
e0183287 H-H' 3.0 7.0
e0225875 A2-A3 3.6 17.3 10.1
TH1 C1-C1' 5.9 8.9 18.0
AVERAGE SPECIFIC YIELD 6.2 8.5 13.4
BASIN AVERAGE (WEIGHTED) 10.5
SAN LUIS VALLEY SUBAREA (WEIGHTED) 8.0
EDNA VALLEY SUBAREA (WEIGHTED) 11.7
Notes: Cross-sections shown in SLO Basin Characterization Report (GS1 Water Solutions, 2018)
Qal = alluvium; QTp = Paso Robles Formation; Pismo = Pismo Formation
Weighted averages based on penetrated thicknesses of aquifer type.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-45 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-18. Groundwater Elevation Contours Spring 1986
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 6-46 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-19. Groundwater Elevation Contours Spring 2019
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-47
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
The water level contour maps and the base of permeable sediments were processed for volume
calculation using Surfer, a grid-based mapping and graphic program. The methodology consisted of
gridding and trimming surfaces to the Basin subarea boundaries, followed by volume calculation
between surfaces. The gross volumes obtained were then multiplied by the representative specific
yield for each subarea. An example of the methodology showing gridded surfaces for Spring 2019
water levels and the base of permeable sediments is presented in Figure 6-20. Estimated total storage
volumes for selected years using the specific yield method are listed in Table Figure 6-14.
Figure 6-20. Storage Volume Grids
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-48
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-14. Spring Groundwater Storage Estimates
YEAR SLO SUBAREA (ACRE-FEET) EDNA SUBAREA (ACRE-FEET) BASIN TOTAL (ACRE-FEET)
1986 36,310 132,840 169,150
1990 31,560 119,950 151,510
1995 36,750 131,020 167,770
1998 36,990 133,010 170,000
2005 38,080 126,210 164,290
2011 35,910 120,220 156,130
2014 34,280 104,950 139,230
2019 34,940 105,630 140,570
The groundwater storage estimates for the Basin are greater than previously reported, which was
23,300 acre-feet for the San Luis Valley subarea and 46,000 acre-feet for the Edna Valley subarea
(Boyle Engineering, 1991). The Draft DWR study estimated an average storage of 16,000 acre-feet for
the San Luis Valley subarea and 34,000 acre-feet for the Edna valley subarea (DWR, 1997). The
increases are due primarily to improvements in characterizing Basin saturated thicknesses, specific
yield, and methodology.
For example, the average saturated thickness of Basin sediments in the Edna Valley is listed as 102.9
feet by Boyle (1991). For Spring 1990, the average thickness of saturated sediments in the Edna Valley
subarea using the base of permeable sediments in the SLO Basin Characterization Report (GSI Water
Solutions, 2018) and Surfer gridding methodology is estimated to be approximately 150 feet, an
increase of 50 percent. The estimated average specific yield value for the Edna Valley subarea is also
close to 30 percent greater for GSP storage calculations (11.7 percent) than the prior estimate (9.1
percent). An additional 30-35 percent decrease in Basin storage areas was also incorporated into the
prior methodology through the application of a subsurface configuration factor, which was not clearly
described. (Boyle Engineering, 1991).
Increases in total groundwater in storage between prior work and current estimates does not imply an
increase in sustainable yield or basin recharge rate. The purpose of total storage estimates for the
water budget is to provide an independent calculation of change in storage over time, which is a critical
part of the water budget equation.
6.4.7. Change in Storage
Balancing the water budget is the final step in water budget development.
As previously mentioned, the water budget equation is as follows:
INFLOW – OUTFLOW = CHANGE IN STORAGE
The annual change in storage for the surface water budget is assumed to be zero, as surface flow
moves quickly through the basin and any differences in storage are minor compared to the total budget.
Therefore, the surface water balance equation can be simplified as INFLOW = OUTFLOW and was
used to estimate the stream outflow component of the surface water budget.
For the groundwater budget, groundwater-surface water interaction (as stream flow seepage) was
adjusted to approximate the change in storage calculated using the specific yield method discussed
above. The difference between the estimated change in storage shown in the water budget and the
measured change in storage using the specific yield method is the mass balance error. Change in
storage is reported between seasonal high (Spring) conditions per GSP regulations.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-49
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Change in storage and mass balance error for the groundwater budget is shown in Table 6-15. Figure
6-21 shows total storage using the water budget and specific yield method.
Table 6-15. Change in Storage Comparison – Historical Base Period 1987 – 2019
SUBAREA
WATER BUDGET SPECIFIC YIELD METHOD MASS BALANCE ERROR
CHANGE IN STORAGE (ACRE-FEET) ACRE-FEET AFY PERCENT*
San Luis Valley subarea 690 -1,370 2,060 62 6
Edna Valley Subarea -27,440 -27,210 -230 -7 0
*Percent of total subarea water budget
The difference in change in storage estimates between the water budget and the specific yield method
is approximately 60 AFY for the San Luis Valley subarea over the historical base. The water budget
estimates a 690 acre-foot gain in storage, compared to a 1,370 acre-foot decline in storage using the
specific yield method. A review of the contour maps indicates that the decline in San Luis Valley
subarea storage shown by the specific yield method is due to the effects of groundwater level declines
in the Edna Valley subarea being contoured across the bedrock high into the San Luis Valley subarea
(Figure 6-18 and Figure 6-19). There are no hydrographs for water levels in the bedrock high area, and
the extent to which water level declines in the Edna Valley subarea have influenced water levels in the
eastern portion of the San Luis Valley subarea is uncertain. Available water level hydrographs do not
show overall water level declines west of the bedrock high (Figure 5-11).
The difference in change in storage estimates between the water budget and the specific yield method
is less than 10 AFY for the Edna Valley subarea over the historical base period. The water budget
estimates a 27,440 acre-foot decline in storage, compared to a 27,210 acre-foot decline in storage
using the specific yield method. The change in storage mass balance error for the Basin historical
groundwater budget is less than 100 acre-feet per year, which is reasonable for the purposes of
preliminary sustainable yield estimates.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-50 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-21. Groundwater Storage Estimate Comparison for Basin Subareas
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-51
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.4.8. Preliminary Sustainable Yield Estimate
The sustainable yield is the maximum quantity of water, calculated over a base period representative of
long-term conditions in the Basin and including any temporary surplus, that can be withdrawn annually
from a groundwater supply without causing an undesirable result. Temporary surplus is the amount of
water that may be pumped from an aquifer to make room to store future water that would otherwise be
evaporated from shallow groundwater table or discharged from the aquifer to area creeks. Undesirable
results will be defined for six sustainable management criteria in Chapter 8 (Sustainable Management
Criteria). Examples of potential undesirable results are related to long-term declines in water levels and
associated loss in groundwater in storage.
Estimating sustainable yield includes evaluating historical, current, and projected water budget
conditions. The analytical water budget method utilized in this analysis evaluates historical and current
conditions and provides a preliminary estimate for the Basin sustainable yield. The projected water
budget will be evaluated using the Basin numerical model presented later in the projected water budget
section of the chapter, at which time the minimum thresholds for the sustainable management criteria
can be incorporated and the final sustainable yield will be determined. The preliminary sustainability
estimate can be used for planning potential projects and management action scenarios for the Basin
numerical model.
The preliminary sustainable yield of the Basin has been estimated separately for each of the subareas.
The Edna Valley subarea has experienced cumulative storage declines since 1998, while the San Luis
Valley subarea experiences minimal storage declines during drought but recovers and is typically close
to full storage capacity (Figure 6-21).
For the Edna Valley subarea, sustainable yield is estimated as the amount of long-term recharge
(groundwater inflow) to the Basin over the historical base period (3,400 AFY) minus subsurface outflow
(100 AFY). The resulting preliminary sustainable yield is estimated at a 3,300 AFY.
The San Luis Valley subarea has not experienced cumulative and persistent storage declines. Long-
term average recharge to groundwater in the San Luis Valley subarea is estimated to be 3,700 AFY, of
which an estimated 1,200 AFY is used by wetlands, leaving 2,500 AFY for withdrawal without long-term
declines in storage (subsurface outflow is supported by wastewater discharges). The historical
recharge to the subarea may be less than the sustainable yield, however, average annual recharge can
increase with storage declines, particularly in a Basin that is at or near storage capacity.
The San Luis Valley subarea did experience significant undesirable results due to land subsidence
during the period of high groundwater use and associated storage decline toward the end of the 1987-
91 drought. Average groundwater production from 1990-1992 was 3,960 AFY. Land subsidence is not
necessarily a risk over the entire subarea and would generally require historical storage declines to be
exceeded in affected areas for additional subsidence to occur. However, without mitigation for land
subsidence or specific projects that increase recharge during dry periods, the preliminary sustainable
yield of the San Luis Valley subarea is estimated at 2,500 AFY, based on the long-term average
recharge of 3,700 AFY minus 1,200 AFY used by wetlands. Figure 6-15 summarizes the preliminary
sustainable yield estimates.
Table 6-16. Preliminary Sustainable Yield Estimate (AFY)
SAN LUIS VALLEY SUBAREA 2,500
EDNA VALLEY SUBAREA 3,300
BASIN TOTAL 5,800
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-52
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
The above values are lower overall than historical estimates by Boyle (1991) and DWR (1997). Boyle
estimated 5,900 AFY of sustainable yield for the Basin while DWR estimated 2,000-2,500 for the San
Luis Valley subarea and 4,000-4,500 for the Edna Valley Subarea.
6.4.9. Quantification of Overdraft
Overdraft is the condition of a groundwater Basin or subbasin where the amount of water withdrawn by
pumping exceeds the amount of water that recharges a Basin over a period of years, during which the
water supply conditions approximate average conditions.
While the 33-year historical base period is representative of the long-term climatic conditions needed
for estimating sustainable yield, a shorter period is appropriate for characterizing water supply
conditions with respect to Basin withdrawals and overdraft. Over the last 10 years the City has
introduced recycled water use at Laguna golf course (historically irrigated by groundwater) and has
stopped pumping groundwater from the San Luis Valley subarea, while total irrigated agriculture in the
Edna Valley subarea has leveled off after increasing from the beginning of the historical base period
through the mid-2000’s (Table 6-5). Overdraft for GSP planning purposes has been estimated as the
difference between sustainable yield and average groundwater withdrawals over the last 10 years
(2010-2019), with an adjustment in the San Luis Valley subarea to account for reductions in agricultural
acreage due to recent development.
Groundwater extractions in the San Luis Valley subarea (adjusted for recent development) have
averaged 1,800 AFY since 2010, which is 700 AFY less than the average recharge of 2,500 AFY over
the same representative period, indicating a surplus of groundwater for the subarea. In the Edna Valley
subarea, groundwater pumping has averaged 4,400 AFY since 2010, which is 1,100 AFY more than
the sustainable yield of 3,300 AFY for the subarea. The Edna Valley subarea is an estimated 1,100
AFY in overdraft. Total Basin overdraft is estimated at 400 AFY. Table 6-16 summarizes the overdraft
estimates.
Table 6-17. Estimated Overdraft (AFY)
SAN LUIS VALLEY SUBAREA -700*
EDNA VALLEY SUBAREA 1,100
BASIN TOTAL 400
*surplus
In comparison, prior work (Boyle Engineering, 1991) concluded that there was short-term overdraft in
the Basin and that withdrawals in excess of sustainable yield was a common occurrence. However,
during the period from 1978-1990, the Basin was not considered in a state of sustained overdraft. The
Draft 1997 DWR study does not address overdraft, although there is a net deficit in the basin water
budget for the 1969-1977 base period, a surplus for the 1983 water budget, and a deficit for the 1990
water budget. The draft DWR report concluded that additional water beyond the long-term dependable
yield could be extracted from the Basin, but that there could be adverse impacts.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-53
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.5. Current Water Budget
The current water budget quantifies inflows and outflows for the Basin based on the last four years of
the historical water budget, from 2016 to 2019. These years provide the most recent population, land
use, and hydrologic conditions. Recent Basin conditions have been characterized by above average
rainfall, along with a decrease in urban extractions and imported surface water supplies assumed to be
associated with greater conservation awareness by the public during the 2012-2016 drought. There
have also been declines in agricultural acreage and associated groundwater extractions in the San Luis
Valley subarea associated with urban development.
Comparisons of the current water budget to the 1987-2019 historical surface water budget used for the
preliminary sustainable yield estimates for the two subareas and total Basin are shown in Table 6-17
through Table 6-19. Bar graphs showing the same information are shown in Figure 6-22 through
Figure 6-26. As expected, the average annual water budget inflows and outflows are greater under
current conditions than the historical base period, primarily due to greater rainfall. There has been
more groundwater inflow than outflow under the current water budget in the San Luis Valley subarea,
leading to increased groundwater in storage. In the Edna valley subarea, the outflow has been slightly
greater than inflow under the current water budget, with relatively little change to groundwater in
storage since the end of the recent drought (Figure 6-21). As noted above, groundwater extractions for
agriculture in the San Luis Valley subarea have declined between the historical and current water
budgets.
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-54
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-18. Current Water Budget - San Luis Valley Subarea
SURFACE WATER BUDGET HISTORICAL AVERAGE (1987-2019) CURRENT (2016-2019)
INFLOW AFY
Precipitation 10,580 12,280
Groundwater extractions (Urban) 740 400
Groundwater extractions (Ag) 1,630 1,370
Stream Inflow at Basin Boundaries 10,720 10,570
Wastewater discharge to streams 4,080 3,910
Local Imported Supplies 5,820 5,430
TOTAL IN 33,580 33,960
OUTFLOW AFY
ET of precipitation 7,770 8,220
ET of Applied Water (Urban) 2,050 1,510
ET of Applied Water (Ag) 1,310 1,100
ET of Lake/Wetland/Riparian 650 690
Surface Water Delivery Offset 4,080 3,910
Infiltration of Precipitation 1,610 3,190
Infiltration of Applied Water (Urban) 440 440
Infiltration of Applied Water (ag) 320 260
GW-SW interaction (net) 970 510
Stream outflow at Basin boundary 14,390 14,120
TOTAL OUT 33,580 33,960
GROUNDWATER BUDGET HISTORICAL AVERAGE (1987-2019) CURRENT (2016-2019)
INFLOW AFY
Infiltration of precipitation 1,610 3,190
Urban water return flow 440 440
Agricultural return flow 320 260
GW-SW interaction (net) 970 510
Subsurface from bedrock 340 340
TOTAL IN 3,670 4,750
OUTFLOW AFY
Groundwater extractions (Urban) 740 400
Groundwater extractions (Ag) 1,630 1,370
Wetland direct ET 1,160 1,190
Subsurface outflow 120 120
TOTAL OUT 3,650 3,080
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-55
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-19. Current Water Budget - Edna Valley Subarea
SURFACE WATER BUDGET HISTORICAL (1987-2019) CURRENT (2016-2019)
INFLOW AFY
Precipitation 9,300 10,780
Groundwater extractions (Urban) 880 820
Groundwater extractions (Ag) 3,210 3,440
Stream Inflow at Basin Boundaries 3,630 3,480
TOTAL IN 17,020 18,520
OUTFLOW AFY
ET of precipitation 6,910 7,200
ET of Applied Water (Urban) 600 610
ET of Applied Water (Ag) 2,650 2,870
ET of Riparian 40 40
Infiltration of Precipitation 1,890 2,800
Infiltration of Applied Water (Urban) 280 210
Infiltration of Applied Water (ag) 560 570
GW-SW interaction (net) 510 490
Stream outflow at Basin boundary 3,580 3,750
TOTAL OUT 17,020 18,520
GROUNDWATER BUDGET HISTORICAL AVERAGE (1987-2019) CURRENT (2016-2019)
INFLOW AFY
Infiltration of precipitation 1,890 2,800
Urban water return flow 290 220
Agricultural return flow 560 570
GW-SW interaction (net) 510 490
Subsurface from bedrock 110 110
TOTAL IN 3,360 4,180
OUTFLOW AFY
Groundwater extractions (Urban) 880 820
Groundwater extractions (Ag) 3,210 3,440
Subsurface outflow 100 100
TOTAL OUT 4,190 4,360
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-56
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Table 6-20. Current Water Budget - Basin Total
SURFACE WATER BUDGET HISTORICAL AVERAGE (1987-2019) CURRENT (2016-2019)
INFLOW AFY
Precipitation 19,880 23,060
Groundwater extractions (Urban) 1,620 1,220
Groundwater extractions (Ag) 4,840 4,810
Stream Inflow at Basin Boundaries 14,350 14,050
Wastewater discharge to streams 4,080 3,910
Local Imported Supplies 5,820 5,430
TOTAL IN 50,600 52,480
OUTFLOW AFY
ET of precipitation 14,680 15,420
ET of Applied Water (Urban) 2,650 2,120
ET of Applied Water (Ag) 3,960 3,970
ET of Lake/Wetland/Riparian 690 730
Surface Water Delivery Offset 4,080 3,910
Infiltration of Precipitation 3,500 5,990
Infiltration of Applied Water (Urban) 720 650
Infiltration of Applied Water (ag) 880 830
GW-SW interaction (net) 1,480 1,000
Stream outflow at Basin boundary 17,970 17,870
TOTAL OUT 50,600 52,480
GROUNDWATER BUDGET HISTORICAL AVERAGE (1987-2019) CURRENT (2016-2019)
INFLOW AFY
Infiltration of precipitation 3,500 5,990
Urban water return flow 730 660
Agricultural return flow 880 830
GW-SW interaction (net) 1,480 1,000
Subsurface from bedrock 450 450
TOTAL IN 7,030 8,930
OUTFLOW AFY
Groundwater extractions (Urban) 1,620 1,220
Groundwater extractions (Ag) 4,840 4,810
Wetland direct ET 1,160 1,190
Subsurface outflow 220 220
TOTAL OUT 7,840 7,440
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-57 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-22. Historical and Current Average Annual Surface Water Budget – San Luis Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-58 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-23. Historical and Current Average Annual Surface Water Budget – Edna Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-59 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-24. Historical and Current Average Annual Surface Water Budget – Basin Total
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-60 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-25. Historical and Current Average Annual Groundwater Budget – San Luis Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-61 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 6-26. Historical and Current Average Annual Groundwater Budget – Edna Valley Subarea
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 6-62 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Annual Groundwater Budget – Basin Total
Water Budget (§354.18) Section 6
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 6-63
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
6.6. Projected Water Budget
SGMA Regulations require the development of a future surface water and groundwater budget to
estimate future baseline conditions of supply, demand, and aquifer response to Basin groundwater use.
The future water budget provides a baseline against which management actions will be evaluated over
the GSP implementation period from 2022 to 2042. Future water budgets were developed using the
GSFLOW model developed for this GSP (Appendix G). Each simulation was run continuously through
the historical calibration period (water years 1987-2019) through the end of the predictive simulation
period (water years 2020 through 2044). Assumptions and details of the model simulations are
provided in the following sections.
6.6.1. Assumptions Used in Future Water Budget Development
SGMA regulations mandate the development of a future groundwater budget to estimate future
baseline conditions of supply, demand, and aquifer response to Basin pumping. The future water
budget provides a baseline against which management actions may be evaluated during the GSP
implementation period. Future water budgets were developed using the Basin GSFLOW integrated
model.
As per Section 354.18(c)(3)(A) of the SGMA regulations, the future water budget should be based on
50 years of historical climate data. The GSP GSFLOW model and historical water budget analysis is
based on 33 years of historical data rather than 50 years of data. As detailed in Section 6.2.1., this is
judged to be a representative historical period spanning a variety of hydrologic year types and is the
best available information for groundwater planning purposes. Therefore, the future water budget is
based on this time series rather than a 50-year time series of data.
Assumptions about future groundwater supplies and demands are described in the following
subsections.
6.6.1.1. Future Water Demand Assumptions
For the purpose of evaluating the effects of climate change and future baseline water budget
development, the assumption is made that there will be no increase in irrigated acreage or agricultural
pumping over the SGMA planning horizon. Agricultural pumping is maintained at Water Year 2019
levels. Representatives of agricultural stakeholders have been involved in the GSP planning process
from the beginning, including representation on the Groundwater Sustainability Commission, active
involvement in public meetings, and significant contributions through the public comment process. In
the Edna Valley, it is understood by the agricultural stakeholders that the path to sustainability likely
requires a decrease in agricultural pumping.. In accordance with Section 354.18 (c)(3)(B) of the SGMA
Regulations, the most recently available land use (in this case, crop acreage) and crop coefficient
information is used as the baseline condition for estimating future agricultural irrigation water demand.
For the GSP, the most recent crop acreage data was obtained from the office of the San Luis Obispo
County Agricultural Commissioner and is described in Section 6.4.2 (Historical Land Use).
The assumption is also made that municipal pumping in the baseline predictive period will remain at
current levels (Water Year 2019 pumping values). The City does not currently pump groundwater to
supply their service area. Although City population may increase, at present this would not require
increased groundwater pumping. The City may resume groundwater use in the future to augment the
water supply for their service area. However, with the San Luis Valley water budget in surplus as
previously described, there is likely available capacity for the aquifer to provide supply in the San Luis
Valley in the future. Also, space for municipal expansion is frequently made possible by retirement of
agricultural land, which results in lower pumping in the Basin. Several areas within the City are currently
under development and transitioning land from agricultural use to residential and commercial uses.
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Sustainability Plan
Additionally, rural domestic de minimis pumping is assumed to remain at current levels; there are no
significant development plans in County-administered parts of the Basin. , Additionally, this is a small
portion of the overall water budget (2-4% of total pumping), and minor revisions to this pumping
category will not significantly affect model results.
6.6.1.2. Future Climate Assumptions
For the baseline predictive scenario, the historical time series of climatological model input parameters
for water years 1995 through 2019 was repeated for the predictive model period of water years 2020
through 2044. The 1995 – 2019 historical period includes several different water year types, including
representation of the recent drought.
For the climate change predictive scenario, SGMA Regulations require incorporating future climate
estimates into the future water budget. To meet this requirement, DWR developed an approach for
incorporating reasonably expected, spatially gridded changes to monthly precipitation and reference
ETo (DWR 2018). The approach for addressing future climate change developed by DWR was used in
the future water budget modeling for the Basin. The changes are presented as separate monthly
change factors for both precipitation and ETo and are intended to be applied to historical time series
within the climatological base period through 2011. Specifically, precipitation and ETo change factors
were applied to historical climate data for the period 1995-2019 for modeling the future water budget.
DWR provides several sets of change factors representing potential climate conditions in 2030 and
2070. The SLO Basin used the 2070 climate conditions to develop a future water budget. Consistent
with DWR recommendations, datasets of monthly 2070 change factors for the SLO Basin area were
applied to precipitation and ETo data from the historical base period to develop monthly time series of
precipitation and ETo, which were then used to simulate future hydrology conditions.
6.6.2. Projected Future Water Budget
6.6.2.1. Future Surface Water Budget
The future surface water budget includes average inflows from local imported supplies, average inflows
from local supplies, average stream outflows, and average stream percolation to groundwater. Table 6-
21 summarize the average components of the historical average and projected surface water budget.
Because the timeline of preparing the GSP chapters required completions prior to the completion of the
integrated surface water/groundwater model, the historical average values and the current values
presented in Tables 6-21 and 6-22 are taken from the analytical water budget analysis presented in this
chapter. The future water budget values presented are taken from the average 2020-2044 GSFLOW
model output for the climate change scenario. These are different methods of analysis, and as a result
some of the future water budget results are different in magnitude and, in some cases, water budget
component categories, from the analytical water budget results. Differences in values between some of
the component categories of the water budget may be attributable to differences in estimation methods
between the analytical approach and the modeling approach. In addition, many of the differences relate
to the surface water/groundwater component of the water budget, which has a lack of reliable data
during the historical period of record. If future water budgets are developed using the model, past and
future estimates will likely be more consistent.
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Table 6-21 Projected Future Annual Surface Water Inflows to Basin (AFY)
SURFACE WATER BUDGET HISTORICAL AVERAGE
(1987-2019)
CURRENT
(2016-2019)
FUTURE (2020-2044 ANNUAL
AVERAGE)
INFLOW AFY
Precipitation 19,880 23,060 18,182
Groundwater extractions (Urban) 1,620 1,220 1,238
Groundwater extractions (Ag) 4,840 4,810 4,374
Stream Inflow at Basin Boundaries 14,350 14,050 9,295
Wastewater discharge to streams 4,080 3,910 3,910
Local Imported Supplies 5,820 5,430 5,430
TOTAL IN 50,600 52,480 42,429
OUTFLOW AFY
ET of Precipitation 14,680 15,420 12,612 ET of Lake/Wetland/Riparian 690 730
ET of Applied Water (Urban) 2,650 2,120 1,919
ET of Applied Water (Ag) 3,960 3,970 3,512
WWTP Effluent Offset 4,080 3,910 3,910
Infiltration of Precipitation 3,500 5,990 2,606
Infiltration of Applied Water (Urban) 720 650 559
Infiltration of Applied Water (ag) 880 830 862
GW-SW interaction (net) 1,480 1,000 2,534
Stream outflow at Basin boundary 17,970 17,870 13,744
TOTAL OUT 50,600 52,480 42,258
Inspection of values in the future surface water budget and groundwater budgets in Tables 6-21 and 6-
22 reveal some differences between the model-generated future water budget values and the
analytically estimated past and current water budgets. As mentioned previously, the two approaches to
analyzing a water budget are quite different. Still, the differences merit some discussion. First, it is
important to remember that the current water budget represents water years 2016-2019, which was a
relatively wet period coming out of the 2012-2016 drought. The historical average period includes a 33-
year period with slightly higher pumping values, and also includes both the period prior to recent water
level declines (prior to the mid-1990s) as well as periods of documented water level declines (mid-
1990s through present). The future water budget encompasses a 25-year period using the assumptions
previously discussed. For the future water budget, the inflows and outflows are approximately
balanced, as one would expect to see. However, the total inflow and outflows for the future water
budget (about 42,300 AFY) are about 17% lower than the historical average (50,600 AFY). The largest
water budget component difference between the two budgets is the surface water inflow value, wherein
the model-derived value for this parameter for the future water budget is 9,295 AFY, which is about
65% of the analytical estimate. The PRMS model may underestimate the baseflow component of
streamflow, which could explain some of this discrepancy, or the analytical approach may have
overestimated runoff when correlating sub-watershed flows to the Andres. However, as has been
discussed previously in this chapter, there was almost no direct flow measurement data available for
model calibration; all data was calculated based on theoretical estimates equating stage to discharge,
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which incorporates a significant degree of uncertainty into this component of the surface water budget.
Improvement in the surface flow monitoring network will help to better define this component.
Additionally, it is important to realize that this discrepancy in the surface water budget does not
translate into a comparable magnitude of discrepancy in the Groundwater budget. Particularly in SLO
Basin, where the Basin is oriented perpendicular to the direction of streamflow, high flows move into
and out of the Basin quickly, and do not have the same magnitude of influence on the groundwater
budget as is expressed in the surface water budget.
6.6.2.2. Future Groundwater Budget
Projected groundwater budget components are computed using the GSFLOW integrated surface
water/groundwater flow model to simulate average conditions over the implementation period. Table 6-
22 summarizes the projected annual groundwater budget for the SLO Basin.
Table 6-22 Future Water Budget
GROUNDWATER BUDGET HISTORICAL AVERAGE (1987-
2019)
CURRENT
(2016-2019) FUTURE
INFLOW AFY
Infiltration of precipitation 3,500 5,990 2,606
Infiltration of Applied Water (Urban) 730 660 559
Infiltration of Applied Water (ag) 880 830 862
GW-SW interaction (net) 1,480 1,000 2,534
Subsurface from bedrock 450 450 1,093
TOTAL IN 7,030 8,930 7,654
OUTFLOW
Groundwater extractions (Urban) 1,620 1,220 1,238
Groundwater extractions (Ag) 4,840 4,810 4,374
Wetland direct ET 1,160 1,190 2,807
Subsurface outflow 220 220 326
TOTAL OUT 7,840 7,440 8,745
CHANGE IN STORAGE -810 1,490 -1,091
Comparison of the three groundwater budgets indicates some differences in the component estimates
between the future model-derived water budget and the analytical estimates for the historical average
and current conditions. For example, infiltration of precipitation is lower in the future water budget
(2,606 AFY) than in the historical water budget (3,500 AFY), but GW-SW interaction is higher in the
future period (2,534 AFY) than in the historical period (1,480). But if these two water budget
components are considered additively as a broader conceptual category of precipitation-based
recharge, the combined value for the historical period is 4,980 AFY and the combined value for the
future period is 5,140 AFY. These values are quite close, so it is the partition of this broader category
where the two water budget estimates differ.
However, if the change in storage is values are inspected, they make intuitive sense given the
understanding of conditions in the Basin since the 1980s. The historical average change in storage (-
810 AFY) is lower than the future change in storage (-1,091 AFY); this corresponds to the fact that the
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historical period includes an 8 to 10 year period prior to the more recent decline of water levels
observed in Edna Valley. The current period water budget indicates an increase of groundwater in
storage of 1,490 AFY, which corresponds to the observed rise in water levels in Edna Valley since the
end of the 2012 to 2016 drought. And finally, the change in storage value for the future water budget (-
1,091 AFY), which assumes current levels of groundwater pumping, approximately corresponds to the
estimate of overdraft for Edna Valley (1,100 AFY) presented in the water budget previously in this
chapter. Differences in the surface water-influenced terms of the water budget may be improved when
the surface water monitor network is expanded during the implementation phase of the GSP.
6.6.2.3. Impact Assessment of Climate Change
In order to assess the effect that climate change may have on groundwater elevations in the Basin, the
following methodology was used. A baseline predictive scenario was simulated in which no projects or
management actions were simulated, Basin pumping was maintained at the levels documented for
water year 2019, and climate conditions from water years 1995 to 2019 were repeated for the predictive
period of water years 2020 through 2044. Then a climate change scenario was incorporated in which a
meteorological input into the GSFLOW model was changed as per guidance from DWR. Comparisons
of these two scenarios provides an indication of potential impacts on Basin conditions from climate
change.
The model was applied to evaluate the possible effects of climate change using the following
methodology. A brief comparison was made between precipitation input and water level results
between the baseline predictive run and the baseline run with climate change factors incorporated into
the future predictive model simulation. Modeled precipitation in the Basin averaged 20.28 inches per
year in the baseline run, and 20.74 inches per year in the climate change run, with DWR factors applied
to climatological inputs. Water level results in the ten RMS well sites in the Basin, discussed further in
Chapter 7 (Monitoring Network). The average of groundwater elevations at the ten RMS wells was 3.4
feet higher in the climate change scenario run than in the baseline run. This indicates that climate
change is not a significant factor that needs to be considered in the Basin over the SGMA planning
horizon.
6.6.2.4. Future Sustainable Yield and Overdraft
The sustainable yield of the Basin was estimated at 5,800 AFY (2,500 AFY for San Luis Valley and
3,300 AFY for Edna Valley) based on a review of data for the period from water year 1987 through
water year 2019. The projects and management actions described in Chapter 9 (Projects and
Management Actions), and implementation plan as described in Chapter 10 (Implementation Plan), are
developed with the objective of reducing groundwater pumping in the Edna Valley such that there is no
overdraft on a long-term basis. Absent any significant changes in land use patterns or climatological
factors, there is no reason to expect that the sustainable yield estimate developed in this chapter will
vary significantly prior to the next scheduled revision and update of this GSP. An update of the water
budget and sustainable yield estimate may be recommended at the next update of the GSP, particularly
if significant drought conditions are experienced in the coming years; if it becomes arguable that we are
entering a new drought of record, that would constitute new climatological conditions that would
necessitate a revision of the sustainable yield estimate. However, for the current planning period it is
assumed that the future sustainable yield estimate will be approximately equal to that presented
previously in this chapter.
7-1
7
GROUNDWATER SUSTAINABILITY PLAN
Monitoring Networks (§354.32 &
§354.34)
This chapter describes the proposed monitoring networks for the GSP in
accordance with SGMA regulations in Sub article 4: Monitoring Networks.
Monitoring is a fundamental component of the GSP necessary to
identify impacts to beneficial uses or Basin users, and to measure
progress toward the achievement of any management goal. The
monitoring networks must be capable of capturing data on a
sufficient temporal and spatial distribution to demonstrate short-
term, seasonal, and long-term trends in groundwater and related
surface water conditions, and to yield representative information
about groundwater conditions for GSP implementation. There are
three monitoring networks for the Basin: a groundwater level
network, a groundwater quality network, and a surface water flow
network.
IN THIS CHAPTER
• Monitoring Networks
• Sustainability
Indicator Monitoring
• Monitoring and
Technical Reporting
Standards
• Assessment and
Improvement of
Monitoring Network
Monitoring Networks (§354.32 & §354.34) Section 7
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7.1. Introduction
Chapter 7 describes the monitoring objectives, rationale, protocols, and data reporting requirements of
the monitoring networks. Monitoring requirements for sustainability indicators are presented, and data
gaps are identified, along with steps to be taken to fill the data gaps before the first five-year
assessment.
The following is a list of applicable SGMA sustainability indicators that will be monitored in the
Basin:
• Chronic lowering of groundwater levels.
• Reduction in groundwater storage.
• Degradation of groundwater quality.
• Land subsidence.
• Depletion of interconnected surface water (includes potential impacts to GDEs).
Sustainability indicators are discussed in detail in Chapter 8 (Sustainability Management Criteria). This
monitoring networks chapter focuses on the monitoring sites and data collection needed to support the
evaluation of each sustainability indicator.
7.2. Monitoring Objectives
The proposed monitoring network must be able to adequately measure changes in groundwater
conditions to accomplish the following monitoring objectives:
• Demonstrate progress toward achieving measurable objectives.
• Monitor impacts to the beneficial uses and users of groundwater.
• Monitor changes in groundwater conditions relative to measurable objectives and minimum
thresholds for sustainability indicators.
• Quantify annual changes in water budget components.
The monitoring network must provide adequate spatial resolution to properly monitor changes to
groundwater and surface water conditions relative to measurable objectives and sustainability
indicators within the Basin. The network must also provide data with sufficient temporal resolution to
demonstrate short-term, seasonal and long-term trends in groundwater and related surface conditions.
7.2.1. Management Areas
Although there are differences in land use and associated water budgets between the San Luis Valley
and Edna Valley subareas, as described in Chapter 6 (Water Budget), separate management areas
have not been formally established. The monitoring network includes representative wells across the
Basin for which minimum thresholds and measurable objective have been selected based on local
conditions, as described in Chapter 8 (Sustainability Management Criteria).
7.2.2. Representative Monitoring Sites
Monitoring sites are the individual locations within a monitoring network and consist of groundwater
wells and stream gages. While a monitoring network uses a sufficient number of sites to observe the
overall groundwater conditions and the effects of Basin management projects, a subset of the
monitoring sites may be used as representative for meeting the monitoring objectives for specific
sustainability criteria.
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Representative Monitoring Sites are the locations at which sustainability indicators are monitored, and
for which quantitative values for minimum thresholds, measurable objectives, and interim milestones
are defined.
The criteria that were used to determine which wells to utilize are as follows:
• A minimum 10-year period of record of historical measurements spanning wet and dry periods.
• Available well information (well depth, screened interval).
• Access considerations.
• Proximity and frequency of nearby pumping wells.
• Spatial distribution relative to the applicable sustainability indicators.
• Groundwater use.
• Impacts on beneficial uses and Basin users.
7.2.3. Scientific Rationale
GSP monitoring program development is based on a combination of SGMA monitoring networks Best
Management Practices (BMPs), local hydrogeology, and the monitoring requirements for individual
sustainability criteria.
Some of the SGMA monitoring network BMPs implemented for this GSP include the following:
• Defining the monitoring objectives.
• Utilizing existing monitoring networks and data sources to the greatest extent possible to meet
those objectives.
• Adjusting the temporal/spatial coverage to provide monitoring data consistent with the need.
• Efficient use of representative monitoring sites to provide data for more than one sustainability
indicator.
County monitoring programs that existed before SGMA include sites that do not meet SGMA monitoring
network BMPs with respect to known construction information, such as wells with no available Well
Construction Report (WCR) and active wells that are used for groundwater supply. While not
prohibiting the use of these wells as a monitoring site, SGMA regulations require that the GSP identify
sites that do not meet BMPs and describe the nature of the divergence. If the monitoring network uses
wells that lack construction information, the GSP shall include a schedule for acquiring monitoring wells
with the necessary information or shall demonstrate that such information is not necessary to
understand or manage groundwater in the Basin.
As discussed in Chapters 4 (Basin Setting) and 5 (Groundwater Conditions), information from available
boring logs indicates that there is no regional or laterally extensive aquitard separating the Alluvial
aquifer, Paso Robles Formation aquifer, and Pismo Formation aquifer in the Basin. In the San Luis
Valley, a physical distinction between Alluvium and Paso Robles Formation sediments is often not
apparent, and information from WCRs indicates that wells are regularly screened across productive
strata in both formations, which effectively function as a single hydrogeologic unit. DWR also
concluded that there are no continuous confining layers, and unconfined groundwater table conditions
essentially prevail throughout the Basin, including the Edna Valley (DWR, 1997). A minor exception is
recognized in Chapter 6 (Water Budget) (Section 6.3.5) near the intersection of Biddle Ranch Road and
Edna Road, where there is a shallow (semi-perched) alluvial aquifer tapped by a former windmill well.
Therefore, with respect to groundwater level monitoring, data collected from wells completed in one or
more of the three principal aquifers (Alluvium, Paso Robles Formation, and Pismo Formation) can be
used collectively for groundwater elevation contouring and storage estimates. Obtaining well
construction information for all monitoring network wells is not an immediate necessity and will be
addressed (see Section 7.6).
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7.2.4. Existing Monitoring Programs
Existing monitoring programs are discussed in Chapter 3 (Description of Plan Area). Figure 3-9 shows
the locations of monitoring wells identified in the GAMA program (publicly available groundwater quality
data), the SLOFCWCD semi-annual groundwater level program, and the CCRWQCB Irrigated Lands
Regulatory Program (groundwater quality data). A total of 12 existing SLOFCWCD monitoring wells
are used as part of the GSP groundwater level monitoring network described in the following sections.
There are also groundwater level and quality data collected for various contaminant investigations and
monitoring programs that are publicly available from the SWRCB Geotracker website.
7.2.5. Groundwater Level Monitoring Network
Groundwater level monitoring is a fundamental tool in characterizing Basin hydrology. Groundwater
levels (often reported as elevations relative to a reference point) in wells are measures of the hydraulic
head in an aquifer. Groundwater moves in the direction of decreasing head (downgradient), and
groundwater elevation contours can be used to show the general direction and hydraulic gradient
associated with groundwater movement. Changes in the amount of groundwater in storage within an
aquifer can also be estimated based on changes in hydraulic head, along with other parameters.
There are 40 monitoring wells in the GSP groundwater level monitoring network, 22 wells in the San
Luis Valley and 18 wells in the Edna Valley (Figure 7-1 and Table 7-1). Construction information is
available for 31 of the 40 wells. Based on the available information, 16 of the wells are interpreted to
be alluvial wells, while the remaining 24 wells tap into the Paso Robles Formation, Pismo Formation, or
are mixed aquifer wells that utilize groundwater from more than one aquifer. Half the wells are used for
irrigation, seven are private domestic wells, and 13 are dedicated monitoring wells.
Groundwater levels may be used as a proxy for monitoring other sustainability indicators (besides
chronic lowering of water levels) provided that significant correlation exists between groundwater
elevations and the sustainability indicator for which the groundwater elevations serve as a proxy. Ten
of the groundwater level monitoring network wells are Representative Monitoring Site (RMS) wells used
for evaluating sustainability criteria. Six representative monitoring site wells are used for evaluating
chronic lowering of groundwater level and reduction of groundwater in storage, which is correlated with
groundwater levels (Chapter 6, Section 6.3.5). Two wells are used for evaluating subsidence, which is
correlated with groundwater levels in the area being monitored (Chapter 4, Section 4.7), and three wells
are used to evaluate depletion of interconnected surface water, which is correlated with groundwater
levels (Chapter 5, Section 5.7). One of the wells used to evaluate depletion of interconnected surface
water is also a representative monitoring site for subsidence. The sustainability criteria and associated
minimum thresholds and measurable objectives are presented in Chapter 8 (Sustainable Management
Criteria).
7.3. Monitoring Networks
This section introduces the proposed GSP monitoring networks and describes the networks in
relation to the following SGMA sustainability indicators applicable to the Basin:
• Chronic lowering of groundwater levels.
• Reduction of groundwater in storage.
• Groundwater quality degradation.
• Land subsidence.
• Depletion of interconnected surface water (includes potential impacts to GDEs).
The GSP monitoring program consists of three separate networks, one for groundwater levels, one for
groundwater quality, and one for surface water flow. Each network is described below.
Monitoring Networks (§354.32 & §354.34) Section 7
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7.3.1. Groundwater Level Monitoring Network
SGMA regulations do not require a specific density of monitoring wells, other than being sufficient to
represent groundwater conditions for GSP Implementation. The monitoring network well density is
roughly 20 wells per 10 square miles, which is 10 times greater density than guidelines for the
statewide CASGEM program. There are currently sufficient wells in the network to provide information
for overall sustainable management of the Basin, although some local data gaps have been identified
that will be addressed during GSP implementation.
A groundwater level monitoring well is recommended in the Foothill Boulevard/O’Conner Way area to
improve groundwater level contour control and associated groundwater storage estimates in the Los
Osos Valley area within the Basin. Other groundwater level monitoring locations are recommended for
their proximity to potential GDEs and are in the vicinity of existing or proposed stream gage locations.
The background and rationale for the GDE-related monitoring sites are presented in Appendix F
(Stillwater Sciences, 2020).
Table 7-1 presents the GSP groundwater level monitoring network wells. Table 7-2 presents additional
areas recommended for groundwater level monitoring. Figure 7-1 shows the location of the existing
groundwater level monitoring wells and the recommended additional monitoring areas.
Monitoring Networks (§354.32 & §354.34) Section 7
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability
Agencies 7-6 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Table 7-1. Groundwater Level Monitoring Network
Notes:
1. Representative Monitoring Sites are in bold. Wells with known State Well Completion Reports are underlined.
2. TRS = Township Range Section and ¼-¼ section listed, State Well ID bolded where applicable.
3. Reference Point elevations from various sources with variable accuracy.
4. Principal Aquifers are Quaternary Alluvium (Qa), Quaternary Paso Robles Formation (Qpr), and Tertiary Pismo Formation (Tps). Other bedrock aquifers (non-Basin sediments) are Tertiary Monterey
Formation (Tm) and Cretaceous-Jurassic Franciscan Assemblage (KJf). Aquifers are inferred where construction information is not available.
5. Representative well criteria include Subsidence (SUB), Chronic Water Level Decline (WL), and Groundwater Storage Decline (GSW). Other criteria are Transducer site (T), and Interconnected Surface
Water (ISW) indicator evaluation site, which may be paired with nearby existing or proposed stream gage. Transducer installations are pending funding and well owner authorization. Measurement
frequency is semi-annual for all wells except Transducer sites (T), which are measured daily.
6. Well Use includes Monitoring Well (MW), Irrigation Well (IRR), Public Supply Well (PS), and Domestic Well (DOM). Modifiers are Active (A) or Inactive (I). Information for some wells inferred pending
confirmation
7. Indicates a the well is currently in the Counties Water Level Program.
LOCAL ID1 TRS / STATE ID2
WELL DEPTH
(FEET)
SCREEN INTERVAL
(FEET)
RP ELEV.3
(FEET AMSL)
FIRST
DATA
YEAR
LAST
DATA
YEAR
DATA
PERIOD
(YEARS)
DATA
COUNT AQUIFER4
WELL
CRITERIA5
WELL
USE6 GSA
SLV-01 30S/12E-23E (pending) (pending) 304 (pending) Qa ISW, T MW County
SLV-02 30S/12E-22G (pending) (pending) 276 (pending) Qa MW City
SLV-03 30S/12E-30P 153 Qa IRR-I County
SLV-04 30S/12E-35B1 48 28-48 215.6 1991 2020 29 38 Qa IRR-A City
SLV-05 30S/12E-35D 52 32-52 187 1990 2018 28 7 Qa ISW, T IRR-A City
SLV-06 31S/12E-04D 85 45-85 150 1989 1 1 Qa T MW City
SLV-07 31S/12E-04K 125 55-125 139.5 1992 2000 8 46 Qpr PS-I City
SLV-08 31S/12E-03K 70 50-70 128 1988 2020 32 2 Qpr IRR-A City
SLV-09 31S/12E-4R1 130 40-130 129.5 1988 2020 32 48 Qa/Qpr SUB PS-I City
SLV-10 31S/12E-3Q 48 131 2017 2020 3 82 Qa MW City
SLV-11 31S/12E-3P1 61 119 1990 2006 16 31 Qa MW City
SLV-12 31S/12E-
10D3 175 50-90; 150-170 109.2 1992 2020 28 72 Qa/Qpr/Tps ISW, SUB, T IRR-A City7
SLV-13 31S/12E-11D 40 5-40 121.75 1996 2020 24 49 Qa T, ISW MW City
SLV-14 31S/12E-12E 20 5-20 144.68 1990 2020 30 60 Qa MW County
SLV-15 31S/12E-
10G2 190 122 1965 2020 55 90 Qpr IRR-A City7
SLV-16 31S/12E-
10H3 165 65-165 122 1984 2020 36 68 Qpr WL DOM-A City7
SLV-17 31S/12E-11M 100 60-100 119.78 1996 2020 24 73 Qpr MW County
SLV-18 31S/12E-11K 30 6-21 133.28 1990 2020 30 59 Qa MW County
SLV-19 31S/12E-
14C1 128 1958 2020 62 98 Qpr WL, ISW, T IRR-A County
SLV-20 31S/13E-18D 202 Qa MW County
SLV-21 31S/12E-13A 60 50-60 178.68 2018 2018 1 Qpr MW County
SLV-22 31S/12E-13C 100 11-100 178 2004 2020 16 2 Qpr/Kjf T IRR-I County
EV-01 31S/13E-
16N1 72 324 1958 2020 62 99 Qa ISW, T DOM-A County7
EV-02 31S/13E-20A 75 305 Qa ISW IRR-I County
EV-03 31S/13E-
19H4 250 178-250 254 Qpr/Tps IRR-A County
EV-04 31S/13E-
19H1 262 1958 2020 62 100 Tps WL, GWS, T IRR-A County7
EV-05 31S/13E-20G 400 120-400 280 Tps IRR-I County
EV-06 31S/13E-19J1 251 1998 2020 22 44 Qpr DOM-I County7
EV-07 31S/13E-19J2 250 1998 2020 22 45 Tps DOM-A County7
EV-08 31S/13E-21L 350 Qa ISW, T IRR-A County
EV-09 31S/13E-
19R3 440 130-190; 290-430 239 1974 2020 46 45 Tps/Tm WL, GWS PS-A County7
EV-10 31S/13E-28F 340 200-330 344 Qpr/Tps IRR-A County
EV-11 31S/13E-20F6 150 55-150 230 2011 2020 9 Qpr/Tm ISW, T MW County7
EV-12 31S/13E-28J3 600 303 1993 2020 27 39 Qpr/Tps IRR-A County7
EV-13 31S/13E-
27M3 400 130-380 289 1993 2020 27 34 Qpr/Tps WL, GWS IRR-A County7
EV-14 31S/13E-27R 300 90-290 319 2017 2020 3 6 Qpr/Tps T MW County
EV-15 31S/13E-27Q 307 1989 2020 31 9 Qpr/Tps DOM-I County
EV-16 31S/13E-35D 260 200-260 323 1988 2020 32 188 Tps WL, GWS PS-A County
EV-17 31S/13E-35F 260 200-260 333 2014 2020 6 66 Tps/Kjf PS-I County
EV-18 31S/13E-
36R1 327 1968 2020 52 99 (out of Basin) IRR-A County
Monitoring Networks (§354.32 & §354.34) Section 7
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Figure 7-1. Water Level Monitoring Network
Monitoring Networks (§354.32 & §354.34) Section 7
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Sustainability Plan
Table 7-2. Recommended Groundwater Level Monitoring Network Additions
WATER LEVEL
DATA GAP ID LOCATION PURPOSE
WL-A Near Foothill Blvd. and O'Connor Way Groundwater elevation contours and storage
WL-B Madonna Road near Laguna Lake ISW evaluation
WL-C Elks Lane south of SLO Creek Bridge ISW evaluation
WL-D South Higuera near old Highway Bridge ISW evaluation
WL-E Davenport Creek east of Crestmont Road ISW evaluation
WL-F Corbett Canyon Road near Canada Verde ISW evaluation
7.3.2. Groundwater Quality Monitoring Network
Groundwater quality monitoring refers to the periodic collection and chemical or physical analysis of
groundwater from wells. As discussed in Chapter 5 (Groundwater Conditions) in Section 5.9, the
quality of groundwater in the Basin is generally good. Groundwater quality trends in the Basin are
stable, with no significant trends of ongoing deterioration of groundwater quality based on the Central
Coast Basin Plan.
Groundwater quality networks should be designed to demonstrate that the degraded groundwater
quality sustainability indicator is being observed for the purposes of meeting the sustainability goal
(DWR, 2016). In other words, the main purpose of the groundwater quality monitoring network is to
support the determination of whether the degradation of groundwater quality is occurring at the
monitoring sites, based on the sustainability indicator constituents and minimum thresholds selected.
This GSP groundwater quality network is also designed to utilize existing monitoring programs to the
greatest degree possible (DWR, 2016).
Sustainability indicator constituents selected for groundwater quality are Total Dissolved Solids (TDS),
Nitrate, and Arsenic. These constituents were introduced in Chapter 5 (Groundwater Conditions) in
Section 5.9.3 as diffuse or naturally occurring in the Basin and are further discussed in relation to
sustainability indicators in Section 7.3.4 and in Chapter 8 (Sustainable Management Criteria). Two
other water quality constituents associated with notable contaminant plumes in the South San Luis
Obispo and Buckley Road areas (Figure 7-2 and Section 7.3.4) will also be monitored within the GSP
water quality network, but not as sustainability indicators.
The groundwater quality network consists of nine sites (Figure 7-2), which are all are Public Water
System supply wells. Water quality for these wells can be accessed using the GAMA Groundwater
Information System. Wells in the Irrigated Lands Regulatory Program were evaluated for potential
inclusion in the GSP monitoring program, however, the irrigation wells have not historically been
sampled for groundwater quality at regular intervals, therefore no historical record of groundwater
quality data exists. In addition, Agricultural Order 4.0 of the Irrigated Lands Regulatory Program is
currently in draft form and under review. Selection of specific wells regulated under that program would
not be recommended until the program is implemented and monitoring data is available for review. By
comparison, the public water system wells have a history of groundwater quality data and specific wells
are sampled at regular intervals for the three indicators recommended for groundwater quality
monitoring in Chapter 8 (Sustainable Management Criteria) – TDS, Nitrate, and Arsenic.
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7.3.2.1. Groundwater Quality Monitoring Data Gaps
Current groundwater quality monitoring within the Basin is sufficient to collect the spatial and historical
data needed to determine groundwater quality trends for groundwater quality indicators. The GAMA
database includes 120 wells within the Basin boundaries that have been monitored for groundwater
quality in the last three years. The nine wells selected (Figure 7-2) provide representative Basin
coverage but can be supplemented with other data if needed to support sustainability indicator
evaluation. The water quality network wells is used collectively to provide the metric for use with the
groundwater quality degradation sustainability indicator in Chapter 8 (Sustainable Management
Criteria). No data gaps in groundwater quality monitoring are currently identified.
Table 7-3 presents the GSP groundwater quality monitoring network. Figure 7-2 show the locations of
the groundwater quality monitoring wells.
Table 7-3. Groundwater Quality Monitoring Network
LOCAL ID STATE ID1
FIRST DATA
YEAR
LAST DATA
YEAR
DATA PERIOD
(YEARS)
DATA
COUNT
(TDS)2
DATA
COUNT (N)3
DATA
COUNT
(AS)4 GSA
WQ-1 4000206-003 2003 2019 16 4 12 5 County
WQ-2 4000780-001 2002 2019 17 5 21 6 City
WQ-3 4010009-004 1989 2019 30 8 42 8 City
WQ-4 4000604-001 2002 2020 18 6 69 6 City
WQ-55 4000734-001 2004 2020 16 4 21 6 County
WQ-6 4000819-001 2017 2020 3 3 4 1 City
WQ-7 4010023-008 1992 2020 28 19 142 148 County
WQ-8 4000202-001 2003 2018 15 5 23 27 County
WQ-9 4000765-001 2002 2019 17 7 19 36 County
Notes: Data accessed on GAMA Groundwater Information System
1. State ID for public water system
2. TDS = Total Dissolved Solids – typically measured every three years
3. N = Nitrate-Nitrogen – typically measured every year or quarterly
4. As = Arsenic – variable from monthly to every three years
5. WQ-5 also used to track TCE (see Section 8.2.4)
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Figure 7-2. Water Quality Monitoring Network
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7.3.3. Surface Water Flow Monitoring Network
Surface water flow monitoring can provide valuable information for the Basin model and for evaluating
potential depletion of interconnected surface water for groundwater dependent ecosystems (GDEs),
which is one of the sustainability indicators. The evaluation of surface water connectivity with the Basin
and relevance to GDEs is described in Appendix F (Stillwater Sciences, 2020) that includes
recommendations for the surface water flow monitoring sites identified in this chapter.
As summarized in Chapter 3 (Description of Plan Area), there are six permanent stream gages in or
adjacent to the Basin, all within the San Luis Valley subarea watershed (Figure 7-3). The existing
gaging stations only provide stage data, and not actual stream flow data. Stream stage is the height of
water level in the stream above an arbitrary point, usually at or below the stream bed. Stage data can
be useful for identifying flow and no-flow conditions, flood stage alerts, and analyzing the timing of
precipitation and runoff in watersheds. Streamflow data is critical for quantifying Basin recharge from
stream seepage as part of the water budget/model and for addressing depletion of interconnected
surface water sustainability indicators related to GDEs.
Stage data can be converted to streamflow through the use of a rating curve, which incorporates
information that is specific to each site, including the cross-sectional area of the channel and the
average surface water velocity for a given flow stage. A description of the methodology for monitoring
surface water flow in natural channels is presented in Appendix H. There are partial rating curve
approximations for three of the sites based on actual streamflow measurements (Section 3.6.1.3). A
modeling approach to estimating rating curves was performed by Questa Engineering (2007), but the
results of that study have not been validated with field measurements.
7.3.3.1. Surface Flow Monitoring Data Gaps
The existing gages are all in the San Luis Valley subarea watershed, where the majority of potential
GDEs have been identified (Figure 5-15). There are currently no surface flow monitoring sites in the
Edna Valley subarea, which is the subarea subject to overdraft as described in Chapter 6 (Water
Budget). Data gaps for surface water flow monitoring with respect to interconnected surface water
depletion, GDEs, and the water budget are identified on Stenner Creek near the upstream Basin
boundary, on San Luis Obispo Creek near the downstream Basin boundary, and on Pismo Creek near
the downstream Basin boundary (Appendix H). Three stream gages are recommended for installation
to fill these data gaps adjacent to the Basin boundaries. In addition, two more stream gage sites are
recommended on East Corral de Piedra Creek and West Corral de Piedra Creek at Orcutt Road to fill a
data gap in the water budget in the Edna Valley. Stream gages on these two principal drainages,
along with a gage downstream of their confluence on Pismo Creek, will provide important information
on stream seepage in the Edan Valley for the water budget/Basin model, and will allow a direct
comparison of streamflow between the two watersheds, one of which has a permitted reservoir
upstream of Orcutt Road as described in Chapter 6 (Water Budget) in Section 6.3.3.1. Rating curve
development is recommended for all stream gages to provide the stream flow information needed for
the water budget/model and sustainability indicator evaluation.
Table 7-4 presents the GSP surface water flow monitoring network. Table 7-5 presents recommended
sites for additional stream gages. For the most robust data collection program, each stream gage
should be paired with an alluvial piezometer to define both groundwater elevation and surface water
elevation simultaneously, which is currently not the case. Figure 7-3 shows the locations of the existing
gages, recommended gages, and the nearby groundwater level monitoring sites (both existing and
recommended) that can be used to evaluate interconnected surface water depletion and GDE
indicators (see Section 7.3.6 and Appendix H).
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Table 7-4. Existing Surface Water Flow Monitoring Network
LOCAL ID WATER COURSE LOCATION
FIRST DATA
YEAR
DATA
INTERVAL
DATA PERIOD
(YEARS) GSA
SG-745 San Luis Obispo Creek Andrews St. Bridge 2006 15-minutes 14 City
SG-781 Stenner Creek Nipomo Street 2005 15-minutes 15 City
SG-790 San Luis Obispo Creek Marsh Street 2019 15-minutes 1 City
SG-740 San Luis Obispo Creek Elks Lane 2005 15-minutes 15 City
SG-778 Prefumo Creek Madonna Road 2005 15-minutes 15 City
SG-783 East Fork Creek Jesperson Road 2005 15-minutes 15 County
Table 7-5. Recommended Surface Water Monitoring Network Additions
SURFACE WATER
FLOW GAP ID LOCATION PURPOSE
SG-A Stenner Creek at Stenner Creek Road Water Budget, Surface water connectivity, GDE
indicator evaluation
SG-B San Luis Obispo Creek at Old Highway Bridge Water Budget, Surface water connectivity, GDE
indicator evaluation
SG-C West Corral de Piedra Creek at Orcutt Road Water Budget
SG-D East Corral de Piedra Creek at Orcutt Road Water Budget
SG-E Pismo Creek at Railroad Crossing Water Budget, Surface water connectivity, GDE
indicator evaluation
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Figure 7-3. Surface Water Flow Monitoring Network
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7.4. Sustainability Indicator Monitoring
Sustainability indicators are the effects caused by groundwater conditions occurring throughout the
Basin that, when significant and unreasonable, become undesirable results.
The SGMA sustainability indicators for GSP implementation are as follows:
• Chronic lowering of groundwater levels.
• Reduction in groundwater storage.
• Seawater Intrusion (this indicator is not applicable to Basin).
• Degraded groundwater quality.
• Land subsidence.
• Depletion of interconnected surface water.
7.4.1. Chronic Lowering of Groundwater Levels
Chronic lowering of groundwater levels can lead to a significant and unreasonable depletion of the
water supply. All of the groundwater level monitoring network wells can be used for evaluating chronic
lowering of groundwater levels, with a selected subset of six RMSs formally assigned to assess
Minimum Thresholds and Measurable Objectives in Chapter 8 (Sustainable Management Criteria).
Groundwater monitoring network wells not included in the subset of RMSs are included in the network
primarily for preparing groundwater level contour maps, which are used for evaluating hydraulic
gradient and groundwater flow direction. Groundwater level contour maps can reveal groundwater
pumping depressions that result from lowering of groundwater levels and can also be used to calculate
change in groundwater storage. The area where chronic lowering of water levels has been occurring is
in the Edna Valley as shown in Chapter 5 (Groundwater Conditions) on Figure 5-11. Four of the six
representative wells focus on this area (Figure 7-1).
Static groundwater level measurements shall be collected at least two times per year, to represent
seasonal low and seasonal high groundwater conditions. Historically, the semi-annual groundwater
level program conducted by SLOFCWCD has measured groundwater levels in April and October of
each year. This schedule will be maintained for the GSP.
In addition, 12 wells have been recommended (based on spatial distribution, equipment access, and
interconnected surface water/GDE applications; Figure 7-1) for pressure transducer installation to
automatically record groundwater levels on a daily basis, providing more detailed information on short-
term trends, seasonal high and low conditions, and interconnected surface water depletion. Pressure
transducers are instruments that record water levels automatically at pre-determined intervals. They
are installed below the water surface in a well and use the pressure of the overlying water column to
produce a depth to water measurement. Pressure transducers are a very efficient means of collecting
groundwater level data at frequent intervals. The recommended transducer locations are listed in Table
7-1.
7.4.2. Reduction of Groundwater Storage
Groundwater storage and water levels are directly correlated, and chronic lowering of water levels also
leads to a reduction of groundwater storage. Change in groundwater storage will be monitored using
the overall monitoring network, while selected representative wells will track reduction of groundwater
storage as the sustainability indicator.
The comprehensive 40-well monitoring network will be used to contour groundwater elevations for
seasonal high conditions, from which annual spring groundwater storage estimates will be estimated
and the annual change in storage reported as required for Annual Reports. Groundwater storage will
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be calculated using the specific yield method, which is the product of total saturated Basin volume and
average specific yield. The saturated Basin volume is the volume between a groundwater elevation
contour map for a specific period (such as Spring 2019) and the base of permeable sediments (Chapter
6; Section 6.3.5). Representative Monitoring Sites that will be used for monitoring reductions in
groundwater storage are listed in Table 7-1 and shown in Figure 7-1. Chapter 8 discusses the
Minimum Thresholds and Measurable Objectives assigned to the representative wells.
7.4.3. Seawater Intrusion
The Basin is not susceptible to seawater intrusion and will not be monitored for that indicator.
7.4.4. Degraded Groundwater Quality
The significant and unreasonable degradation of water quality would be an undesirable result. As
discussed in Section 7.2.2, groundwater quality constituents in the Basin that have been selected for
groundwater quality indicator monitoring include TDS, Nitrate, and Arsenic. Selenium has been
observed at concentrations that affect well operations at individual wells in the Basin, but it does not
appear to be a widespread issue throughout the Basin (Chapter 5; Section 5.9.3.5). The selected water
quality indicators represent common constituents of concern in relation to groundwater production for
domestic, municipal and agricultural use that will be assessed by the monitoring network. TDS is
selected as a general indicator of groundwater quality in the Basin. Nitrate is a widespread
contaminant in California groundwater and selected due to its presence across the Basin associated
with agricultural activities, septic systems, landscape fertilizer and wastewater treatment facilities.
Arsenic is selected to represent naturally occurring contaminants in the Basin. Other constituents of
concern may be added to the list during GSP implementation. The sites currently best suited for
evaluating trends over time are public supply wells. Sampling intervals vary by well and by constituent,
ranging from every three years to monthly, but longer historical records are available, compared to
other types of wells.
The significant and unreasonable degradation of water quality includes the migration of contaminant
plumes that impair water supplies. There are two anthropogenic contaminant plumes that underly
multiple properties and are under investigation within the Basin. These include a tetrachloroethylene
(PCE) plume, also known as the South San Luis Obispo (SLO) PCE Plume, and a trichloroethylene
(TCE) plume, also known as the Buckley Road Area plume (Figure 7-2).
7.4.4.1. South SLO PCE Plume
PCE is primarily used as a solvent at dry cleaning establishments and has a maximum contaminant
level in drinking water of 5 micrograms per liter. Dissolved PCE in groundwater has been detected
underlying portions of the City of San Luis Obispo, mainly south of the confluence of San Luis Obispo
Creek and Stenner Creek. There have been several site investigations and documented PCE releases
at various locations within the City. Historical site investigations date to the early 1990’s, with regional
investigations in 2005 (QPS, 2005) and 2013-2015 (URS, 2013), (URS, 2015). The Department of
Toxic Substance Control (DTSC) and the Regional Water Quality Control Board (RWQCB) have
provided most of the regulatory oversight related to site investigations and clean-up efforts since the
early 1990’s. Currently, the City has initiated a comprehensive PCE investigation, including monitoring
well constructions, with Proposition 1 grant funding. Representative wells from the future PCE
monitoring well network will be selected for inclusion with the GSP groundwater quality network
specifically for tracking PCE in the Basin.
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7.4.4.2. Buckley Road Area TCE Plume
TCE has a variety of uses, typically as an industrial solvent/degreaser. The maximum contaminant
level for TCE in drinking water is 5 micrograms per liter. In 2013, the RWQCB initiated an investigation
into the source of TCE detected in two supply wells in the industrial area of Buckley Road and Thread
Lane. County of San Luis Obispo Environmental Health Services also began a sampling program
following TCE detection above the maximum contaminant level in groundwater from a residential
supply well in 2015. Information from these and subsequent investigations, including investigation at
the San Luis Obispo County Airport north of Buckley Road, indicated that the likely source of TCE was
the industrial area of Buckley Road and Thread Lane. These investigations were summarized in a
public notice from the RWQCB dated January 15, 2019. One of the supply wells selected for the
groundwater quality network (WQ-5) is in the industrial area and both historically and currently reports
TCE concentrations above the maximum contaminant level (24 micrograms per liter TCE reported in
April 2020). Currently, the RWQCB is enforcing a replacement water program to provide treatment for
wells impacted by the TCE plume. A web page has been established by the Water Board to provide
the latest information to the public and can be accessed at https://www.waterboards.ca.gov/
centralcoast/water_issues/hot_topics/tce_pce_info/tce_pce_index.html. The TCE plume will be
monitored for the GSP through tracking the concentration reported at WQ-5 and observing published
plume maps over time. A general trend of decreasing TCE concentration, along with plume
containment, would be measures of success in plume management.
7.4.5. Land Subsidence
Land subsidence can lead to undesirable results when it interferes with surface land uses. Land
subsidence is frequently associated with groundwater pumping and has been documented in the San
Luis Valley subarea (see Chapter 4; Section 4.7 and Chapter 6; Section 6.7.3). The purpose of land
subsidence monitoring is to identify the rate and extent of land subsidence and to provide data for
sustainability criteria thresholds. DWR maintains a land subsidence dataset derived from
Interferometric Synthetic Aperture Radar (InSAR) data from satellite imagery. InSAR is a remote
sensing method used to measure land-surface elevations over large areas, with accuracy on the order
of centimeters to millimeters. InSAR uses satellites that emit and measure electromagnetic waves that
reflect off of the earth’s surface to produce synthetic aperture radar images with a spatial resolution of
about 100 meters by 100 meters. Vertical displacement values associated with land subsidence can be
estimated by comparing these images over time.
The DWR land subsidence dataset shows vertical displacement from 2015-2019 in California
groundwater basins. The raster GIS dataset covers the entire Basin, with no data gaps. The dataset
shows minimal vertical displacement of less than an inch from 2015-2019 throughout the Basin.
Continued evaluation of Basin land subsidence through monitoring the available InSAR data is planned.
In addition, two representative monitoring site wells have been identified for land subsidence monitoring
based on the historical area of land subsidence in the Basin (Chapter 4; Section 4.7) and are included
in Table 7-2. Groundwater level can be a proxy for land subsidence because the process is typically
not reversible and maintaining groundwater levels above historic lows in areas susceptible to land
subsidence can protect against future undesirable results and is described in Chapter 8 (Sustainable
Management Criteria).
7.4.6. Depletion of Interconnected Surface Water
Surface water provides beneficial uses, and depletion of interconnected surface water due to
groundwater pumping can result in undesirable results by impacting these beneficial uses.
The purpose of monitoring for depletion of interconnected surface water is to characterize the
following:
• Flow conditions including surface water discharge, surface water head, and baseflow contribution.
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• Identifying the approximate date and location where ephemeral or intermittent flowing streams
cease to flow.
• Historical change in conditions due to variations in stream discharge and regional groundwater
extraction.
• Other factors that may be necessary to identify adverse impacts on beneficial uses of the surface
water.
One of the beneficial uses of surface water is the environmental water demand which supports riverine,
riparian, and wetland ecosystems. Locations where surface water is interconnected with groundwater
have the potential for supporting GDEs, which are ecological communities or species that depend on
groundwater emerging from aquifers (rising into streams or lakes) or on groundwater occurring near
ground surface where it may be used by riparian vegetation, wetland vegetation, or oak woodlands.
Depending on location and time of year, GDEs that overlie the Basin can be supported by a range of
water sources including direct precipitation, surface runoff, shallow subsurface flow, and groundwater.
Shallow subsurface flow can vary from short-term precipitation and runoff driven flow (e.g. bank storage
and other macro-pores filled during a precipitation event that drain on the order of days to weeks) to
flow that is directly connected to groundwater (e.g. baseflow as groundwater discharge into streams
during the dry season). Because GDEs overlying the Basin are supported by a wider range of surface
and groundwater hydrological processes in the wet season, monitoring of sustainability indicators that
support GDEs (i.e., conditions near interconnected surface water) should focus on the late spring
baseflow period and summer/early fall dry season. Primary groundwater dependence for GDEs is
more likely during the late summer and early fall dry season, although in some reaches irrigation return
flow may also be a factor. If the groundwater conditions that support GDEs are met in the late spring
and dry summer and fall seasons, sufficient groundwater is more likely also be available in the wet
season to sustain GDEs (see Appendix H).
There are six existing County stream gages within, or adjacent to the Basin (Table 7-4, Figure 7-3).
The existing gages only report stage, as discussed in Section 7.2.3. An additional five stream gages
are proposed, both for water budget and interconnected surface water flow data gaps (Table 7-5).
Rating curves, which correlate stage with stream flows, should be developed for all RMS stream gage
sites. In addition, groundwater level monitoring is recommended near the stream gages sites, and at
additional sites for riparian and wetland/marsh GDE types (Figure 7-3). Table 7-6 shows the pairing
between the stream gages and the nearby water level monitoring sites for interconnected surface water
that supports potential GDEs evaluation (both existing and recommended).
The wells in Table 7-6 used for monitoring of ISW that may support GDEs need to be in locations that
are representative of groundwater levels in the riparian zones. A few of the existing wells (SLV-5, SLV-
19, EV-11) are not immediately adjacent to their paired stream gage but may have a sufficient hydraulic
connection to local riparian conditions to be useful for GDE indicator evaluation. The data for each
paired monitoring well and stream gage would be supplemented with field surveys (discussed below),
to evaluate the suitability of the monitoring sites.
In addition to streamflow and groundwater level monitoring, streamflow surveys are recommended
across a range of seasons and water year types to identify losing and gaining reaches with the Basin.
Identifying losing and gaining reaches is fundamental to understanding surface water-groundwater
connectivity. Losing reaches occur in Basin recharge areas that are typically dry during the summer
and late fall. Gaining reaches occur in Basin discharge areas where groundwater is contributing to
surface water flow. Groundwater pumping that lowers groundwater levels in an aquifer beneath a creek
channel may deplete surface water by either expanding a losing reach or contracting a gaining reach,
depending on the depth of the water table and the permeability of the stream bed. The streamflow
surveys characterize the extent of gaining and losing reaches and help evaluate depletion of
interconnected streamflow. This type of data collection is conducted by measuring instream flow in
multiple locations along a reach of creek in a short period of time and examining the loss or gain of
stream flow rates along the length of the stream channel.
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Table 7-6. Proposed Interconnected Surface Water Monitoring Locations
7.5. Monitoring Technical and Reporting Standards
Monitoring technical and reporting standards include a description of the protocols, standards for
monitoring sites, and data collection methods.
7.5.1. Groundwater Levels
Monitoring protocols and data collection methods for groundwater level monitoring and reporting are
described in the attached Appendix H, and are based on SGMA monitoring protocols, standards and
sites BMPs, USGS data collection methods, and practical experience. Wells used for monitoring
program sites have been constructed according to applicable construction standards, although not all
the information required under the BMPs is available for every site. Table 7-2 lists the pertinent
information available for the monitoring sites.
7.5.2. Groundwater Quality
Monitoring protocols and standards for groundwater quality sampling sites are those required for public
water systems from which the groundwater quality data is obtained. Sample collection and field tests
shall be performed by appropriately trained personnel as required by California Code of Regulations
Title 22, Section 64415. All wells used for public supply are expected to meet applicable construction
standards.
7.5.3. Surface Water Flow
As previously discussed, the existing gaging stations only provide stage data, and not actual stream
flow data. Stage data can be converted to streamflow through the use of a rating curve, which
STREAM GAGE MONITORING WELL AREA
SG–745 (none - bedrock) SLO Creek near upstream Basin boundary
SG-781 SLV-5 Stenner Creek above SLO Creek confluence
SG-790 SLV-5 SLO Creek below Stenner Creek confluence
SG-740 WL-C SLO Creek at Elks Lane
SG-778 WL-B Prefumo Creek at Laguna Lake outlet
SG-783 SLV-19 East Fork SLO Creek at Jesperson Lane
SG-A SLV-01 Stenner Creek near upstream Basin boundary
SG-B WL-D SLO Creek near downstream Basin boundary
SG-C EV-2 West Corral de Piedra at Orcutt Road
SG-D EV-8 East Corral de Piedra at Orcutt Road
SG-E EV-11 Pismo Creek at downstream Basin boundary
(none) SLV-12 Calle Joaquin
(none) SLV-13 Tank Farm Road
(none) WL-E Davenport Creek near Crestmont Road
(none) WL-F Corbett Canyon Road near Canada Verde
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incorporates information that is specific to each site, including the cross-sectional area of the channel
and the average surface water velocity for a given flow stage. These rating curves are developed using
depth profiles and flow velocity measurements during storm-runoff events (Appendix H). Rating curves
may need to be revised periodically as they can shift due to changes in channel geometry. Protocols
and data collection methods will be based on applicable USGS standards and SLOFCWCD standards.
7.5.4. Monitoring Frequency
Monitoring frequency is the time interval between data collection. Seasonal fluctuations relating to
groundwater levels or quality are typically on quarterly or semi-annual cycles, correlating with seasonal
precipitation, recharge, groundwater levels, and well production. The monitoring schedule for
groundwater levels collected under the GSP groundwater level monitoring program will coincide with
seasonal groundwater level fluctuations, with higher levels (i.e. elevations) in April (Spring) and lower
levels in October (Fall). A semi-annual monitoring frequency provides a measure of seasonal cycles,
which can then be distinguishable from the long-term trends. At the transducer-monitored locations,
groundwater level measurements will be recorded automatically on a daily basis and downloaded
during the regular semi-annual groundwater level monitoring events. Daily measurements provide the
same time-step as the Basin model, and will also allow direct correlation with daily stream flow data.
Ultimately, more of the wells in the monitoring network will be equipped with continuous measurement
transducers than are currently equipped.
The monitoring frequency for groundwater quality sampling is variable and based on the schedule
determined by the regulating agency (County Environmental Health Services for small public water
systems and the State Division of Drinking Water for large public systems). TDS is typically monitored
every three years, while nitrate and arsenic may be monitored annually, quarterly, or even monthly at
vulnerable systems. The frequency selected for monitoring individual constituents at each system is
sufficient to protect public health, and therefore considered sufficient for Basin management purposes.
Surface monitoring network frequency is a near-continuous record of flow stage, collected at 15-minute
intervals. The stage data can then be converted to average daily flow (cubic feet per second) using a
rating curve. Automatic gaging equipment (e.g. radar sensors or bubbler gages) at proposed flow
monitoring locations will maintain the near-continuous monitoring frequency. Rating curves are needed
at all gage sites, which requires manual flow measurements over a range of stream stages. New and
existing wells listed in Table 7-6 used for interconnected surface water that could affect GDEs may also
be equipped with groundwater level transducers, either upon construction (for network additions) or
when the recommended nearby stream gage is installed. If continuous groundwater elevation data is
collected at these sites, the data will be reviewed to determine if revisions to the undesirable results or
sustainability management criteria should be revised.
7.6. Data Management System
SGMA requires development of a Data Management System (DMS). The DMS stores data relevant to
development of a groundwater Basin’s GSP as defined by the GSP Regulations (California Code of
Regulations, Title 23, Division 2, Chapter 1.5, Subchapter 2). To comply with SGMA, the Basin DMS
was developed in this GSP and will store data that is relevant to development and implementation of
the GSP as well as for monitoring and reporting purposes. Appendix H describes the data management
plan associated with the DMS.
7.7. Assessment and Improvement of Monitoring Network
The current assessment of the monitoring networks has identified data gaps that will be filled during the
implementation phase of the GSP and prior to the first five-year assessment. These data gaps,
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consisting of six groundwater level monitoring sites and five surface water flow monitoring sites, are
listed in Table 7-2 and Table 7-4 and shown in Figure 7-1 and Figure 7-3.
As previously mentioned, obtaining well construction information for all monitoring network wells is not
an immediate necessity or a requirement for Basin management purposes, provided the lack of
information does not affect the usefulness of the monitoring results toward Basin management. Over
time, wells for which construction information is not known will be inspected with a video camera to
document construction, either within the next five years or at the earliest practical opportunity, such as
when the well pump is being serviced. The monitoring networks will be re-evaluated at each five-year
assessment. If required, it may be necessary to install a group of paired piezometers with different
screened intervals to confirm the HCM assessment that vertical hydraulic gradients between geologic
formations are not significant to the Basin hydrogeologic system.
7.8. Annual Reports and Periodic Evaluation by the GSAs
Reporting requirements for the Annual Report and for periodic evaluation of the GSP are contained in
Article 7 of the GSP regulations. The GSAs will submit an Annual Report that meets Article 7
regulations by April 1 of each year following adoption of the GSP, with the first Annual Report
anticipated in 2022. Periodic evaluations of the GSP, including the monitoring networks, will be
performed at least every five years and whenever the GSP is amended, with the first written evaluation
anticipated no later than 2027.
8-1
8
GROUNDWATER SUSTAINABILITY PLAN
Sustainable Management Criteria
(§354.22)
This chapter defines the conditions specified at each of the Representative
Monitoring Sites (RMSs) that constitute Sustainable Management Criteria
(SMCs), discusses the process by which the GSAs in the Basin will
characterize undesirable results, and establishes minimum thresholds and
measurable objectives for each Sustainability Indicator.
This chapter defines sustainability in the Basin for the purposes of
managing groundwater in compliance with SGMA, and it
addresses the regulatory requirements involved. The Measurable
Objectives (MOs), Minimum Thresholds (MTs), and undesirable
results presented in this chapter define the future sustainable
conditions in the Basin and guide the GSAs in development of
policies, implementation of projects, and promulgation of
management actions that will achieve these future conditions.
IN THIS CHAPTER
• Sustainability Goals
and Definitions
• Groundwater
Reduction and
Degradation
• Management Areas
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8.1. Introduction
Defining Sustainable Management Criteria (SMC) requires technical analysis of historical data, and
input from the affected stakeholders in the Basin. This chapter presents the data and methods used to
develop the SMC and demonstrate how they influence beneficial uses and users. The SMCs presented
in this chapter are based on currently available data and application of the best available science. As
noted in this GSP, data gaps exist in the hydrogeologic conceptual model. Uncertainty caused by these
data gaps was considered when developing the SMC. Due to uncertainty in the hydrogeologic
conceptual model, these SMCs are considered initial criteria and will be reevaluated and potentially
modified in the future as new data become available.
The discussion of SMC in this chapter is organized by Sustainability Indicators. The following
Sustainability Indicators are applicable in the Basin:
• Chronic lowering of groundwater elevations
• Reduction in groundwater storage
• Degraded water quality
• Land subsidence
• Depletion of interconnected surface water
The sixth Sustainability Indicator, sea water intrusion, only applies to coastal basins, and is not
applicable in the Basin.
To maintain an organized approach throughout the text, this chapter follows the same structure
for each Sustainability Indicator. The description of each SMC contains all the information
required by Section 354.22 et. seq of the SGMA regulations and outlined in the Sustainable
Management Criteria BMP (DWR, 2017), including:
• How undesirable results were developed, including:
− The criteria defining when and where the effects of the groundwater conditions that cause
undesirable results based on a quantitative description of the combination of minimum
threshold exceedances (§354.26 (b)(2))
− The potential causes of undesirable results (§354.26 (b)(1))
− The effects of these undesirable results on the beneficial users and uses (§354.26 (b)(3))
• How minimum thresholds were developed, including:
− The information and methodology used to develop minimum thresholds (§354.28 (b)(1))
− The relationship between minimum thresholds and the relationship of these minimum
thresholds to other Sustainability Indicators (§354.28 (b)(2))
− The effect of minimum thresholds on neighboring basins (§354.28 (b)(3))
− The effect of minimum thresholds on beneficial uses and users (§354.28 (b)(4))
• How minimum thresholds relate to relevant Federal, State, or local standards (§354.28 (b)(5))
− The method for quantitatively measuring minimum thresholds (§354.28 (b)(6))
• How measurable objectives were developed, including:
− The methodology for setting measurable objectives (§354.30)
− Interim milestones (§354.30 (a), §354.30 (e), §354.34 (g)(3))
The SGMA regulations address minimum thresholds before measurable objectives. This order was
maintained for the discussion of all applicable Sustainability Indicators
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8.2. Definitions (§351)
The SGMA legislation and regulations contain a number of new terms relevant to the SMCs. These
terms are defined below using the definitions included in the SGMA regulations (§ 351, Article 2).
Where appropriate, additional explanatory text is added in italics. This explanatory text is not part of the
official definitions of these terms. To the extent possible, plain language, including limited use of overly
technical terms and acronyms, was used so that a broad audience will understand the development
process and implications of the SMCs.
1. Interconnected surface water (ISW) refers to surface water that is hydraulically connected at any
point by a continuous saturated zone between the underlying aquifer and the overlying surface
water. Interconnected surface waters are parts of streams, lakes, or wetlands where the
groundwater table is at or near the ground surface and there is water in the lakes, streams, or
wetlands.
2. Interim milestone (IM) refers to a target value representing measurable groundwater conditions, in
increments of five years, set by an Agency as part of a Plan. Interim milestones are numeric targets
such as groundwater elevations that will be achieved every five years to demonstrate progress
towards sustainability.
3. Management area refers to an area within a basin for which the Plan may identify different minimum
thresholds, measurable objectives, monitoring, or projects and management actions based on
differences in water use sector, water source type, geology, aquifer characteristics, or other factors.
4. Measurable objectives (MOs) refer to specific, quantifiable goals for the maintenance or
improvement of specified groundwater conditions that have been included in an adopted Plan to
achieve the sustainability goal for the basin. Measurable objectives are goals that the GSP is
designed to achieve.
5. Minimum thresholds (MTs) refer to numeric values for each Sustainability Indicator used to define
undesirable results. Minimum thresholds are established at representative monitoring sites.
Minimum thresholds are indicators of where an unreasonable condition might occur. For example, a
particular groundwater elevation might be a minimum threshold if lower groundwater elevations
would result in a significant and unreasonable reduction in groundwater storage.
6. Representative monitoring site (RMS) refers to a monitoring site within a broader network of sites
that typifies one or more conditions within the basin or an area of the basin.
7. Sustainability Indicator refers to any of the effects caused by groundwater conditions occurring
throughout the basin that, when significant and unreasonable, cause undesirable results, as
described in Water Code Section 10721(x). The five Sustainability Indicators relevant to the Basin
are listed in the introductory section of Chapter 8.
8. Uncertainty refers to a lack of understanding of the basin setting that significantly affects an
Agency’s ability to develop sustainable management criteria and appropriate projects and
management actions in a Plan, or to evaluate the efficacy of Plan implementation, and therefore
may limit the ability to assess whether a basin is being sustainably managed.
9. Undesirable Result Section 10721 of the Sustainable Groundwater Management Act states that
Undesirable result means one or more of the following effects caused by groundwater conditions
occurring throughout the basin:
− Chronic lowering of groundwater levels indicating a significant and unreasonable depletion of
supply if continued over the planning and implementation horizon. Overdraft during a period of
drought is not sufficient to establish a chronic lowering of groundwater levels if extractions and
groundwater recharge are managed as necessary to ensure that reductions in groundwater
levels or storage during a period of drought are offset by increases in groundwater levels or
storage during other periods.
− Significant and unreasonable reduction of groundwater storage.
− Significant and unreasonable seawater intrusion.
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− Significant and unreasonable degraded water quality, including the migration of contaminant
plumes that impair water supplies.
− Significant and unreasonable land subsidence that substantially interferes with surface land
uses.
− Depletions of interconnected surface water that have significant and unreasonable adverse
impacts on beneficial uses of the surface water.
10. Section § 354.26 of the SGMA regulations states that “The criteria used to define when and where
the effects of the groundwater conditions cause undesirable results shall be based on a quantitative
description of the combination of minimum threshold exceedances that cause significant and
unreasonable effects in the basin.”
8.3. Sustainability Goal (§354.24)
The sustainability goal for the Basin is a comprehensive statement that describes the important factors
to be considered during the SGMA planning horizon. The sustainability goal was developed over a
series of public meetings and public workshops with input from the City, County, and affected
stakeholders. The June 10, 2020 Stakeholder Workshop, Groundwater Management Vision, was
dedicated to obtaining information to be used to develop a sustainability goal for the Basin. In the
workshop, stakeholders participated in an interactive visioning exercise where they helped populate a
virtual white board to answer the question, “What is our shared vision of what a ‘sustainable SLO Basin’
means?”
Stakeholders added ideas, perceptions, outcomes, and values onto the white board across the
following categories:
• Available Groundwater Supply: What needs/uses does our groundwater supply always need to be
able to serve?
• Available Groundwater Storage: What needs/uses does our stored groundwater need to serve or
prepare us for?
• Groundwater Dependent Ecosystem Health: What outcomes do we want for surface water
ecosystems and prevention of land subsidence?
• Cost to Users: If we achieve a “sustainable Basin,” how does it look to ratepayers?
• Groundwater Quality: What is the quality of groundwater we aim to sustain?
During the September 9, 2020 GSC meeting, the results of the interactive exercise from the June
workshop were presented in an organized fashion to stakeholders. Significant concepts from the
visioning exercise are incorporated into the Sustainability Goal presented herein and are represented
as guiding principles that underpin the Basin sustainability goal. The SGMA regulations require the
sustainability goal to culminate in the absence of undesirable results within 20 years of the applicable
statutory deadline.
Per Section § 354.24 of the SGMA regulations the Sustainability goal has three parts:
• Description of the sustainability goal
• A discussion of the measures that will be implemented to ensure the Basin will be operated within
sustainable yield, and
• An explanation of how the sustainability goal is likely to be achieved.
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8.3.1. Description of Sustainability Goal
The sustainability goal for the Basin is to manage the Basin to ensure beneficial uses and basin users
have access to a safe and reliable groundwater supply that meets current and future demand without
causing undesirable results.
Guiding principles of this goal are:
• Available groundwater supply supports diverse needs reliably and equitably.
• Stored groundwater equitably supports supply resilience and evolving needs.
• Groundwater levels support the sustained health of groundwater dependent ecosystems.
• Cost of maintaining sustainable groundwater levels is equitably distributed.
• Groundwater quality is maintained to a safe standard to meet diverse basin needs.
8.3.2. Sustainability Strategy
The sustainability strategy was developed and discussed at numerous public meetings of the GSC.
Projects and management actions were developed collaboratively with GSA Staff, GSC members, and
the public utilizing the guiding principles of the Sustainability Goal. A total of seven (7) projects are
evaluated in Chapter 9 (Projects and Management Actions) and are centered around supplemental
water sources that could be brought into the SLO Basin to mitigate the overdraft. In addition to the
projects, three (3) management actions will be implemented. The implementation of a combination of
projects and the management actions listed below will ensure that the SLO Basin will operate within the
sustainable yield and achieve sustainability as described in the following sections of this Chapter.
• State Water Project for Edna Valley Agricultural Irrigation
• Stater Water Project Recharge Basin within the Edna Valley area.
• State Water Project to the Golden State Water Company
• State Water Project to the Edna and Varian Ranch Mutual Water Companies
• City of SLO Recycled Water for Edna Valley Agriculture
• Varian Ranch Mutual Water Company Arroyo Grande Subbasin Wells
• Price Canyon Discharge Relocation
• Expand Monitoring Network
• Develop and Implement Groundwater Extraction Metering Plan
• Develop Demand Management Plan
The projects and management actions will be implemented using an adaptive management strategy.
Adaptive management allows the GSAs to react to the success or lack of success of actions and
projects implemented in the Basin and to make management decisions to redirect efforts in the Basin to
more effectively achieve sustainability goals. The implementation of the projects and management
actions is described in additional detail in Chapter 10 (Implementation Plan).
8.4. Generalized Process for Establishing Sustainable Management
Criteria (§354.22-30)
SMCs for the Basin were developed after technical analysis of hydrogeologic and geotechnical data by
the consulting team, input from the GSC members, public input received in public meetings, written
public comments in response to GSC meeting and workshop presentations, and meetings with GSA
staff and GSC members. Public comments on alternative SMCs discussed during GSC meetings and
responses to those comments are included in Appendix I. All presentations made at public meetings
are available for review at the SLO Basin web site created for this GSP, www.slowaterbasin.com. The
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process further built on the Basin Groundwater Sustainability Agencies’ history of involving interested
parties – including the City, the County, environmental stakeholders, rural residents, agricultural
stakeholders, water purveyors, and mutual water companies – in public meetings focused on
groundwater resource planning.
The general process for establishing minimum thresholds and measurable objectives for the
SMC and assessing significant and unreasonable conditions constituting undesirable results in
the Basin was iterative and included the following:
• Evaluating historical data on groundwater elevations from wells monitored by the City and County.
• Evaluating water budget information presented in Chapter 6, including sustainable yield estimates
and average deficits for the San Luis Valley and Edna Valley parts of the basin.
• Holding a series of public outreach meetings that outlined the GSP development process and
introduced stakeholders to SMC, MOs, MTs, and other related information.
• Soliciting public comment and input on several alternative minimum threshold and measurable
options based upon preliminary technical analysis presented at GSC meetings and the five guiding
principles agreed upon.
• Evaluating public comment to assess what are significant and unreasonable effects relevant to
SMC. Public comments from outreach meetings were analyzed to assess if different areas in the
Basin had different perspectives for what constitutes an undesirable result in the Basin and how
minimum thresholds and measurable objectives are established.
• Combining public comment, outreach efforts, hydrogeologic data and considering the interests of
beneficial uses and groundwater users, land uses, and property interests in the Basin to describe
undesirable results and setting preliminary conceptual MTs and MOs.
• Performing groundwater model simulations that incorporate projects and management actions
discussed in Chapter 9 to assess if the SMC are achievable.
• Conducting public meetings to present recommended preliminary conceptual minimum thresholds
and measurable objectives that are technically sound and reasonable, and receiving additional
public input. Presentations and discussion of SMCs occurred at eleven meetings in the Basin
between March 2020 and May 2021.
• Reviewing and considering public and GSC input on recommended preliminary SMCs with GSA
staff.
• GSC recommended final SMCs to GSAs for approval.
A number of alternative options for both MTs and MOs were considered for each RMS after evaluation
of the historical record of groundwater elevations at each well, assessment of trends of groundwater
elevation decline (where applicable), and input from stakeholders regarding their desired conditions.
Details regarding the specific SMCs for each Sustainability Indicator are included in the following
sections of this chapter describing each indicator.
For all applicable Sustainability Indicators except for water quality (I.e., chronic lowering of groundwater
levels, reduction of storage, land subsidence, and depletion of interconnected surface water), this GSP
uses water levels as a proxy measurement metric to assess the SMCs for each indicator. Water levels
are measured directly at each RMS. For the land subsidence Sustainability Indicator, direct
measurement of changes in land surface elevation data (InSAR data) published by DWR define the
SMCs, and water levels will be monitored in an RMS in the area of documented past subsidence to
monitor groundwater conditions (SLV-09), and to manage such that water levels do not approach the
levels observed in 1991-1992.
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Figure 8-1. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) SLV-19
Figure 8-2. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) SLV-16
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Figure 8-3. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) SLV-09
Figure 8-4. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) SLV-12
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Figure 8-5. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) EV-12
Figure 8-6. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) EV-04
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Figure 8-7. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) EV-09
Figure 8-8. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) EV-16
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Figure 8-9. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) EV-01
Figure 8-10. HYDROGRAPH, MINIMUM THRESHOLD (MT), MEASURABLE OBJECTIVE (MO), AND INTERIM
MILESTONES (IM) FOR REPRESENTATIVE MONITORING SITE (RMS) EV-11
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8.5. Chronic Lowering of Groundwater Levels Sustainability Indicator
This section of the GSP describes the SMC for the Chronic Lowering of Groundwater Levels
Sustainability Indicator. The definition of Undesirable Results is presented, and MTs and MOs are
presented for each RMS in the monitoring network.
8.5.1. Undesirable Results (§354.26)
The definition of undesirable results for the Chronic Lowering of Groundwater Indicator for the purposes
of this GSP is as follows:
The Basin will be considered to have undesirable results if two or more RMSs for water levels
within a defined area of the Basin (i.e., San Luis Valley or Edna Valley) display exceedances of
the minimum threshold groundwater elevation values for two consecutive fall measurements.
Geographically isolated exceedances (i.e., conditions in a single well) will require investigation
to determine if local or basin wide actions are required in response.
Details addressing specific MTs and MOs are presented in the following sections. A summary of MTs
and MOs used in the definition of Undesirable Conditions for the Chronic Lowering of Groundwater
Sustainability Indicator are presented along with other indicators in Table 8-1.
Table 8-1. Summary of MTs, MOs, and IMs for SLO Basin RMSs
RMS MT MO 2020 WL 2027 IM 2032 IM 2037 IM SUSTAINABILITY INDICATOR
SAN LUIS VALLEY
SLV-09 102 110 119 110 110 110 Subsidence/Water Levels
SLV-16 70 100 111 100 100 100 Water Levels/Storage
SLV-19 80 110 123 110 110 110 Water Levels/Storage
SLV-12 96 105 105 105 105 105 SW-GW Interaction/Water Levels
EDNA VALLEY
EV-09 82 164 146 150 155 160 Water Levels/Storage
EV-04 160 247 209 219 229 239 Water Levels/Storage
EV-13 172 248 215 223 231 238 Water Levels/Storage
EV-16 150 190 180 175 180 185 Water Levels/Storage
EV-01 263 314 290 314 314 314 SW-GW Interaction /Water levels
EV-11 177 227 219 227 227 227 SW-GW Interaction /Water levels
Note: All water level and interim milestone measurements refer to fall measurements.
8.5.1.1. Criteria for Establishing Undesirable Results (§354.26(b)(2))
Significant and unreasonable Chronic Lowering of Groundwater Levels in the Basin are those that:
• Reduce the ability of existing domestic wells of average depth to produce adequate water for
domestic purposes (drought resilience).
• Cause significant financial burden to those who rely on the groundwater basin.
• Interfere with other SGMA Sustainability Indicators.
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8.5.1.2. Possible Causes for Undesirable Results (§354.26(b)(1))
Conditions that could theoretically lead to an undesirable result include the following:
• Continuation of current levels of Edna Valley groundwater pumping without development of
additional water supply projects, or development of additional municipal or agricultural pumping at
significantly higher rates than are currently practiced. Maintenance of current or additional non-de
minimis pumping may result in continued decline in groundwater elevations and exceedance of the
proxy minimum threshold.
• Expansion of de minimis pumping. Adding domestic de minimis pumpers in the areas of the Basin
administered by the County may result in lower groundwater elevations, and an exceedance of the
proxy minimum threshold.
• Extensive, unanticipated drought. Minimum thresholds are established based on reasonable
anticipated future climatic conditions. Extensive, unanticipated droughts more severe than those on
record may lead to excessively low groundwater recharge and unanticipated high pumping rates
that could cause an exceedance of the proxy minimum threshold.
8.5.1.3. Effects of Undesirable Results on Beneficial Users and Land Uses (§354.26(b)(3))
The primary effects on the beneficial users occurs from allowing multiple exceedances of the MTs in a
small geographic area. Allowing two exceedances in a network of 10 RMS wells is reasonable if the
exceedances are distributed throughout the Basin. If the exceedances are clustered in a limited area, it
indicates that significant unreasonable effects are being experienced by a localized group of
landowners. Any single exceedance will require investigation to determine the significance and causes
of the observed conditions.
8.5.2. Minimum Thresholds (§354.28(c)(1))
Section §354.28(c)(1) of the SGMA regulations states that “The minimum threshold for chronic lowering
of groundwater levels shall be the groundwater elevation indicating a depletion of supply at a given
location that may lead to undesirable results”.
After the 10 RMS had been selected and discussed at public meetings, numerous alternative draft MTs
were developed based on the evaluation of historical groundwater elevations over the available period
of record (including consideration of average water levels over various time periods, long term trends,
response to the recent drought, etc.), consideration of likely future use of groundwater, well
construction data, assessment of remaining available saturated thickness, and public input from
stakeholders. The following sections present details on the development of MTs for specific RMSs in
the Basin.
8.5.2.1. Information and Methods Used for Establishing Chronic Lowering of Groundwater Level
Minimum Thresholds (§354.28(b)(1))
The primary source of data that was evaluated for the Sustainability Indicator of chronic lowering of
groundwater levels is historical groundwater elevation data collected by the County.
The information used for establishing the MOs and MTs for the chronic lowering of groundwater
levels Sustainability Indicator included:
• Historical groundwater elevation data from wells monitored by the County of San Luis Obispo.
• Depths and locations of existing wells.
• Maps of current and historical groundwater elevation data.
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• Input from stakeholders regarding significant and unreasonable conditions and desired current and
future groundwater elevations communicated during public meetings and solicitation of public
comment on various options of MTs and MOs presented in the public forum.
• Results of modeling of various project scenarios of future groundwater level conditions.
It is observed that historical trends of water levels are significantly different in the San Luis Valley and
the Edna Valley. For this reason, the approach for setting MTs is different in the San Luis Valley than in
the Edna Valley.
San Luis Valley
In the San Luis Valley, there have been no long-term water level declines in any of the monitoring wells
or RMS (Figure 5-11). All four of the RMS hydrographs in San Luis Valley (SLV-09, SLV-12, SLV-16,
and SLV-19) display a significant temporary decline in water levels in the early 1990s. This corresponds
to the period when the City and other groundwater users increased pumping from their wells during the
drought of the late 1980s and early 1990s. After 1992-1993groundwater pumping was reduced, and
water levels have been in relative equilibrium since. While seasonal fluctuations continue as would be
expected, year-to-year water levels have been essentially stable. In addition, the water budget analysis
presented in Chapter 6 (Water Budget) documents that the San Luis Valley portion of the Basin is in
surplus. City staff and City GSA participants have communicated their desire to maintain flexibility to
develop groundwater in the future to potentially augment their water supply portfolio to supply the public
with drinking water in their service area. Therefore, the City wishes to avoid the definition of MTs that
would prevent future development of groundwater. For this reason, MTs for chronic lowering of
groundwater levels at RMSs in the San Luis Valley that have not experienced any historical declines
are set 10 to 20 feet lower than previously observed low water levels, to allow for potential future
groundwater development by the City. (An exception to this approach is made for RMS SLV-12, due to
its location proximate to Prefumo and San Luis Obispo Creeks, and its additional use as an RMS for
depletion of interconnected surface water; the MT for this RMS is set at the historically observed lowest
water level.) The GSAs will coordinate during GSP implementation to ensure such future development
does not lead to undesirable results in the Basin. The GSAs considered historical groundwater
elevations, available saturated thickness, proximity of nearby wells, and general hydrogeologic
judgement when setting these MTs. Figure 7-1 displays the locations of RMSs for water levels and
groundwater in storage in the Basin. MTs are presented in Table 8-1. Figure 8-1 through Figure 8-4
present historically observed water levels in the four RMS in the San Luis Valley portion of the basin,
and the MTs set at these wells.
Edna Valley
In Edna Valley, by contrast, four wells show water level declines over the past 20-30 years (EV-04, EV-
09, EV-13, and EV-16). Various alternative approaches were considered to establish MTs including
designation of current water levels, water levels higher than current water levels, historical low water
levels (usually those that occurred in 2015 at the end of the recent drought), and levels lower than the
historical low. Not all of the Edna Valley hydrographs show the same trends. Evaluations were made
allowing consideration for the human right to water by de minimis users in the Basin, as well as
accommodations for agricultural stakeholders in the Basin. Each hydrograph has unique characteristics
depending on the local hydrogeologic setting in the immediate vicinity of the well, and this leads to the
consideration of different definitions of MTs for different wells, as discussed below.
RMS EV-13, EV-04, and EV-09 display declining water levels over the past 20-25 years, with historical
low elevations occurring around Fall 2015 at the end of the recent drought, followed by some degree of
recovery since then. The hydrographs for all three of these wells display recovery of water levels since
then (Figure 8-5, Figure 8-6, Figure 8-7). Agricultural stakeholders in the Edna Valley communicated
concern that setting the MT at the 2015 water levels in these wells would not provide them adequate
operational flexibility to protect their long investments in the production of agriculture in the area. De
minimis users communicated concern about lowered water levels affecting their ability to pump water
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for their domestic use. At the April 7, 2021 GSC meeting the agricultural stakeholders requested
consideration of an MT for these three RMSs to be defined 10 feet lower than 2015 drought water level.
They communicated their desire for a slightly greater factor of safety for their operations and
investments in the event of another drought during the planning horizon of SGMA activities. Members
of the GSC were polled, and a majority of the GSC members agreed that this was a reasonable request
to protect the significant investments in vineyard agriculture in the valley and would not be considered
an undesirable condition in this part of Edna Valley. Therefore, for these three wells, the MTs were
defined to be 10 feet lower than the historical low groundwater elevation observed in 2015, at the height
of the recent drought. (The measurement for EV-04 represents the Spring 2015 measurement; the Fall
measurement was not collected. It is assumed that the Fall measurement would be lower than the
Spring measurement, so the MT is set slightly lower than the Spring measurement.)
In order to assess the risk on private domestic well owners of having groundwater elevations lower than
recent drought low levels, an analysis was performed to evaluate potential water level of MTs
compared to the depths of private domestic wells identified in County data. The basin-wide Fall 2015
groundwater elevations were mapped and compared to the total depths of domestic wells in the
County’s well permitting database. Then the 2015 groundwater elevation arrays were reduced by 10
feet, 25 feet, and 50 feet, to project conditions of lowered water levels. These revised lowered
groundwater elevations were then compared to the total depths of the identified domestic wells. If in
any of these comparison evaluations, the water level was below the total depth of a domestic well, that
well was marked as “dry” in the analysis and is summarized in Table 8-2 below. The objective of this
analysis is to assess the level of impact to domestic wells associated with water level reduction of these
magnitudes. This is not intended to be a definitive analysis, given that depth and location data of the
domestic wells are imperfect (many wells in the database are placed on the same point location, an
artifact of the practice of assigning locations to the center of a section if better information is not
available.) However, it is intended to provide a general indication of how many additional domestic
wells might be impacted if water levels were decreased.
For the analysis of 2015 water levels, the data indicated 15 wells as “dry”, out of 155 wells in the
database. (In reality, anecdotal information indicates local knowledge of three to four known wells that
needed to be replaced or stopped being used during the recent drought in Edna Valley). For water
levels 10 feet lower than 2015 water levels, no additional domestic wells in the County database were
indicated as “dry”, beyond those identified as dry using 2015 water levels. For water levels 25 feet
lower than 2015 water levels, 29 wells were identified as “dry”, an increase of 14 additional wells. For
water levels 50 feet lower than 2015 water levels, 40 wells were identified as “dry”, an increase of 25
additional wells. This evaluation was performed to give a relative idea as to the potential impact on
domestic wells of lowered water levels. The conclusion of this analysis was that water levels 25 feet
and 50 feet lower than the drought minimums would result in an unacceptable condition in which the
number of domestic supply wells at risk of adverse operating conditions was too high. Therefore, the
conclusion of this analysis is that lowering water levels 25 to 50 feet below 2015 conditions constitutes
an unreasonable risk to domestic well owners, but that water levels 10 feet below the 2015 drought
levels constitutes an acceptable level of risk for all stakeholders, and the definition of MTs for wells in
this area 10 feet lower than 2015 levels does not constitute unreasonable or undesirable conditions.
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Table 8-2. Groundwater Levels in Domestic Wells During the 2015 Drought (Edna Valley)
GROUNDWATER LEVEL CONDITION TOTAL WELLS NUMBER OF “DRY” WELLS
2015 Groundwater Levels 155 15
2015 Groundwater Levels -10 feet 155 15
2015 Groundwater Levels -25 feet 155 29
2015 Groundwater Levels -50 feet 155 40
RMS EV-16 displays a relatively steady decline in water levels of about 3.25 feet/year at the Varian
Ranch Mutual Water Company (VRMWC) service area since the year 2000. The 2011-2015 drought is
not apparent in this hydrograph as a period of historical low groundwater elevations. For this well, the
MT was set at an elevation of 150 feet, which is lower than current groundwater elevations of about 180
feet, to allow for the various stakeholders (both agricultural interests and mutual water companies) in
the area to implement projects to slow and stabilize the observed water level declines (Figure 8-10).
Consideration of the recent rate of groundwater elevation decline, amount of available saturated
thickness, and hydrogeologic judgement regarding the amount of time likely required to mitigate this
trend, were used in defining the MTs at this well. (VRMWC owns property and wells in the adjacent
Arroyo Grande sub-basin of the Santa Maria Valley Groundwater Basin, which may be useful in
reversing this trend, and will be discussed in Chapter 9 (Projects and Management Actions)).
8.5.2.2. Relationship between Individual Minimum Thresholds and Relationship to Other
Sustainability Indicators (§354.28(b)(2))
Section 354.28 of the SGMA regulations requires that the description of all MTs include a discussion of
the relationship between the MTs for each Sustainability Indicator. In the SMC Best Management
Practices document (DWR, 2017), DWR has clarified this requirement. First, the GSP must describe
the relationship between each Sustainability Indicator’s MT by describing why or how a water level MT
set at a particular RMS is similar to or different to water level thresholds in a nearby RMS. Second, the
GSP must describe the relationship between the selected MT and MTs for other Sustainability
Indicators; in other words, describe how (for example) a water level minimum threshold would not
trigger an undesirable result for land subsidence.
Groundwater elevation MTs are derived from examination of the historical record reflected in
hydrographs at the RMS. They were tested for achievability through model simulations (as described in
Chapter 9 (Projects and Management Actions). Because the MOs are largely based on observed
historical groundwater conditions, the minimum thresholds derived from these objectives are not
expected to conflict with each other. Groundwater elevation MTs can theoretically influence other
Sustainability Indicators.
Examples are listed below:
1. Change in groundwater storage. Changes in groundwater elevations are directly correlated to
changes in the amount of stored groundwater. Pumping at or less than the sustainable yield will
maintain or raise average groundwater elevations in the Basin. The groundwater elevation MTs are
set to establish a minimum elevation that will not lead to undesirable conditions, and that are
acceptable to the stakeholders in the area. Therefore, if the groundwater elevation MTs are met,
they will not result in long term significant or unreasonable changes in groundwater storage.
2. Subsidence. A significant and unreasonable condition for subsidence is permanent pumping-
induced subsidence that substantially interferes with surface land use. One cause for subsidence is
dewatering and compaction of clay-or peat-rich sediments in response to lowered groundwater
levels. As discussed in Chapter 5 (Groundwater Conditions), significant subsidence was observed
along Los Osos Valley Road in the early 1990s, which resulted in the City paying for significant
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damage to affected local businesses. No observed subsidence has been reported in the Edna
Valley. If MTs are maintained higher than the historically low water levels that were observed during
the subsidence episode, this will minimize the risk of additional subsidence in the Basin. The
groundwater elevation MT in RMS SLV-09 along Los Osos Valley Road is set 15 feet higher than
the historically low groundwater elevation observed in the early 1990s. Therefore, if this MT is met,
it should minimize the risk of further subsidence along Los Osos Valley Road. No subsidence MTs
based on water levels are established in Edna Valley (the actual MTs for subsidence will be based
on InSAR data provided annually by DWR, and are discussed later in this chapter). Should new
subsidence be observed due to lower groundwater elevations, the groundwater elevation MTs will
be raised to mitigate this subsidence and avoid future subsidence.
3. Degraded water quality. Protecting groundwater quality is critically important to all groundwater
users in the Basin, particularly for drinking water and agricultural uses. Maintaining groundwater
levels protects against degradation of water quality or exceeding regulatory limits for constituents of
concern in supply wells due to actions proposed in the GSP. Water quality in the Basin could
theoretically be affected through two processes:
− Low groundwater elevations in an area could theoretically cause deeper, poorer-quality
groundwater to flow upward from bedrock into existing supply wells. Should groundwater
quality degrade due to lowered groundwater elevations, the groundwater elevation MTs may be
raised to avoid this degradation. However, since MTs are set to avoid significant declines of
groundwater elevations below historically observed levels, and the historical low water levels
did not result in water quality degradation, this is not expected to occur.
− Changes in groundwater elevation due to actions implemented to achieve sustainability could
change groundwater gradients, which could cause poor quality groundwater to flow towards
supply wells that would not have otherwise been impacted. However, MTs are established so
as not to change the basin patterns or gradients of groundwater flow, so this is not expected to
occur in the Basin.
4. Depletion of Interconnected Surface Water. Groundwater levels measured at RMSs (SLV-
12, EV-01, EV-11) will serve as a proxy for depletion of interconnected surface water. In addition,
stream flow gages along SLO Creek will continue to measure surface water conditions in San Luis
Valley, and proposed stream gages along Corral de Piedras Creek will serve to generate
information on surface water inflow and outflow in Edna Valley, allowing for direct measurement of
surface water gains and losses to the groundwater systems based on future hydrologic and
pumping conditions in the Basin. However, MTs along the Creeks are defined at levels designed to
avoid significant water declines in these areas, with the goal of minimizing any potential significant
depletion of interconnected surface water flows.
5. Seawater intrusion. This Sustainability Indicator is not applicable to this Groundwater Basin.
8.5.2.3. Effect of Minimum Thresholds on Neighboring Basins (§354.28(b)(3))
Two neighboring groundwater basins share a boundary with the San Luis Obispo Basin; the Los Osos
Basin to the northwest, and the Arroyo Grande Subbasin of the Santa Maria Valley Groundwater Basin
to the southeast. The shared boundary with both of these basins is not extensive, and the
Hydrogeologic Conceptual Model (HCM) posits that a groundwater divide separates the groundwater
between those basins and the San Luis Obispo Basin. In the San Luis Valley there have been no
trends indicating groundwater declines that would affect the Los Osos Basin. In Edna Valley the areas
with observed declines are over two miles downgradient from the Arroyo Grande Subbasin boundary. It
is not anticipated that actions associated with the GSP will have any significant impact on either the Los
Osos Basin or the Arroyo Grande Subbasin.
Additionally, the SLO Basin GSAs have developed a cooperative working relationship with both the Los
Osos Groundwater Basin – Basin Management Committee and the GSAs working in the Arroyo Grande
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Subbasin. Hydrogeologic conditions near the basin boundaries will be monitored, and any issues
potentially affecting those basins will be communicated.
8.5.2.4. Effects of Minimum Thresholds on Beneficial Users and Land Uses (§354.28(b)(4))
Agricultural land uses and users
The agricultural stakeholders in the Edna Valley have maintained an active role during the development
of this GSP. The groundwater elevation MTs place a practical limit on the acceptable lowering of
groundwater levels in the Basin, thus conceptually restricting the current level of agriculture in the
region without projects to supplement water supply to the Basin, or management actions to reduce
current pumping. In the absence of other mitigating measures, this has been the practical effect of
potentially limiting the amount of groundwater pumping in the Basin. Limiting the amount of
groundwater pumping could limit the additional amount and type of crops that can be grown in the
Basin, which could result in a reduction of economic viability for some properties. The groundwater
elevation MTs could therefore limit the Basin’s agricultural economy.
This could have various effects on beneficial users and land uses:
• There could be an economic impact to agricultural employees and suppliers of agricultural
production products and materials, as well as the tourism industry supported by the wineries in the
Basin. Many parts of the local economy rely on a vibrant agricultural industry and they too will be
hurt proportional to the losses imparted to agricultural businesses.
• Growth of city, county, and state tax rolls could be slowed or reduced due to the limitations imposed
on agricultural growth and associated activities.
However, it should be noted that projects and management actions discussed in Chapter 9 will be
pursued to allow for alternatives to reductions in agricultural pumping.
Urban land uses and users
The groundwater elevation MTs effectively limit the amount of groundwater pumping in the Basin.
However, the MTs for the RMSs in the San Luis Valley are established below currently observed
groundwater elevations to allow for reasonable future development of groundwater for potable supply to
City residents. If groundwater elevations experience significant and sustained decline in the immediate
vicinity of SLO Creek, this could potentially result in less groundwater discharge to the creek due to
areas of interconnected groundwater and surface water. Impacts to stream flows will be monitored with
the augmentation of current data collection programs in San Luis Valley, and the addition of new
stream gauges in the Basin.
Domestic land uses and users
The groundwater elevation MTs are established to protect as many domestic wells as possible.
Therefore, the MTs will likely have an overall beneficial effect on existing domestic land uses by
protecting the ability to pump from domestic wells within the Edna Valley portion of the Basin. However,
limited saturated thickness in some localized areas in the Basin of the shallowest domestic wells may
require owners to drill deeper wells if water levels are decreased. Additionally, the groundwater
elevation MTs may limit the increase of non-de minimis groundwater use in order to limit future declines
in groundwater levels caused by non-de minimis domestic pumping.
Ecological land uses and users
Groundwater elevation MTs protect the groundwater resource and the existing ecological habitats that
rely upon it because they are set to avoid significant and unreasonable declines in groundwater levels.
As noted above, groundwater level MTs may limit increases in non-de minimis and agricultural
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groundwater uses. Ecological land uses and users may benefit by this reduction in non-de minimis and
agricultural groundwater uses.
8.5.2.5. Relevant Federal, State, or Local Standards §354.28(b)(5)
No Federal, State, or local standards exist for chronic lowering of groundwater elevations.
8.5.2.6. Method for Quantitative Measurement of Minimum Thresholds §354.28(b)(6)
Conformance of Basin conditions to the established groundwater elevation MTs will be assessed
through direct measurement of water levels from existing RMS. During planned 5-year revisions to this
GSP, additional RMS may be stablished for the SMC evaluations, and direct water level measurements
at these wells will be the method for quantitative measurement of MTs in the future. Groundwater level
monitoring will be conducted in accordance with the monitoring plan outlined in Chapter 7 (Monitoring
Network) and will comply with the requirements of the technical and reporting standards included in
SGMA regulations.
As noted in Chapter 7 (Monitoring Network), the existing groundwater monitoring network in the Basin
includes 12 wells. The GSP monitoring network developed in Chapter 7 increases the groundwater
monitoring network to 40 wells to be used for water level measurements.
8.5.3. Measurable Objectives §354.30(a)(g)
The MOs for chronic lowering of groundwater levels represent target groundwater elevations that are
established to achieve the sustainability goal by 2042. MOs are groundwater levels established at each
RMS. MO groundwater levels are higher than MT groundwater levels and provide operational flexibility
above MTs to ensure that the Basin be sustainably managed over a range of climate and hydrologic
variability. MOs are subject to change by the GSAs after GSP adoption as new information and
hydrologic data become available.
8.5.3.1. Information and Methods Used for Establishing Chronic Lowering of Groundwater Level
Measurable Objectives §354.30(b)
Preliminary MOs were established based on historical groundwater level data, along with input and
desired future groundwater levels from domestic groundwater users, agricultural interests,
environmental interests, and other Basin stakeholders. The input and desired conditions were used to
formulate a range of alternative MO options, which were discussed by the GSAs and the GSC. Final
MOs were voted on by the GSC members to recommend to the GSAs for approval as part of the full
GSP.
Preliminary MOs were established based on historical groundwater level data and input regarding
desired future groundwater levels from domestic groundwater users, agricultural interests,
environmental interests, and other public stakeholders. The input and desired conditions were used to
formulate a range of conceptual MO scenarios. These scenarios were evaluated using the groundwater
model developed during this GSP preparation to project the effects of future Basin operation and to
select measurable objectives for the GSP.
As previously discussed in Chapter 5 (Groundwater Conditions) and Section 8.4.2, groundwater
conditions in San Luis Valley and Edna Valley are significantly different. Therefore, as with the MTs, the
approach to the MOs is different in the two valleys.
San Luis Valley
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In San Luis Valley, definition of MOs within the historically observed range of groundwater elevations,
but about 20 feet lower than fall 2020 water levels, was considered to preserve the City’s desired
flexibility to resume reasonable and managed groundwater use to augment its potable water supply
portfolio to serve its customer base. MOs for SLV-09, SLV-16, SLV-19, and SLV-12 were set within the
range of historical data, but lower than current water levels (Table 8-1) (Figure 8-1 through Figure 8-4).
Edna Valley
In Edna Valley, if recovery from drought levels is evident (EV-04, EV-09, EV-12), MOs were set at the
high-water levels observed immediately prior to the drought (Spring 2011, in most cases) (Figure 8-5
through Figure 8-7). The rationale for this selection was that if the antecedent conditions before the
recent drought are replicated, and no significant new groundwater pumping is occurring in the Basin,
then the water level declines observed from 2012-2015 in the Basin will not be significantly exceeded in
a similar drought. To the extent that groundwater elevations can recover to levels higher than the 2011
levels, the Basin will be more resilient to drought.
For the wells in Edna Valley to monitor surface water/groundwater conditions (EV-01, EV-11), MOs
were set at approximately the average of seasonal high water levels over the period of record (Figure
8-8, Figure 8-91). RMS EV-01 shows that similar high water levels occur with regularity during wet
periods, going back to the late 1950s. Therefore, this level was selected for the MOs for these wells
because they are the naturally occurring water levels that have been observed for decades.
The MO for RMS EV-16, located in the southeast area of Tiffany Ranch Road near the upgradient
extent of the Basin, was set slightly below current water levels (Figure 8-7). This approach is to try to
prevent further significant reductions in water levels at this location, since it does not appear to have
experienced any recovery of water levels since 2015, and needs to maintain sufficient saturated
thickness to sustain production for the service area.
Since there is data uncertainty due to significant data gaps, MTs and MOs will be reviewed every 5
years during GSP updates throughout the twenty-year SGMA implementation horizon to assess if the
RMSs and the assigned MOs and MTs remain protective of sustainable conditions in the Basin. MTs
and MOs may be modified in the future as hydrogeologic conditions are monitored through the
implementation phase of SGMA.
8.5.3.2. Interim Milestones §354.30(a)(e)
Interim milestones (IMs) are required to be included in the GSP. IMs at 5-year intervals for the MOs
established at each RMS are included on Table 8-1.
Preliminary IMs were developed for the 10 RMS established for the basin. In San Luis Valley, because
there have been no historic declines in water levels, IMs were simply defined as being numerically
equivalent to the MO throughout the SGMA period. In Edna Valley, Interim milestones were generally
selected to define a smooth linear increase in water levels between the observed groundwater
elevation at the RMS in 2020, and the MO as presented in Table 8-1.
IMs may be adjusted at any time during the SGMA timeline. It is expected that they will be reconsidered
at 5-year intervals when the Basin GSP is revised and updated. The monitoring of basin conditions
during the initial 5-year period will provide good indicators on if the IMs are close to being met. Failure
to meet IMs is not in and of itself an indication of undesired conditions but is meant to provide
information determining whether the 20-year goals are on track to being achieved. Alternative projects
and management actions may be considered or pursued if the IMs are not being met. Table 8-1
summarizes the interim milestones for the RMS.
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8.6. Reduction of Groundwater Storage Sustainability Indicator
§354.28(c)(2)
8.6.1. Undesirable Results
As per §354.26 of the SGMA regulations, locally defined significant and unreasonable conditions were
assessed based on review of historical groundwater data and stakeholder input during numerous public
meetings, analysis of available data, and discussions with GSA staff. It is recognized based on well-
established hydrogeologic principles that the Reduction of Groundwater Storage Sustainability Indicator
is directly correlated to the lowering of water level Sustainability Indicator.
Significant and unreasonable changes in groundwater storage in the Basin are those that:
• Lead to long-term reduction in groundwater storage.
• Interfere with other Sustainability Indicators.
Assessment of groundwater in storage will initially be evaluated with the same RMS and associated
water level MTs and MOs as the chronic lowering of groundwater levels sustainability criteria. As
additional data is collected in the monitoring network described in Chapter 7 (Monitoring Network), new
RMS may be established, and appropriate SMCs determined by the GSAs.
For the purposes of this GSP, the definition of undesired conditions for the Reduction of Groundwater
Storage Sustainability Indicator is as follows:
The Basin will be considered to have undesirable results if two or more than two RMS for groundwater
storage within a defined area of the Basin (I.e., San Luis Valley or Edna Valley) display exceedances of
the MTs for two consecutive Fall measurements. Geographically isolated exceedances will require
investigation to determine if local or basin wide actions are required in response.
8.6.1.1. Criteria for Establishing Undesirable Results §354.2(b)(2)
Significant and unreasonable Reduction of Groundwater Storage in the Basin are those that:
• Reduce the ability of existing domestic wells of average depth to produce adequate water for
domestic purposes (drought resilience).
• Cause significant financial burden to those who rely on the groundwater basin.
• Interfere with other SGMA Sustainability Indicators.
8.6.1.2. Potential Causes of Undesirable Results §354.2(b)(1)
Conditions that could theoretically lead to an undesirable result include the following:
• Continuation of current levels of Edna Valley pumpage without development of additional water
supply projects, or development of additional municipal or agricultural pumping at significantly
higher rates than are currently practiced. Maintenance of current or additional non-de minimis
pumping may result in continued decline in groundwater elevations and exceedance of the proxy
minimum threshold.
• Expansion of de minimis pumping. Adding domestic de minimis pumpers in the areas of the Basin
administered by the County may result in lower groundwater elevations, and an exceedance of the
proxy minimum threshold.
• Extensive, unanticipated drought. Minimum thresholds are established based on reasonable
anticipated future climatic conditions. Extensive, unanticipated droughts more severe than those on
record may lead to excessively low groundwater recharge and unanticipated high pumping rates
that could cause an exceedance of the proxy minimum threshold.
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8.6.1.3. Effects of Undesirable Results on Beneficial Users and Land Uses §354.2(b)(3)
The effects of these undesirable results on the beneficial users and uses are the same effects as those
discussed for the Chronic Lowering of Groundwater Levels Sustainability Indicator.
The primary effects on the beneficial users (§354.26 (b)(3)) occurs from allowing multiple exceedances
of the MTs in a small geographic are. Allowing a minimum of two exceedances in a network of 10 RMS
wells is reasonable if the exceedances are distributed throughout the Basin. If the exceedances are
clustered in a limited area, it indicates that significant unreasonable effects are being experienced by a
localized group of landowners. Any exceedances will require investigation to determine the significance
and causes of the observed conditions.
8.6.2. Minimum Thresholds §354.28(c)(2)
Section §354.28(c)(2) of the SGMA regulations states that “The minimum threshold for reduction of
groundwater storage shall be a total volume of groundwater that can be withdrawn from the basin
without causing conditions that may lead to undesirable results. Minimum thresholds for reduction of
groundwater storage shall be supported by the sustainable yield of the basin, calculated based on
historical trends, water year type, and projected water use in the basin.”
This GSP will monitor changes in groundwater level at the RMSs as a proxy for the change in
groundwater storage metric. As allowed in §354.36(b)(1) of the SGMA regulations, groundwater
elevation data at the RMS will be reported annually as a proxy to track changes in the amount of
groundwater in storage.
Based on well-established hydrogeologic principles, stable groundwater elevations maintained above
the MTs will limit depletion of groundwater from storage. Therefore, using groundwater elevations as a
proxy, the MT is that the groundwater surface elevation averaged across all the wells in the
groundwater level monitoring network will remain stable above the MT for chronic lowering of
groundwater levels.
In accordance with the SGMA regulation cited above, GSAs have the option of defining the MT metric
as a calculated volume of groundwater in storage. As discussed in Chapter 6 (Water Budget), separate
estimates for total groundwater in storage were generated for the San Luis Valley and Edna Valley
using methodology described in Chapter 6 (Water Budget) and shown in Figure 6-21. After the
monitoring network described in Chapter 7 is established, and several years of water level data have
been collected, a robust and repeatable method for directly quantifying groundwater in storage using
the monitoring network may be developed and finalized. It is possible that in future versions of the GSP,
the MT may be changed to be defined as the directly calculated amount of groundwater in storage.
However, for the current 5-year implementation period, water levels at the RMS will be used as a proxy
for the groundwater in storage Sustainability Indicator.
8.6.2.1. Information and Methods for Establishing Reduction of Storage Minimum Thresholds
§354.28(b)(1)
As with the chronic reduction of groundwater levels Sustainability Indicator, the primary source of data
that was evaluated for the Sustainability Indicator of reduction of groundwater storage is historical
groundwater elevation data maintained by the County.
The information used for establishing the MOs and MTs for the chronic lowering of groundwater
levels Sustainability Indicator included:
• Historical groundwater elevation data from wells monitored by the County of San Luis Obispo.
• Depths and locations of existing wells.
• Maps of current and historical groundwater elevation data.
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• Input from stakeholders regarding significant and unreasonable conditions and desired current and
future groundwater elevations communicated during public meetings and solicitation of public
comment on various options of MTs and MOs presented in the public forum.
• Results of modeling various project scenarios of future groundwater level conditions.
Storage MTs will be measured by collecting water level measurements at the RMS sites in the
monitoring network. The monitoring network and protocols used to measure groundwater elevations at
the RMS are presented in Chapter 7 (Monitoring Network). The Water Level Monitoring Network is
presented in Figure 7-1. This data will be used to monitor groundwater elevations and assess changes
in groundwater storage.
8.6.2.2. Relationship between Individual Minimum Thresholds and Other Sustainability Indicators
§354.28(b)(2)
The MTs for reduction in groundwater storage is a single value of average groundwater elevation over
the entire Basin. Therefore, the concept of potential conflict between MTs at different locations in the
Basin is not applicable. The reduction in groundwater storage MT could influence other Sustainability
Indicators.
The reduction in groundwater storage MT was selected to avoid undesirable results for other
Sustainability Indicators, as outlined below:
• Chronic lowering of groundwater levels. Because groundwater elevations will be used as a proxy for
estimating groundwater pumping and changes in groundwater storage, the reduction in
groundwater storage would not cause undesirable results for this Sustainability Indicator.
• Seawater intrusion. This Sustainability Indicator is not applicable to this Basin.
• Degraded water quality. The minimum threshold proxy of stable groundwater levels is not expected
to lead to a degradation of groundwater quality.
• Subsidence. Because future average groundwater levels will be stable, they will not induce any
additional subsidence.
• Depletion of interconnected surface waters. Groundwater levels measured at representative
monitoring wells (SLV-12, EV-01, EV-11) will serve as a proxy for depletion of interconnected
surface water. In addition, stream flow gages along SLO Creek will continue to measure surface
water conditions in San Luis Valley, and proposed stream gages along Corral de Piedras Creek will
serve to generate information on surface water inflow and outflow in Edna Valley, allowing for direct
measurement of surface water gains and losses to the groundwater systems based on future
hydrologic and pumping conditions in the Basin. However, MTs along the creeks are defined to
avoid significant water declines in these areas, with the goal of minimizing any potential significant
depletion of interconnected surface water flows.
8.6.2.3. Effects of Minimum Thresholds on Neighboring Basins §354.28(b)(3)
Two neighboring groundwater basins share a boundary with the SLO Basin; the Los Osos Basin to the
northwest, and the Arroyo Grande sub-basin of the Santa Maria Valley Groundwater Basin to the
southeast. Neither of these shared boundaries are extensive, and the HCM posits that a groundwater
divide separates the groundwater between them and the SLO Basin. In the San Luis Valley there have
been no trends indicating groundwater declines that would affect the Los Osos Basin. In Edna Valley
the areas with observed declines are one to two miles from the Arroyo Grande Basin boundary in a
downgradient direction. It is not anticipated that actions associated with the GSP will have any
significant impact on either the Los Osos Basin or the Arroyo Grande Basin.
The SLO Basin GSAs have developed a cooperative working relationship with the Los Osos
Groundwater Basin – Basin Management Committee and the GSAs working in the Arroyo Grande
Subbasin. Groundwater conditions near the borders with these basins will be monitored and shared.
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8.6.2.4. Effects of Minimum Thresholds on Beneficial Uses and Users §354.28(b)(4)
The MT for reduction in groundwater storage will maintain stable average groundwater elevations but
may require a reduction in the amount of groundwater pumping in the Basin, or development of sources
of supplemental water as discussed in Chapter 9 (Projects and Management Actions). Reducing
pumping may impact the beneficial uses and users of groundwater in the Basin.
The practical effect of this GSP for protecting against the reduction in groundwater storage undesirable
result is that it encourages minimal long-term net change in groundwater elevations and storage.
Seasonal and drought cycle variations are expected, but during average conditions and over the long-
term, beneficial users will have access to adequate volumes of water from the aquifer to service the
needs of all water use sectors. The beneficial users of groundwater are protected from undesirable
results.
Agricultural Land Uses and Users
The MT for reduction in groundwater storage may limit or reduce non-de minimis production in the
Basin by reducing the amount of available water. The practical effect of these MTs on agricultural users
is that current levels of agricultural pumping may not be sustainable without development of additional
sources of water to the Basin. Owners of undeveloped agricultural lands that are currently not irrigated
may be particularly impacted because the additional groundwater pumping needed to irrigate these
lands could increase the Basin pumping beyond the sustainable yield, violating the MT. Existing
agricultural operations may also be limited in their use of more water-intensive crops, expansion of
existing irrigated lands, and by periods of extended drought that decrease the quantity of water
naturally returning to the basin.
Urban Land Uses and Users
Potential future increases of groundwater pumping in the City of San Luis Obispo could decrease the
cost of water for municipal users in the City, because groundwater may be the cheapest water supply
alternative. However, in order to avoid undesirable results, the City is unlikely to pursue groundwater
pumping in the quantity that it did during the 1980s-90s drought without the use of groundwater
recharge.
Domestic Land Uses and Users
Existing domestic groundwater users may generally benefit from this MT. Many domestic groundwater
users are de-minimis users whose pumping may not be restricted by the projects and management
actions adopted in this GSP. By restricting the amount of groundwater that is pumped from the Basin,
the de-minimis users would be protected from overdraft that could impact their ability to pump
groundwater or require them to drill deeper wells.
Ecological Land Uses and Users
Groundwater dependent ecosystems would generally benefit from this MT. Maintaining groundwater
levels close to current levels keeps groundwater supplies near present levels, which will continue to
support groundwater dependent ecosystems.
8.6.2.5. Relation to State, Federal, or Local Standards §354.28(b)(5)
No federal, state, or local standards exist for reductions in groundwater storage.
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8.6.2.6. Methods for Quantitative Measurement of Minimum Thresholds §354.28(b)(6)
The quantitative metric for assessing compliance with the reduction in groundwater storage MT is
monitoring groundwater elevations. The approach for quantitatively evaluating compliance with the MT
for reduction in groundwater storage will be based on evaluating groundwater elevations semi-annually.
All groundwater elevations collected from the groundwater level monitoring network will be analyzed
and averaged.
In the future, after the monitoring network is established and multiple years of data are available for
analysis, a robust and repeatable method for calculating groundwater in storage utilizing the monitoring
well network may be developed and finalized. At that time, the metric for defining the SMC of reduction
of groundwater in storage may possibly be changed to direct calculation of groundwater in storage for
the two areas of the basin, but this will be reviewed after additional data has been collected during the
implementation phase of the GSP.
8.6.3. Measurable Objectives §354.30(a)(g)
The change in storage Sustainability Indicator uses groundwater levels as a proxy for direct calculation
of groundwater in storage. The same MTs and MOs are used as are defined in the chronic lowering of
groundwater level indicator to protect against significant and unreasonable reduction in groundwater
storage.
8.6.3.1. Information and Methods Used for Establishing Reduction of Groundwater Storage
Measurable Objectives §354.30(b)
Input from stakeholders suggested that they would prefer more groundwater in storage to maintain
resiliency against future droughts. Therefore, the conservative approach of simply maintaining stable
groundwater levels was adopted for the MO. MOs for the RMS are identical to the MOs for the chronic
lowering of groundwater elevations MOs (Table 8-1).
8.6.3.2. Interim Milestones §354.30(a)(e)
Interim milestones for groundwater storage are the same as those established for chronic lowering of
groundwater elevations. Achieving the groundwater elevation interim milestones will also eliminate long
term reductions in groundwater in storage. Interim milestones are included on Table 8-1.
8.7. Seawater Intrusion Sustainability Indicator §354.28(c)(3)
This Sustainability Indicator does not apply to the Basin since the Basin is not a coastal basin.
8.8. Degradation of Groundwater Quality Sustainability Indicator
§354.28(c)(4)
The purpose of the Degraded Water Quality Indicator in SGMA is to prevent any degradation in
groundwater quality as a result of groundwater management under the GSP. SGMA is not intended to
serve as impetus to improve water quality within the Basin. The Basin’s current water quality is not
considered degraded. For these reasons, the SMC in this section are set to maintain current conditions
in the Basin, protecting from potential degradation as a result of groundwater management under this
GSP.
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8.8.1. Undesirable Results §354.26(a)(d)
Section §354.28(c)(2) of the SGMA regulations states that “The minimum threshold shall be based on
the number of supply wells, a volume of water, or a location of an isocontour that exceeds
concentrations of constituents determined by the Agency to be of concern for the basin.”
By SGMA regulations, the Degraded Groundwater Quality undesirable result is a quantitative
combination of groundwater quality minimum threshold exceedances. The undesirable results for the
Degraded Water Quality Sustainability Indicator as defined for the purposes of this GSP are as follows:
The Basin will be considered to have Undesirable Results if, for any 5-year GSP Update period, an
increase in groundwater quality minimum threshold exceedances is observed at 20 percent or more of
the RMSs in the Basin, as a result of groundwater management implemented as part of the GSP.
The undesirable conditions for degraded water quality in the Basin are based on the goal of fewer than
20% of the RMSs for water quality exceedances that can occur as a result of GSP groundwater
management activities over the next 5-year management period. Based on the current number of wells
in the existing water quality monitoring network described in Chapter 7, the percentage defined equates
to a maximum of two wells that can exceed the minimum thresholds.
Specifics regarding the definition of the MTs used in defining the Undesirable Results are detailed in
the following sections. A summary of the MTs defined for the Degradation of Water Quality
Sustainability Indicator are presented in Table 8-3.
Table 8-3. San Luis Obispo Valley Basin Groundwater Basin Water Quality Minimum Thresholds
ID TDS MT (PPM) NO3 MT (PPM) ARSENIC MT (PPB) TCE, PCE (PPB)
WQ-1 900 10 10 5
WQ-2 900 10 10 5
WQ-3 900 10 10 5
WQ-4 900 10 10 5
WQ-5 900 10 10 5
WQ-6 900 10 10 5
WQ-7 900 10 10 5
WQ-8 900 10 10 5
WQ-9 900 10 10 5
8.8.1.1. Criteria for Establishing Undesirable Results §354.26(b)(2)
Criteria used to establish the Undesirable Results for Degraded Water Quality Sustainability
Indicator are observed water quality data and trends that:
• Reduce capacity of public water supply systems or unreasonably increase costs for public or private
water supply.
• Reduce crop production.
• Result in constituent concentrations above regulatory primary drinking water standards at supply
wells.
• Results in constituent concentrations above the RWQCB Basin Objectives for secondary standards
(TDS)
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8.8.1.2. Potential Causes for Undesirable Results §354.26(b)(1)
Conditions that may lead to an undesirable result include the following:
• Changes to Basin Pumping: If the location and rates of groundwater pumping change as a
result of projects implemented under the GSP, these changes could cause movement of one of the
constituents of concern towards a supply well at concentrations that exceed relevant water quality
standards.
• Groundwater Recharge: Active recharge with imported water or captured runoff could cause
movement of one of the constituents of concern towards a supply well in concentrations that exceed
relevant water quality standards.
• Recharge of Poor-Quality Water: Recharging the Basin with water that exceeds a primary or
secondary MCL or concentration that reduces crop production could lead to an undesirable result.
However, permitting requirements generally preclude this circumstance.
8.8.1.3. Effects of Undesirable Results on Beneficial Users and Land Uses §354.26(b)(3)
As defined in this GSP, undesirable results are established to prevent degradation of water quality
within the Basin prior to the implementation of any actions inherent in the management of groundwater
in the Basin.
This limits the potential impacts of undesirable water quality on beneficial users in the Basin.
However, potential effects of undesirable results include:
• Increased water treatment costs for public or private supply wells
• Reduced agricultural production
8.8.2. Minimum Thresholds §354.28(c)(4)
8.8.2.1. Effects of Undesirable Results on Beneficial Users and Land Uses §354.28(b)(1)
Locally defined significant and unreasonable conditions were assessed based on federal and state
mandated drinking water and groundwater quality regulations, the Sustainable Management Criteria
survey, public meetings, and discussions with GSA staff.
Significant and unreasonable changes in groundwater quality in the Basin are increases in a
chemical constituent that either:
• Result in groundwater concentrations in a public supply well above an established primary or
secondary MCL, or
• Lead to reduced crop production.
The information used for establishing the degraded groundwater quality minimum thresholds
included:
• Historical groundwater quality data from production wells in the Basin
• Federal and state primary drinking water quality standards
• RWQCB Basin objectives for groundwater quality (2019) for TDS
• Feedback about significant and unreasonable conditions from GSC members, GSA staff members,
and public stakeholders
The historical groundwater quality data used to evaluate groundwater quality minimum thresholds are
presented in Chapter 5 (Groundwater Conditions) on Figure 5-19 through Figure 5-21.
As stated in Section 8.7.1, the SGMA regulations allow three options to develop an approach for setting
degraded water quality minimum thresholds (number of wells, volume of water, or location of
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concentration isocontour). In the Basin, degraded water quality minimum thresholds are based on EPA-
published water quality standards (EPA, 2018) for constituents of concern with a primary or secondary
MCL to avoid degrading the existing water quality with respect to these constituents in the Basin.
(Primary standards refer to chemical constituents in groundwater with a potential impact on human
health; secondary standards refer to constituents that may affect taste or odor of drinking water.)
As noted in Section 354.28 (c)(4) of the SGMA regulations, minimum thresholds are based on a
degradation of groundwater quality, not an improvement of groundwater quality. Therefore, this GSP
was developed to avoid taking actions that may inadvertently move groundwater constituents that have
already been identified in the Basin in such a way that they have a significant and unreasonable impact
that would not otherwise occur.
Based on the review of groundwater quality in Chapter 5 (Groundwater Conditions), water
quality in the basin is generally good. The primary constituents of concern that exist for both
agricultural wells and public supply wells are:
• Total Dissolved Solids (TDS)
• Nitrate
• Arsenic
• Volatile Organic Compounds (PCE and TCE)
As noted in Section 5.6.3, based on available information there are two known groundwater
contamination plumes in the Basin: The TCE plume along Buckley Road south of the airport, and a
PCE plume within the City. Both of these cases are under active investigation with oversight by the
RWQCB.
The MTs for the constituents of concern are presented in Table 8-3.
8.8.2.2. Relation of Minimum Thresholds to Other Sustainability Indicators §354.28(b)(2)
The groundwater quality minimum thresholds were set for each of four constituents previously
discussed. These minimum thresholds were derived from existing data measured at individual wells
and applicable regulatory criteria. There are no conflicts between the existing groundwater quality data.
Because the underlying groundwater quality distribution is reasonable and realistic, there is no conflict
that prevents the Basin from simultaneously achieving all minimum thresholds.
No actions regarding the MTs for Water Quality will directly influence other Sustainability
Indicators. However, preventing migration of poor groundwater quality (for example, actions
required to prevent additional migration of contaminant plumes) could theoretically limit
activities needed to achieve minimum thresholds for other Sustainability Indicators, as
discussed below:
• Change in groundwater levels. Groundwater quality minimum thresholds could influence
groundwater level minimum thresholds by limiting the types of water that can be used for recharge
to raise groundwater levels or locations where it could be recharged. Water used for recharge
cannot exceed any of the groundwater quality minimum thresholds.
• Change in groundwater storage. Nothing in the groundwater quality minimum thresholds
promotes pumping in excess of the sustainable yield. The groundwater quality minimum thresholds
will not result in an exceedance of the groundwater storage minimum threshold.
• Seawater intrusion. This Sustainability Indicator is not applicable to this basin.
• Subsidence. Nothing in the groundwater quality minimum thresholds promotes a condition that will
lead to additional subsidence and therefore, the groundwater quality minimum thresholds will not
result in a significant or unreasonable level of subsidence.
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• Depletion of interconnected surface waters. Nothing in the groundwater quality minimum
thresholds promotes additional pumping or lower groundwater elevations in areas where
interconnected surface waters may exist. Therefore, the groundwater quality minimum thresholds
will not result in a significant or unreasonable depletion of interconnected surface waters.
8.8.2.3. Effects of Minimum Thresholds on Neighboring Basins §354.28(b)(3)
Because there is a groundwater divide between the SLO Basin and the adjacent Los Osos Basin and
Arroyo Grande sub-basin, there is no anticipated effect of the degraded groundwater quality minimum
thresholds on each of the two neighboring Basins.
8.8.2.4. Effects of Minimum Thresholds on Beneficial Users and Land Uses §354.28(b)(4)
The practical effect of the MTs for the Degraded Groundwater Quality Sustainability Indicator is that it
deters any significant long-term changes to groundwater quality in the Basin. Therefore, Basin
management that prevents the undesirable results from occurring will not constrain the use of
groundwater, nor have a negative effect on the beneficial users and uses of groundwater.
Agricultural Land Uses and Users
The degraded groundwater quality minimum thresholds generally benefit the agricultural water users in
the Basin by maintaining groundwater quality suitable for use in agriculture. For example, limiting the
number of additional agricultural supply wells that may exceed constituent of concern concentrations
(for example, TDS) that could reduce crop production ensures that a supply of usable groundwater will
exist for beneficial agricultural use.
Urban Land Uses and Users
The degraded groundwater quality minimum thresholds generally benefit the urban water users in the
Basin. Limiting the number of additional wells where constituents of concern could exceed primary or
secondary MCLs ensures an adequate supply of quality groundwater for municipal use. Management of
the Basin to prevent occurrences of these MTs may also result in lowered costs for water treatment.
Existing State, Federal, Public Health or Municipal regulations may require that a well not be used if
MCLs are exceeded and may supersede any actions related to SGMA-related MT exceedances. Wells
in violation of federal, state, and local water quality regulations will have to comply with the specific
regulations.
Domestic Land Uses and Users
The degraded groundwater quality minimum thresholds generally benefit the domestic water users in
the Basin by maintaining current and acceptable water quality.
Ecological Land Uses and Users
Although the groundwater quality minimum thresholds do not directly benefit ecological uses, it can be
inferred that the degraded groundwater quality minimum thresholds generally benefit the ecological
water uses in the Basin. Preventing constituents of concern from migrating will prevent unwanted
contaminants from impacting ecological groundwater supply.
8.8.2.5. Relevant Federal, State, or Local Standards §354.28(b)(5)
The Degraded Groundwater Quality minimum thresholds specifically incorporate federal and state
drinking water standards.
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8.8.2.6. Methods for Quantitative Measurement of Minimum Thresholds §354.28(b)(6)
The Degraded Groundwater Quality minimum thresholds will be directly measured using analytical
laboratory results of sampling conducted at the RMSs of the Water Quality Monitoring Network
presented in Chapter 7 (Monitoring Network). Groundwater quality will initially be measured using
existing monitoring programs.
Exceedances of primary or secondary MCLs will be monitored by reviewing water quality reports
submitted to the California Division of Drinking Water by municipalities and small water systems for the
wells that are included in the Water Quality Monitoring Network.
8.8.3. Measurable Objectives §354.30(a)(g)
Groundwater quality should not be degraded due to actions taken under this GSP and, therefore, the
measurable objectives are defined as zero exceedances as a result of groundwater management, in
samples from the Water Quality Monitoring Network wells over the 20-year SGMA implementation
horizon.
8.8.3.1. Information and Methods for Establishing Degradation of Water Quality Measurable
Objectives §354.30(b)
Because protecting groundwater quality is important to the beneficial users and uses of the resource,
the measurable objective for the Degradation of Water Quality Sustainability Indicator is defined as zero
exceedances of the MTs over the 20-year SGMA implementation period. Any exceedance will be
reviewed by the GSAs to determine its significance and potential responses.
8.8.3.2. Interim Milestones §354.30(a)(e)
Interim milestones show how the GSAs anticipate moving from current conditions to meeting the
measurable objectives. For water quality, measurable objectives are set at the current number of water
quality exceedances, which in this case is zero. Interim milestones are set for each five-year interval
following GSP adoption. The interim milestones for degraded groundwater quality are defined as zero
exceedances of the MT for each constituent of concern for 5, 10 and 15 years after GSP adoption.
8.9. Land Subsidence Sustainability Indicator §354.28(c)(5)
8.9.1. Undesirable Results §354.26(a)(d)
Locally defined significant and unreasonable conditions for the Land Subsidence Sustainability
Indicator were assessed based on public meetings and discussions with GSA staff. Significant and
unreasonable rates of land subsidence in the Basin are those that lead to a permanent subsidence of
land surface elevations that impact infrastructure.
For clarity, this Sustainable Management Criterion references two related concepts:
• Land subsidence is a gradual settling of the land surface caused by, among other processes,
compaction of subsurface materials due to lowering of groundwater elevations from groundwater
pumping. Land subsidence from dewatering subsurface clay layers can be an inelastic process, and
the potential decline in land surface could be permanent.
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• Land surface fluctuation is the periodic or annual measurement of the ground surface elevation.
Land surface may rise or fall in any one year. Declining land surface fluctuation may or may not
indicate long-term permanent subsidence.
Subsidence was documented in the Los Osos Valley in the early 1990s. Currently, InSAR data
provided by DWR shows that there has been a 0.01 to 0.02 foot gain in ground surface elevation along
Los Osos Valley Road between June 2015 and October 2020. Therefore, there has been no recent
significant land subsidence in the Basin.
By regulation, the ground surface Land Subsidence undesirable result is a quantitative combination of
subsidence minimum threshold exceedances. For the Basin, no long-term subsidence that impacts
infrastructure (including commercial buildings, homes, utility infrastructure, etc.) is acceptable. The
Undesirable Results for the land subsidence Sustainability Indicator as defined for the purposes of this
GSP are as follows:
The Basin will be considered to have Undesirable Results if measured subsidence using InSAR data,
between June of one year and June of the subsequent year is greater than 0.1 foot in any 1-year, or a
cumulative 0.5 foot in any 5-year period, as a result of groundwater management under the GSP, or
any long-term permanent subsidence is attributable to groundwater management.
Should potential subsidence be observed, the GSAs will first assess whether the subsidence may be
due to elastic processes. If the subsidence is not elastic, the GSAs will undertake a program to
correlate the observed subsidence with measured groundwater levels, and ultimately implement
changes to local groundwater management if the subsidence is judged to be the cause of the
subsidence.
8.9.1.1. Criteria for Establishing Undesirable Results §354.26(b)(2)
Criteria used to establish the Undesirable Results for Land Subsidence Sustainability Indicator are
satellite-measured subsidence data (InSAR data) collected by DWR.
8.9.1.2. Potential Causes of Undesirable Results §354.26(b)(1)
Conditions that may lead to an undesirable result include:
• A shift in pumping locations, which could lead to a substantial decline in groundwater levels.
• Shifting a significant amount of pumping and causing groundwater levels to fall in an area that is
susceptible to subsidence, such as certain areas underlaying the City, could trigger subsidence in
excess of the minimum threshold.
8.9.1.3. Effects of Undesirable Results on Beneficial Users and Land Uses §354.26(b)(3)
The effects of these undesirable results on the beneficial users and uses (§354.26 (b)(3)) include the
damage of critical infrastructure, and the damage of private or commercial structures that would
adversely affect their uses. Staying above the minimum threshold will avoid the subsidence undesirable
conditions.
8.9.2. Minimum Thresholds §354.28(c)(5)
Section 354.28(c)(5) of the SGMA regulations states that “The minimum threshold for land subsidence
shall be the rate and extent of subsidence that substantially interferes with surface land uses and may
lead to undesirable results.”
Based on an analysis of potential errors in the InSAR data, as discussed in the following section, the
subsidence minimum threshold is: The InSAR measured subsidence between June of one year and
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June of the subsequent year shall be no more than 0.1 foot in any single year and a cumulative 0.5 foot
in any five-year period, resulting in no long-term permanent subsidence.
Although InSAR data is the official minimum threshold value for the land subsidence Sustainability
Indicator, the GSAs have included one well to monitor for water levels as a proxy for potential
subsidence. Regular data collection from this well could alert the GSAs to conditions that may lead to
subsidence before InSAR data are available. RMS SLV-09 along Los Osos Valley Road is in the area
of the basin that experienced significant subsidence in the early 1990s. Therefore, this well has been
selected to monitor for conditions that could lead to subsidence. The minimum threshold for this well is
set at 102 feet, 15 feet higher than the observed low water level in the early 1990s.
8.9.2.1. Information and Methods Used for Establishing Land Subsidence Minimum Thresholds
§354.28(b)(1)
Minimum thresholds were established to protect groundwater supply, land uses and property interests
from substantial subsidence that may lead to undesirable results. Changes in surface elevation are
measured using InSAR data available from DWR. The general minimum threshold is the absence of
long-term land subsidence due to pumping in the Basin.
The InSAR data provided by DWR, however, are subject to measurement error. DWR has stated
that, on a statewide level, for the total vertical displacement measurements between June 2015
and June 2018, the errors are as follows (GSP, Paso Robles Basin, 2020):
1. The error between InSAR data and continuous GPS data is 16 mm (0.052 feet) with a 95%
confidence level.
2. The measurement accuracy when converting from the raw InSAR data to the maps provided by
DWR is 0.048 feet with 95% confidence level.
For the purposes of this GSP, the errors for InSAR data is considered the sum of errors 1 and 2,
combined total error of 0.1 foot. Thus, measured land surface change of greater than 0.1 feet will be
assessed as potential subsidence. As discussed previously, land surface elevations can fluctuate
naturally. Therefore, subsidence will be monitored at the same time each year to reduce the effect of
general fluctuations of elevation on observed data. Additionally, if subsidence is observed, a correlation
to lowered groundwater elevations at RMS SLV-09 must exist for the minimum threshold to be
exceeded.
Locally defined significant and unreasonable conditions are assessed based on historically observed
water levels in areas of known past land subsidence, satellite-based measurements of land subsidence
provided by DWR, public meetings, and discussions with GSA staff.
8.9.2.2. Relation of Minimum Thresholds to Other Sustainability Indicators §354.28(b)(2)
Land Subsidence minimum thresholds have little or no impact on other minimum thresholds, as
described below:
• Chronic lowering of groundwater elevations. The Land Subsidence minimum thresholds
will not result in significant or unreasonable groundwater elevations.
• Change in groundwater storage. The Land Subsidence minimum thresholds will not change
the amount of pumping, and will not result in a significant or unreasonable change in groundwater
storage.
• Seawater intrusion. This Sustainability Indicator is not applicable in the Basin.
• Degraded water quality. The Land Subsidence minimum thresholds will not change the
groundwater flow directions or rates, and therefore and will not result in a significant or
unreasonable change in groundwater quality.
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• Depletion of interconnected surface waters. The Land Subsidence minimum thresholds will
not change the amount or location of pumping and will not result in a significant or unreasonable
depletion of interconnected surface waters.
8.9.2.3. Effect of Minimum Thresholds on Neighboring Basins §354.28(b)(3)
The ground surface subsidence minimum thresholds are set to prevent any long-term subsidence that
could harm infrastructure. Therefore, the subsidence minimum thresholds will not prevent the Los Osos
Basin or the Arroyo Grande Basin from achieving sustainability.
8.9.2.4. Effect of Minimum Thresholds on Beneficial Users and Land Uses §354.28(b)(4)
The Land Subsidence minimum thresholds are set to prevent subsidence that could harm
infrastructure. Available data indicate that there is currently no subsidence occurring in the Basin that
affects infrastructure, and reductions in pumping are already required by the reduction in groundwater
storage Sustainability Indicator. Therefore, the Land Subsidence minimum thresholds do not require
any additional reductions in pumping. However, in general the amount of pumping in the Los Osos
Valley Road area must be kept at levels significantly lower than implemented in the 1990s.
Staying above the minimum threshold will avoid the Land Subsidence undesirable result and protect
the beneficial uses and users from impacts to infrastructure and interference with surface land uses.
8.9.2.5. Relevant Federal, State, or Local Standard §354.28(b)(5)
There are no federal, state, or local regulations related to subsidence.
8.9.2.6. Method for Quantitative Measurement of Minimum Thresholds §354.28(b)(6)
Minimum thresholds will be assessed using DWR-supplied InSAR data.
8.9.3. Measurable Objectives §354.30(a)(g)
The measurable objectives for subsidence represent target subsidence rates in the Basin. Long-term
ground surface elevation data do not suggest the occurrence of permanent subsidence in the Basin.
Therefore, the measurable objective for subsidence is maintenance of current ground surface
elevations.
8.9.3.1. Information and Methods Used for Establishing Subsidence Measurable Objectives
§354.30(b)
The measurable objectives are set based on maintaining current conditions and changes are measured
by DWR-supplied InSAR data.
8.9.3.2. Interim Milestones §354.28(a)(e)
Interim milestones show how the GSAs anticipate moving from current conditions to meeting the
measurable objectives. Interim milestones are set for each five-year interval following GSP adoption.
Land Subsidence measurable objectives are set at current conditions of no long-term subsidence.
There is no change between current conditions and sustainable conditions. Therefore, the interim
milestones are identical to the minimum thresholds and measurable objectives.
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8.10. Depletion of Interconnected Surface Water Sustainability
Indicator §354.28(c)(6)
Natural hydraulic connections can exist between shallow groundwater systems and some surface water
bodies. These surface water bodies can be gaining (receiving discharge from the alluvial aquifer) or
losing (discharging water to the alluvial aquifer). These relationships may change in magnitude and
direction across seasonal wet and dry cycles, longer term drought cycles, and in response to changes
in surface water operations or groundwater management practices. The total volume or rate of
streamflow in a creek is dependent upon many factors other than contributions from groundwater.
Precipitation, temperature, evapotranspiration, and influent streamflow from the upper contributing
watershed area each individually have a much greater influence on streamflow than groundwater
pumping.
This GSP is designed as a groundwater management plan for the Basin. It is not within the scope or
capability of this plan to mandate specific instream flow requirements deemed necessary for the
recovery of native steelhead populations, such as minimum instream flows or minimum pool depths.
Rather, it is the objective to plan for management of groundwater resources such that depletion of
interconnected surface water is not significantly increased due to projects or management actions
proposed in the plan.
Depletions of interconnected surface water occurs when there are decreased gains or increased losses
in volumes of streamflow caused by lowered groundwater elevations associated with groundwater use.
At certain levels, depletions may have adverse impacts on beneficial uses of the surface water and may
lead to undesirable results.
Flux between a stream and the surrounding aquifer may be theoretically calculated using
Darcy’s Law:
Q = KIA, WHERE
Q = rate o the flux (ft3/d)
K = Hydraulic conductivity of Aquifer (ft/day)
i = Hydraulic gradient between groundwater elevation and surface water elevations
A = Cross Sectional Area of Groundwater Flow (ft2)
If the groundwater elevation in the aquifer is greater than the elevation of the water surface in the
stream, then the direction of flow is from the aquifer to the stream. If the water surface elevation of the
stream is higher than the groundwater elevations, the direction of flow is from the stream to the
surrounding aquifer. In order to accurately make this calculation, surveyed elevations of groundwater
and surface water are necessary, as well as an estimate of hydraulic conductivity of the alluvial aquifer.
If groundwater elevations in the vicinity of a stream are maintained such that the direction and
magnitude of hydraulic gradient between the creek and the aquifer are not significantly changed, it
follows that the flux between stream and aquifer will not be significantly impacted. Therefore,
groundwater levels in appropriate wells are judged to be a valid proxy for the quantification of depletion
of interconnected surface water.
Direct measurement of flux between an aquifer and an interconnected stream is not feasible using
currently available data. A number of proposals to improve the collection of surface water and
interconnected groundwater data are discussed in Chapter 7 (Monitoring Networks), and proposed
details for these tasks are discussed in Chapter 10 (Implementation Plan). Among these
recommendations is to accurately survey stream channel elevations and monitoring well measuring
point elevations, so that the direction of flow may be characterized. In addition, monitoring wells used to
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assess the potential for depletion of surface water may be prioritized for installation of transducers for
continuous monitoring of water levels to more accurately capture the temporal fluctuation of both
stream water surface and groundwater elevations. Until such time as this data is available, this GSP
uses water level measurements in representative wells located immediately adjacent to Basin creeks
as the SMCs for the Depletion of Interconnected Surface Water Sustainability Indicator.
In an effort to demonstrate the relationship between streamflow and groundwater pumping (and
associated water levels), the following modeling exercise is presented. The GSFLOW model is used to
estimate streamflow depletion due to groundwater pumping in the San Luis Valley watershed over the
past 20 years (all streams tributary to San Luis Creek are included in this exercise). The sensitivity of
streamflow to pumping is evaluated as a comparison of two different model simulations. The first
scenario is the historical calibration run, wherein Basin pumping was estimated as described in the
water budget and applied to the historically calibrated model. This scenario was run, and model results
were extracted for streamflow exiting the Basin during the months of July through September (critical
low flow months important to steelhead habitat conditions). including both groundwater and surface
water runoff contributions to streamflow. The results are presented in the top graph of Figure 8-1.
Average streamflow during this time period was 2.7 cfs, with an average groundwater contribution to
streamflow of 1.1 cfs. In the second scenario, all pumping in the Basin was eliminated, and the same
model output was extracted. These results are presented in the bottom graph of Figure 8-1. Average
streamflow increased to 4.1 cfs, with an average groundwater contribution of 1.6 cfs. So, these results
indicate that streamflow depletion of 1.4 cfs, and a decrease of groundwater contribution to streamflow
of 0.5 cfs, has occurred due to historical groundwater pumping in the Basin. It is important to
acknowledge that this is a conceptual modeling exercise intended as a sensitivity analysis, and that
streamflow in the Basin is not well documented or calibrated. As a result, there is a large amount of
uncertainty in these results. Adding to the uncertainty is that the conditions of this scenario are outside
the bounds of the conditions under which the model was calibrated (i.e., removing all pumping). As
such, these results are intended to demonstrate an estimate of historical depletion, the results should
not be used to inform any quantitative criteria at present, nor should any linear correlation between
pumping volume and streamflow be inferred.
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Figure 8-11. Comparison of Modeled Discharge to Streams in San Luis Creek Watershed During Low Flow
Months (July through September) with Baseline and No Pumping Scenarios
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8.10.1. Undesirable Results §354.26(a)(d)
The undesirable result for Depletions of Interconnected Surface Water is a significant and
unreasonable depletion of interconnected surface water flows caused by groundwater management
and pumping in the Basin. The metric for depletion of interconnected surface water is defined in SGMA
as a volume or rate of surface water depletion. As discussed in Section 8.9, measurement of the fluxes
between the aquifer and Basin creeks is not feasible with currently available data. SGMA regulations
allow for the use of groundwater elevations as a proxy for the volume or rate of surface water depletion
when defining MTs and MOs. To use groundwater elevation as a proxy, there must be significant
correlation between groundwater elevation measurements and the sustainability indicator for which
groundwater elevation measurements are to serve as a proxy. Significant correlation is difficult to prove
due to the fact that streamflow due to groundwater pumping is so small compared to the other
streamflow factors discussed above (rainfall, temperature, etc.). Theoretical correlation may be
estimated using Darcy’s Law as previously described; if groundwater elevations are prevented from
excessive permanent declines near the streams, then the direction and magnitude of flux between the
stream and the surrounding aquifer will not be substantially changed from the past 30-year period of
record. Therefore, water level measurements at the RMSs designated for the Depletion of
Interconnected Surface Water Sustainability Indicator will be used as the basis of MTs and Undesirable
Results until better data becomes available under future monitoring activities. The statement defining
undesirable results for the Depletion of Interconnected Surface Water for this GSP is as follows:
The Basin will be considered to have undesirable results if any of the representative wells monitoring
interconnected surface water display exceedances of the minimum threshold values for two consecutive
Fall measurements.
8.10.1.1. Criteria for Establishing Undesirable Results §354.26(b)(2)
Criteria used to define undesired conditions for this Sustainability Indicator are those that:
• Significantly or unreasonably reduce the groundwater levels in the vicinity of the creeks such that
significant depletion of streamflow results.
• Impact the ability to provide surface water supplies to direct diverters
• Interfere with other SGMA Sustainability Indicators.
The information used for establishing the criteria for undesirable results for the Depletion of
Interconnected Surface Water Sustainability Indicator is water levels data collected from three RMS
wells (i.e., SLV-12 and EV-01, and EV-11) that are located immediately adjacent to San Luis Obispo
and Corral de Piedras Creek systems. For the present, water levels in these wells will be used as a
proxy indicator of undesirable results.
8.10.1.2. Potential Causes of Undesirable Results §354.26(b)(1)
Potential causes of undesirable results include increases in pumping in the proximity of a Basin creeks,
or instream projects that could alter the natural flow regimes of the creeks.
8.10.1.3. Effects of Undesirable Results on Beneficial Users and Land Uses §354.26(b)(3)
If depletions of interconnected surface water were to reach undesirable results, adverse effects could
include the reduced ability of the stream flows to meet instream flow requirements for local fisheries
and critical habitat, or reduced ability to deliver surface water supplies to direct users of surface water in
the Basin.
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8.10.2. Minimum Thresholds §354.28(c)(6)
Section 354.28(c)(6) of the SGMA regulations states that “The minimum threshold for depletions of
interconnected surface water shall be the rate or volume of surface water depletions caused by
groundwater use that has adverse impacts on beneficial uses of the surface water and may lead to
undesirable results.”
Current data are insufficient to determine the rate or volume of surface water deletions in the creeks.
Therefore, groundwater elevations in the RMSs intended to monitor surface water/groundwater
interaction (SLV-12, EV-01, EV-11) are used as a proxy for the Depletion of Interconnected Surface
Water Sustainability Indicator. Because there have been no historical groundwater level declines in
the ISW RMS wells, the MTs are defined at these three RMSs as the lowest historically observed water
level in the period of record. Minimum thresholds for these representative wells are presented in Table
8-1 and Figure 8-4, Figure 8-9, and Figure 8-10. If in the future, data from a more comprehensive
monitoring program (as discussed in Chapter 7 (Monitoring Network) and Chapter 10 (Implementation
Plan)) succeed in more robustly quantifying surface water depletions, those data may be used to re-
define minimum thresholds for areas of interconnection.
RMS EV-01 is located along West Corral de Piedras Creek just where it enters the Basin, and EV-11
(Greengate) is located near the junction of East and West Corral de Piedras, near the outlet of the
Basin. These wells are screened at least partially in the alluvial sediments associated with the creek,
and therefore, reflect groundwater conditions in the alluvial sediments. Hydrographs for these wells
display seasonal fluctuation of about 50 feet, which occur during wet and dry climatic periods. To avoid
management conditions that allow for lower groundwater elevations than those historically observed,
MTs for these wells were set at the historic low water levels indicated on the hydrographs, which occur
with regularity during every extended dry period evident in the record (Figure 8-9, Figure 8-10).
San Luis Obispo Creek is a significant feature in the Basin. It is an unregulated (I.e., undammed) creek.
Some reaches of San Luis Obispo Creek in the Basin have been observed to maintain flow year-round,
and some reaches go dry in the summer. A more extensive description and quantification of the
stream/aquifer interaction is included in Chapter 5 (Groundwater Conditions) and Chapter 6 (Water
Budget). The water budget shows that flow conditions in the creek are highly variable depending on
rainfall events and the hydrologic year type. In wetter years, when flows in the San Luis Obispo Creek
are high there is greater amounts of discharge from the creek to the groundwater system. In drier
years, when flows in the San Luis Obispo Creek are low, there is less stream recharge to the
groundwater system. In both cases the amount of flux between the surface water and the groundwater
system is small compared to the volume of water flowing down the creek. Inspection of hydrographs for
RMS SLV-12, intended to monitor conditions along near San Luis Obispo Creek (Figure 8-4) do not
indicate any significant declines of water levels since the drought of the early 1990s. Therefore, this
data suggests that the mechanisms of surface water/groundwater interaction at this location have not
been negatively impacted since the early 1990s.
East and West Corral de Piedras Creeks meet to form Pismo Creek just south of the basin boundary in
Edna Valley. Corral de Piedras Creeks are significant features in the Edna Valley portion of the SLO
Basin. West Corral de Piedras is affected by a private dam that impounds water at the Righetti
Reservoir upstream from the basin. To the extent that captured flows impounded in Righetti Reservoir
do not naturally flow downstream, the amount of stream flow is reduced and ancillary basin recharge
via streamflow percolation is less than it would be under natural (I.e., undammed) conditions in the
Edna Valley. East and West Corral de Piedras Creeks in the Basin are not observed to maintain flow
year-round in most of the Basin. Inspection of hydrographs for RMS EV-01, intended to monitor
conditions near West Corral de Piedras Creeks where it enters the Basin (Figure 8-9, Figure 8-10)
indicate highly seasonal groundwater conditions which fluctuate between well-established high points
near ground surface and low points significantly deeper than the assumed creek bed elevation, and do
not reflect any significant long-term declines of water levels in the observed period of record dating
back to the late 1950s. This hydrograph pattern indicates that surface water in Corral de Piedras
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Creeks recharges the underlying aquifer when the creek is flowing and is disconnected from the
underlying aquifer system when the creek is dry.
As described in Chapter 4 (Basin Setting) and Chapter 5 (Groundwater Conditions), there are
insufficient data to quantitatively assess the extent of the connection between surface water and
groundwater in the Basin. As described in Chapter 7 (Monitoring Networks), a more expansive
monitoring network will be developed during GSP implementation to improve understanding of
interconnection between surface water and groundwater in the Basin. Chapter 10 (Implementation
Plan) addresses details of the plan to accumulate better data for this Sustainability Indicator. If in the
future, better data are generated to quantify the connection between surface water and groundwater,
undesirable results may be revised to reflect this data. For example, if pressure transducers are
installed to generate continuous monitoring data, the definition of undesirable results, which is currently
predicated on the assumption of only two water level measurements per year, will no longer be
applicable, and may need to be revised. However, for this GSP, groundwater elevations in SLV-12, EV-
01, and EV-11 will be used as a proxy for the Depletion of Interconnected Surface Water Sustainability
Indicator.
8.10.2.1. Information and Methods Used for Establishing Depletion of Interconnected Surface
Water Minimum Thresholds
As with the other Sustainability Indicators, the primary methods for development of SMCs for this
Sustainability Indicator is monitoring of groundwater elevations in the three RMSs established for the
purpose of monitoring hydrogeologic conditions in the adjacent creeks.
As with the chronic reduction of groundwater levels Sustainability Indicator, the primary source of data
that was evaluated for the Depletion of Interconnected Surface Water Sustainability Indicator is
historical groundwater elevation data maintained by the GSAs.
The information used for establishing the MOs and MTs for the chronic lowering of groundwater
levels Sustainability Indicator included:
• Historical groundwater elevation data from wells monitored by the County of San Luis Obispo.
• Construction details of RMS wells
• Long-term trends displayed in hydrographs of the RMS wells identified for this Sustainability
Indicator.
The use of groundwater elevation as a proxy metric for the Depletion of Interconnected Surface Water
Sustainability Indicator is adopted given the challenges and cost of direct monitoring of depletions of
interconnected surface water. The depletion of interconnected surface water is driven by the gradient
between water surface elevation in the surface water body and groundwater elevations in the
connected, shallow groundwater system. By defining minimum thresholds in terms of groundwater
elevations in shallow groundwater wells near surface water, the GSAs will monitor and manage this
gradient, and in turn, manage potential changes in depletions of interconnected surface.
8.10.2.2. Relationship between Individual Minimum Thresholds and Other Sustainability Indicators
The MTs for the Depletion of Interconnected Surface Water Sustainability Indicator are defined as the
lowest water levels observed in the period of record for each of the three RMSs. Therefore, the concept
of potential conflict between MTs at different locations in the Basin is not applicable. The Depletion of
Interconnected Surface Water Sustainability Indicator could influence other Sustainability Indicators.
The Depletion of Interconnected Surface Water Sustainability Indicator MTs was selected to
avoid undesirable results for other Sustainability Indicators, as outlined below:
• Chronic lowering of groundwater levels. Because groundwater elevations will be used as a
proxy for estimating Depletion of Interconnected Surface Water Sustainability Indicator, and the
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definitions of the MTs are set at historically observed conditions, the MTs will not cause undesirable
results for this Sustainability Indicator.
• Depletion of Groundwater Storage. Because groundwater elevations will be used as a proxy
for estimating Depletion of Interconnected Surface Water Sustainability Indicator, and the definitions
of the MTs are set at historically observed conditions, the MTs will not cause undesirable results for
this Sustainability Indicator.
• Seawater intrusion. This Sustainability Indicator is not applicable to this Basin.
• Degraded water quality. The minimum threshold proxy of stable groundwater levels is not
expected to lead to a degradation of groundwater quality.
• Subsidence. Because future groundwater levels will be above historically observed conditions,
they will not induce any additional subsidence.
8.10.2.3. Effects of Minimum Thresholds on Neighboring Basins
Two neighboring groundwater basins share a boundary with the SLO Basin; the Los Osos Basin to the
northwest, and the Arroyo Grande Subbasin of the Santa Maria Valley Groundwater Basin to the
southeast. Neither of these shared boundaries are extensive, and the HCM posits that a groundwater
divide separates the groundwater between them and the SLO Basin. In addition, the Basin streams are
relatively far from the Basin boundaries shared with the neighboring basins. In the San Luis Valley
there have been no trends indicating groundwater declines that would affect the Los Osos Basin. In
Edna Valley the areas with observed declines are one to two miles from the Arroyo Grande Basin
boundary in a downgradient direction. It is not anticipated that actions associated with the GSP will
have any significant impact on either the Los Osos Basin or the Arroyo Grande Subbasin.
The SLO Basin GSAs have developed a cooperative working relationship with the Los Osos
Groundwater Basin – Basin Management Committee and the GSAs working in the Arroyo Grande
Subbasin. Groundwater conditions near the borders with these basins will be monitored and shared.
8.10.2.4. Effects of Minimum Thresholds on Beneficial Uses and Users
The MT for Depletion of Interconnected Surface Water is defined to maintain historically observed
groundwater elevations.
The practical effect of this GSP for protecting against the Depletion of Interconnected Surface Water
MTs is that it encourages minimal long-term net change in groundwater elevations in the vicinity of the
Basin streams. Seasonal and drought cycle variations are expected, but during average conditions and
over the long-term, beneficial users will have access to adequate volumes of water from the aquifer to
service the needs of all water use sectors. The beneficial users of groundwater are protected from
undesirable results.
Agricultural Land Uses and Users
The water levels set as MTs are within the historical range of data, implying that surface
water/groundwater interaction will be within historical norms. Therefore, existing agricultural operations
are not expected to be affected by the Depletion of Interconnected Surface Water MTs.
Urban Land Uses and Users
Development of real estate along streams and creeks is generally constrained by prohibiting
development in mapped floodplains in the Basin. Therefore, the Depletion of Interconnected Surface
Water MTs are not anticipated to affect urban land users in the Basin.
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Domestic Land Uses and Users
Development of real estate along streams and creeks is generally constrained by prohibiting
development in mapped floodplains in the Basin. Therefore, the Depletion of Interconnected Surface
Water MTs are not anticipated to affect urban land users in the Basin.
Ecological Land Uses and Users
Groundwater dependent ecosystems would generally benefit from this MT. Maintaining groundwater
levels close to within historically observed ranges will continue to support groundwater dependent
ecosystems. More detailed mapping of GDEs, installation of gages in Edna Valley, and development of
discharge rating curves for the San Luis Creek gages, all will clarify the effects of these MTs on
ecological uses.
8.10.2.5. Relation to State, Federal, and Local Standards
Agreements with NOAA mandate a minimum delivery for environmental flows of 1.6 MGD of effluent
flow from the City Wastewater Treatment Plant located along San Luis Obispo Creek near the outlet of
the Basin in San Luis Valley.
SWRCB permit requirements with respect to outflow from Righetti Reservoir may impact flow conditions
along West Corral de Piedras Creek.
8.10.2.6. Methods for Quantitative Measurement of Minimum Threshold
The quantitative metric for assessing compliance with the Depletion of Interconnected Surface Water
MTs is monitoring groundwater elevations at the three RMSs designated for this Sustainability Indicator
(SLV-12, EV-01, EV-11). The approach for quantitatively evaluating compliance with the MT for
reduction in groundwater storage will be based on evaluating groundwater elevations semi-annually. All
groundwater elevations collected from the groundwater level monitoring network will be analyzed and
averaged.
8.10.3. Measurable Objectives
Similar to minimum thresholds, measurable objectives were defined using water level data based on
the historical water level data observed in RMSs intended to monitor streamflow conditions.
Measurable objectives for these wells are presented in Table 8-1and Figure 8-4, Figure 8-9, and Figure
8-10. If future data from a more comprehensive surface water monitoring program documents
quantitative estimates of stream flow depletion, those data may be used to re-define the measurable
objectives for areas of interconnection.
8.10.3.1. Method for Quantitative Measurement of Measurable Objectives
The measurable objectives are set based on maintaining current conditions of seasonal high water
level elevations observed in the RMS wells during rainy periods. The quantitative method for assessing
compliance with the MOs is monitoring of groundwater elevations at the selected RMSs.
8.10.3.2. Interim Milestones
Interim milestones show how the GSAs anticipate moving from current conditions to meeting the
measurable objectives. Interim milestones are set for each five-year interval following GSP adoption.
MOs for the Depletion of Interconnected Surface Water are set at historically observed conditions of
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high groundwater elevations during wet climatic periods. Therefore, the interim milestones are defined
to be identical to the water levels associated with the MOs.
8.11. Management Areas
Management areas are not established in the Basin. The GSAs and GSC members did not find it
necessary to sub-divide the Basin into smaller management areas with specific administrative
requirements.
9-1
9
GROUNDWATER SUSTAINABILITY PLAN
Projects and Management Actions
(§354.44)
This chapter describes the Projects, Management Actions, and Adaptive
Management information that satisfies Sections 354.42 and 354.44 of the
SGMA regulations.
These projects, actions, and their benefits are intended to help
achieve the sustainable management goals in the Basin.
IN THIS CHAPTER
• Projects Overview
• Integrated Model
Scenarios
• Management Actions
• Adaptive
Management
Projects and Management Actions (§354.44) Section 9
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9.1. Introduction
Under the Regulations, § 354.44, the Groundwater Sustainability Plan (GSP, Plan) is to include
the following:
• Each Plan shall include a description of the projects and management actions the Agency has
determined will achieve the sustainability goal for the basin, including projects and management
actions to respond to changing conditions in the basin.
• Each Plan shall include a description of the projects and management actions that include the
following:
− A list of projects and management actions proposed in the Plan with a description of the
measurable objective that is expected to benefit from the project or management action. The
list shall include projects and management actions that may be utilized to meet interim
milestones, the exceedance of minimum thresholds, or where undesirable results have
occurred or are imminent. The Plan shall include the following:
• A description of the circumstances under which projects or management actions shall be
implemented, the criteria that would trigger implementation and termination of projects or
management actions, and the process by which the Agency shall determine that conditions
requiring the implementation of particular projects or management actions have occurred.
• The process by which the Agency shall provide notice to the public and other agencies that
the implementation of projects or management actions is being considered or has been
implemented, including a description of the actions to be taken.
− If overdraft conditions are identified through the analysis required by Section 354.18, the Plan
shall describe projects or management actions, including a quantification of demand reduction
or other methods, for the mitigation of overdraft.
− A summary of the permitting and regulatory process required for each project and management
action.
− The status of each project and management action, including a timetable for expected initiation
and completion, and the accrual of expected benefits.
− An explanation of the benefits that are expected to be realized from the project or management
action, and how those benefits will be evaluated.
− An explanation of how the project or management action will be accomplished. If the projects or
management actions rely on water from outside the jurisdiction of the Agency, an explanation
of the source and reliability of that water shall be included.
− A description of the legal authority required for each project and management action, and the
basis for that authority within the Agency.
− A description of the estimated cost for each project and management action and a description
of how the Agency plans to meet those costs.
− A description of the management of groundwater extractions and recharge to ensure that
chronic lowering of groundwater levels or depletion of supply during periods of drought is offset
by increases in groundwater levels or storage during other periods.
• Projects and management actions shall be supported by best available information and best
available science.
• An Agency shall take into account the level of uncertainty associated with the basin setting when
developing projects or management actions.
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9.2. Overview of Potential Projects and Management Actions
9.2.1. Project and Management Actions Development
The projects and management actions concepts were developed over a series of working sessions with
GSA staff, meetings with GSC members and in six public GSC meetings between December 9, 2020
and June 21, 2021. The projects and management actions are focused in the Edna Valley (Figure 9-1)
where the overdraft was documented in Chapter 6 (Water Budget). The effectiveness of the projects
and management actions will be assessed by the ability to mitigate undesirable results such as
groundwater level declines in the Edna Valley Representative Monitoring Sites (RMS) described in
Chapter 8 (Sustainable Management Criteria).
9.2.1.1. Screening and Ranking of Projects
An initial screening of the projects was performed using the evaluation criteria shown in Table 9-1. The
Evaluation Criteria developed collaboratively with the GSC members were applied to the list of projects
deliberated by the GSA Staff, GSC members, and the public. The scoring of each project was weighted
to better represent the ease/likelihood of implementation and the impacts of the project on the
sustainability goals described in Chapter 8 (Sustainable Management Criteria).
Two projects that were included in the initial project screening process were not selected for inclusion in
the GSP for further evaluation. The projects that were not included were a project that considered the
City San Luis Obispo delivering potable water to Golden State Water Company and a project for
stormwater capture and recharge of East Corral de Piedra. The City of San Luis Obispo Potable Water
to Golden State Water Company Project was removed after receiving comments from the City GSA
during the public comment period for Chapters 9 and 10. The City GSA provided feedback at a public
study session on July 20, 2021, designating that they were not supportive of this projecting being
included within the GSP due to its infeasibility as it is in direct conflict with existing City General Plan
policy that prohibits the delivery of potable water outside of City limits, as well as City Municipal Code
that prohibits the sale of potable water outside of City limits. Due to this input from the City GSA, and
acknowledgement that this project was infeasible from a policy basis, this project was removed from the
GSP. The East Corral de Piedra Stormwater Capture and Recharge project was not selected for further
evaluation due to the high unit cost ($6,000/AF) due to a small contributing watershed to East Coral de
Piedras Creek, lack of ability to capture a significant amount of stormwater, and sensitivity to Steelhead
passage. These technical limitations resulted in the project being removed from the GSP and deemed
infeasible and impractical. The results of the screening and ranking of the final GSP projects are
displayed in Table 9-2.
9.2.1.2. Summary of Projects
Table 9-3 provides a summary of the projects and management actions considered in this GSP. The
table shows the status, timing for implementation (years), capital costs ($), annual Operations and
Maintenance (O&M) ($/Year), quantity of water delivered (AFY), and the unit cost ($/AFY) for each
project and management action. The projects discussed in this GSP are centered around
supplemental water sources that could be brought into the SLO Basin to mitigate the overdraft. The
projects considered supplemental water from three sources all of which have existing conveyance
infrastructure within or in close proximity to the Basin; State Water Project, City of SLO recycled water,
and Price Canyon discharge. The projects and management actions presented in this GSP are not an
exhaustive list and during the implementation of the GSP additional projects or management actions
may be developed and will be described in the annual and five-year evaluation reports.
The project costs included in this GSP were prepared in conformance with industry practice and, as
planning level cost opinions, and ranked as a Class 4 Conceptual Opinion of Probable Construction
Cost as developed by the Association for the Advancement of Cost Engineering (AACE) (Association
for the Advancement of Cost Engineering, 2011). The AACE classification system is intended to
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classify the expected accuracy of planning level cost opinions and is not a reflection on the effort or
accuracy of the actual cost opinions prepared for the GSP. According to AACE, a Class 4 Estimate is
intended to provide a planning level conceptual effort with an accuracy that will range from ‐30% to
+50% and includes an appropriate contingency for planning and feasibility studies. The conceptual
nature of the projects and associated costs presented in this Chapter are based upon limited design
information available at this current stage of the projects.
At this planning‐level stage, two percentages were applied to the estimated construction costs, 30% for
construction contingency and 25% for implementation costs (which incorporates anticipated Design,
Construction Management, and Environmental and Construction Engineering costs). In order to
estimate annual payments, a loan period of 30 years at a 5% interest rate was assumed. The $/AFY
values were calculated using the total annual cost, which include capital repayment and operations and
maintenance costs, divided by the estimated yield from each project, see Section 9.4 for further detail.
It is important to note that the cost estimates shown in Table 9-3 do not include the cost of the water as
the costs to purchase the water are subject to negotiation between the supplier and the purchasing
party.
The projects were further evaluated with the integrated model to quantify the benefit of the projects
respect to the SMCs in the Edna Valley. Model results are described in more detail in Section 9.4.
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Table 9-1. Initial Project Screening Evaluation Criteria
CRITERIA SCORING
Quantity of Water
1- <250 AFY
2- 250-500 AFY
3- 500-750 AFY
4- 750-1000 AFY
5- > 1,000 AFY
Capital Cost
1->$5M
3- $2.5 M -$ 5 M
5- $0 – 2.5M
Water Cost
1- >$4,000/AFY
2- $3,000 - $4,000/AFY
3- $2,000 - $3,000/AFY
4- $1,000 - $2,000/AFY
5- < $1,000/AFY
O&M Cost
1- >$2,000/AFY
2- $1,000 - $2,000/AFY
3- $500 - $1,000/AFY
4- $100 - $500/AFY
5- < $100/AFY
GW Water Quality Impact
1- Higher TDS to ambient groundwater
3- Equivalent TDS than ambient groundwater
5- Lower TDS than ambient groundwater
Reliability/Resiliency
1- Highly variable
3- Moderately reliable
5- Highly reliable
Timeline to Implement
1- > 10 years
2- 7 years
3- 5 years
4- 3 years
5- < 1 year
Feasibility/Complexity
1- Significant regulatory, environmental, political, or social challenges
3- Potential significant regulatory, environmental, political, or social challenges
5- Limited regulatory, environmental, political, or social challenges
Environmental Impacts
1- Detrimental Environmental impacts
3- Neutral Environmental impacts
5- Beneficial Environmental impacts
Socioeconomic Impacts
1- Detrimental Socioeconomic impacts
3- Neutral Socioeconomic impacts
5- Beneficial Socioeconomic impacts
Eligible for Grant Funding
1- Limited grant funding opportunities
3- Moderate grant funding opportunities
5- Significant grant funding opportunities
Groundwater Level Benefit
1- Minimal Effect on Groundwater Levels
3- Average Effect on Groundwater Levels
5- Highest Effect on Groundwater Levels
Projects and Management Actions (§354.44) Section 9
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Table 9-2. Project Evaluation Scoring Results
PROJECTS
WEIGHTING FACTOR 3 2 2 2 1 1 1 2 1 1 1 4
DESCRIPTION
QUANTITY OF WATER
(AFY) QUANTITY OF WATER CAPITAL COST WATER COST O&M COST GW WATER QUALITY BENEFITS RELIABILITY/RESILIENCY TIMELINE TO IMPLEMENT FEASIBILITY/COMPLEXITY ENVIRONMENTAL IMPACTS SOCIOECONOMIC IMPACTS ELIGIBILITY FOR GRANT FUNDS GROUNDWATER LEVEL BENEFIT TOTAL
SCORE
SWP to Ag Irrigation Connection to SWP to offset Ag groundwater pumping through direct delivery
of SWP Water 1000 5 2 3 4 5 3 3 3 3 4 4 3 73
SWP Recharge Connection to SWP to provide water for groundwater recharge basin 500 3 2 3 4 5 3 3 3 3 4 4 4 71
City of SLO Recycled Water to Ag
Irrigation
Connection to City of SLO Recycled Water System to offset Ag groundwater
pumping through direct delivery 500-800 3 3 1 4 4 5 4 4 3 4 4 3 69
Price Canyon Discharge Relocation Relocation of Sentinel Peak Produced Water Discharge location to upper
Corral de Piedra Creek or direct delivery to agriculture 500 2 2 5 4 5 5 4 2 4 3 4 3 69
Varian Ranch MWC AG Subbasin
Wells
Connection to Varian Ranch MWC wells in Arroyo Grande Subbasin to offset
Varian Ranch groundwater pumping through direct delivery of imported
groundwater
50 1 3 5 4 3 4 4 3 3 4 4 3 67
SWP to GSWC Connection to SWP project to offset GSWC groundwater pumping through
direct delivery of SWP Water 50 1 2 3 4 5 3 4 3 3 4 4 4 66
SWP to Mutual Water Companies Connection to SWP to offset Edna and Varian Ranch MWC groundwater
pumping through direct delivery of SWP Water 50 1 4 3 4 5 3 3 3 3 4 4 3 65
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Table 9-3. Projects and Management Actions Strategies
PROJECTS AND MANAGEMENT
ACTIONS STATUS IMPLEMENTATION TIMING CAPITAL COST
ANNUAL CAPITAL
PAYMENT ANNUAL O&M
TOTAL ANNUAL
PAYMENT
QUANTITY OF
WATER (AF) UNIT COST ($/AF)1
SWP to Ag Irrigation Not begun yet Feasibility study: 0 to 1 years
Design/Construction: 1 to 5 years $ 890,000 $ 58,000 $ 5,000 $ 63,000 1,000 $ 60
City of SLO Recycled Water to Ag
Irrigation
Conceptually evaluated as
part of the City of SLO
Recycled Water Study
(2017)
Feasibility study: 0 to 1 years
Design/Construction: 1 to 3 years $ 1,004,000 $ 65,000 $ 88,000 $153,000 600 $ 260
SWP Recharge Not begun yet Feasibility study: 0 to 1 years
Design/Construction: 1 to 5 years
$ 3,624,000 $ 236,000 $ 101,000 $ 337,000 500 $ 670
SWP to GSWC Not begun yet Feasibility study: 0 to 1 years
Design/Construction: 1 to 5 years $ 2,685,000 $ 175,000 $ 17,000 $ 192,000 200 $ 960
Varian Ranch MWC AG Subbasin
Wells Not begun yet Feasibility study: 0 to 1 years
Design/Construction: 1 to 3 years $ 2,701,000 $ 176,000 $ 34,000 $ 210,000 50 $ 4,200
SWP to Mutual Water Companies Not begun yet Feasibility study: 0 to 1 years
Design/Construction: 1 to 5 years $ 835,000 $ 54,000 $ 5,000 $ 59,000 50 $ 1,180
Price Canyon Discharge Relocation Mitigated Negative Dec
Completed in 2015
Feasibility study: 0 to 1 years
Design/Construction: 1 to 3 years $ 4,909,000 $ 319,000 $ 56,000 $ 375,000 5002 $ 750
Groundwater Extraction Metering
Plan Not begun yet 1 year
Demand Management Strategies Not begun yet As needed
1. Does not include the cost of the water.
2. Quantity of water at the discharge point.
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Figure 9-1. Project Location Map
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9.2.2. Addressing Sustainability Indicators (§ 354.44 (1))
Table 9-4 shows the project and management action benefits and impacts on specific sustainability
indicators and associated measurable objectives and minimum thresholds.
Table 9-4 Summary of Project and Management Action Benefits and Impacts on Sustainability Indicators.
Notes:
Chronic Lowering of Groundwater Levels
Reduction of Groundwater Storage
Depletion of Interconnected Surface Water
Degradation of Groundwater Quality
9.2.3. Overdraft Mitigation (§ 354.44 (2))
The proposed projects and management actions are intended to maintain groundwater levels above
minimum thresholds through in-lieu pumping reductions or increased recharge. Overdraft is caused
when pumping exceeds recharge and inflows in the Basin over a long period of time. Improving the
management of groundwater in the Basin will help to mitigate overdraft.
Projects and Management Actions Benefits Measurable Objective Exceedance of Minimum
Thresholds
SWP to Ag Irrigation Increases water levels in the Edna Valley to avoid minimum
thresholds Yes
City of SLO Recycled Water to Ag Irrigation
Increases water levels in the Edna Valley to avoid minimum
thresholds
Supplemental Water to Edna Valley
Yes
SWP Recharge Increases water levels in the Edna Valley to avoid minimum
thresholds Yes
SWP to GSWC Reduces localized groundwater production
Supplemental Water to the Edna Valley Yes
Varian Ranch MWC AG Subbasin Wells Reduces localized groundwater production
Supplemental Water to the Edna Valley Yes
SWP to Mutual Water Companies Reduces localized groundwater production
Supplemental Water to the Edna Valley Yes
Price Canyon Discharge Relocation Increases recharge to the Edna Valley
Increases streamflow in West Corral de Piedras for Steelhead Yes
Groundwater Extraction Metering Plan Improve understanding of the Basin
Ability to manage the Basin No
Voluntary Fallowing of Agricultural Land Reduces groundwater production in the Edna Valley Yes
Improved Irrigation Efficiency Reduces groundwater production in the Edna Valley Limited
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9.3. Integrated Surface Water and Groundwater Modeling
As part of the development of this GSP, the GSAs incorporated the development of an integrated
groundwater-surface water model of the Basin. A brief overview of the development and application of
the model is presented herein. This discussion is not intended to be complete; more detailed
documentation of the model is included in Appendix G, Surface Water/Groundwater Modeling
Documentation.
The integrated model was developed using GSFLOW, a modeling code developed and maintained by
the United States Geological Survey (USGS). GSFLOW incorporates two existing USGS modeling
codes under a single structure. The first is the Precipitation Runoff Modeling System (PRMS), which
models rainfall, plant uptake, evapotranspiration, and runoff to streams, using a water budget approach
applied to a gridded domain of the model area. The second is MODFLOW, which simulates
groundwater flow and surface water/groundwater interaction in the aquifers of the model area.
GSFLOW operates by first running PRMS, using climatological input and daily time steps to calculate
the movement of rainfall that falls onto the Basin area through plant canopy, root zone, runoff to
streams, and deep percolation to the groundwater environment. GSFLOW then transmits necessary
data to MODFLOW (e.g., streamflow, deep percolation, etc.) at times and locations significant to the
simulation of groundwater flow for the completion of the GSFLOW run.
The areal model grid was established utilizing 500-foot square model grid cells that cover the entire
contributing watershed of the Basin. The vertical grid was discretized into three layers to correspond to
the three water bearing formations in the Basin (Alluvium, Paso Robles Formation, and Pismo
Formation). The bedrock in the contributing watershed area was also discretized into three layers so
that lateral hydraulic communication could be simulated between the bedrock and all three formations
in the Basin.
A historical calibration period from water years 1987 through 2019 was selected to correspond to the
period of the historical water budget analysis documented in Chapter 6 (Water Budget). The pumping
estimates developed in the water budget analysis were used in the model calibration runs. Surface
water flow data is unavailable for creeks in either the San Luis Valley or Edna Valley, but flow estimates
were made for San Luis Obispo Creek based on flow stage or height data from the City’s gages. The
PRMS model was calibrated to achieve acceptable results for peak flow and flow volume on San Luis
Obispo Creek. The MODFLOW model was calibrated to achieve acceptable results for groundwater
elevations at wells in the Basin. The model calibration was found to meet industry criteria of a relative
error of less than 10% (relative error is the mean error divided by the range of observed groundwater
elevations). Therefore, the model was judged to be appropriate to perform predictive simulations to
assess the impacts of proposed projects and management actions on water levels at RMS in the Basin.
The model was applied to evaluate the GSP projects and management actions using the following
methodology. To maintain continuity of results between the historical calibration period (water years
1987 – 2019) and the predictive period (water years 2020-2044), each simulation was run continuously
from the historical calibration period through the end of the predictive simulation period, from water
years 1987 through 2044. The monthly 2019 pumping time series that was developed in the water
budget analysis and used in the MODFLOW historical calibration was repeated for each year in the
predictive simulation period. The climatological time series data from 1995-2019 used as input for
PRMS historical calibration was repeated for the predictive simulation period. Thus, the pumping
conditions reflect the most recent year for which data is available, and climatological conditions for the
predictive simulations replicated the observed conditions from 1995-2019, including the recent drought
period. It is assumed that there will be no significant increase in agricultural pumping or acreage during
this time period.
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In order to assess the effect that a simulated project would have on groundwater elevations in the
Basin, the following methodology was used. A baseline scenario was simulated in which no projects or
management actions occurred. Pumping was maintained at recent levels, and climate conditions were
repeated for the recent time series as previously discussed. Then a project scenario was incorporated
in which a specific project or management action was represented in the model, either through
reduction of pumping or introduction of a new source of recharge, as appropriate. The modeled RMS
hydrographs for the baseline scenario and the project scenario are then plotted on the same chart, so
the effect of the project can be assessed by the difference in water levels between the baseline and
project scenario over the predictive period of the project implementation. The projects discussed herein
were represented with only the project under consideration represented in the model, in order to
quantify the effect of the individual project discussed. So each of the first four model simulations each
represent a particular change in pumping or recharge specific to the project or projects being described.
Then a final run was performed at the end that included all changes simultaneously. It is likely that
more than one of these projects will be required to achieve sustainability, which will be discussed later
in this chapter.
Four separate project scenarios were modeled. However, some of these project model scenarios are
intended to represent multiple projects as described in the following sections, but with different options
for source water. It is assumed that the groundwater pumping reductions in the modeled project
scenarios are offset by supplemental water supplies. For example, one of the project scenarios
simulates a 1,000 AFY reduction in agricultural pumping. This reduction could conceivably be offset
through import of State Water Project (SWP) water or short-term delivery of City of San Luis Obispo
recycled water,. So, this single model simulation could potentially represent the effects of more than
one project, or a combination of projects, depending on the ultimate disposition and feasibility of
obtaining the various possible sources of water or implementation of management actions. When this is
the case, it will be noted in the text of the specific project descriptions. Additionally, a final project
scenario was run in which four projects are represented simultaneously.
Table 9-5 Description of Modeled Scenarios
Scenario Description Applies to Projects
Baseline 2019 Production No Projects
1 Reduce Agricultural pumping by 1,000 AFY SWP for Ag Use
City of SLO Recycled Water to Ag
2 500 AFY to Recharge Basin SWP Recharge Basin
3
Reduce Golden State pumping by 200 AFY.
Reduce ERMWC and VRMWC pumping by
50 AFY (combined).
SWP to GSWC,
Varian Ranch MWC AG Subbasin Wells,
SWP to Mutual Water Companies
4
Discharge 500 AFY as input into West Corral
de Piedras Creek at its entrance to the SLO
Basin
Price Canyon Discharge Relocation
5 Scenarios 1 through 4 All Projects Listed Above
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9.4. Projects
9.4.1. State Water Project for Agricultural Irrigation
The Coastal Branch of the SWP conveys water from the California Aqueduct to San Luis Obispo and
Santa Barbara Counties (Figure 9-1). The California Aqueduct is operated by the California Department
of Water Resources (DWR). The Coastal Branch provides water to two SWP Contractors: the Santa
Barbara County Flood Control and Water Conservation District (via the Central Coast Water Authority
(CCWA), a Joint Powers Authority) and the SLOCFCWCD. The CCWA owns, operates, and maintains
the Polonio Pass Water Treatment Plant (PPWTP) and operates the portion of the Coastal Branch that
is downstream of Polonio Pass.
The Coastal Branch transects the Edna Valley subarea and runs along Orcutt Road as shown in Figure
9-1. This project includes the construction of a new turnout to the Coastal Branch along Orcutt Rd south
of the Energy Dissipation Valve and 200 feet of 10-inch pipeline to connect to the existing Edna Valley
Growers Mutual Water Company distribution system. The project would allow for approximately 1,000
AFY of SWP water based on the availability and cost of SWP water and will offset an equivalent
amount of the irrigation demands currently met by groundwater. The SWP water is a treated water
supply and may require dechlorination before being used for agricultural purposes.
SWP water for irrigation use to offset pumping could be purchased from 1) District subcontractors that
receive their SWP water through Lopez and Chorro Valley Participants, 2) Santa Barbara County
Participants or 3) a portion of the SLOCFCWCD's unsubscribed Table A amount (14,463 AFY). Any
necessary agreements/terms would need to be identified, negotiated and developed amongst relevant
parties, and environmental review would need to be conducted, to facilitate the transfers. The recent
adoption of the Water Management Tools Amendment to the SWP Contracts by the SLOCFCWCD and
the Santa Barbara County Flood Control and Water Conservation District (SBCWCFCD) presents new
opportunities for obtaining SWP water supply and delivery capacity to Edna Valley.
9.4.1.1. Project Benefits (§ 354.44.5)
In order to assess this project’s benefits to water levels in the aquifer and effect on sustainability of the
Basin, a project scenario was simulated using the integrated GSFLOW model developed as part of the
GSP efforts. A baseline simulation was performed in which agricultural pumping was kept constant at
water year 2019 volumes, and climatological conditions for the predictive time period (water years
2020-2044 was defined as a repetition of the historical time series used for 1995-2019.
The model was run continuously for the time period from water years 1987 through 2044. This project
simulation assumes that 1,000 AFY of SWP water is available for agriculture to offset irrigation supply
currently supplied by groundwater.
For the predictive time period, agricultural pumping was reduced by 1,000 AFY in Edna Valley for the
period starting in water year 2026 (these reductions were not applied to San Luis Valley, because no
water level declines have been observed in that area). The 2026 starting period assumes it will take
five years to implement the project or combination of projects required to make up the water for the
pumping reduction. The 1,000 AFY in-lieu pumping reduction was distributed equally among all
identified agricultural wells starting in 2026.
Figure 9-2 displays the baseline and Project Scenario 1 hydrographs for this project for the four Edna
Valley wells identified as the RMS for the Chronic Lowering of Groundwater Levels Sustainability
Indicator. Data from these hydrographs indicate that the increase in water levels over the baseline
scenario in year 2042 at these wells ranges from 5 feet at EV-04 to 31 feet at EV-16. These results are
summarized in Table 9-6.
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It should be noted that it is recognized that some model results in the vicinity of RMS EV-04 seem
anomalous; the well at this location is relatively insensitive to changes in pumping, and the magnitude
of the seasonal and drought water level fluctuations is not fully captured. This was identified in the
model documentation as an area where the model may be improved, but in general the model results
are instructive.
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Figure 9-2. SWP with In-Lieu Agricultural Pumping Reduction - 1,000 AFY – Project Scenario 1
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9.4.1.2. Supply Reliability (§ 354.44.6)
The latest estimates of anticipated SWP availability under future conditions are included in the
Department of Water Resources 2019 SWP Delivery Capability Report (DCR) (DWR, 2019). The 2019
DCR anticipates approximately 58% of the SLOCFCWCD’s and 59% of the SBCFCWCD’s Table A and
other contract amounts will be available on average under anticipated future conditions. These
estimates are based on outputs from the CALSIM-2 Operations model (DWR, 2019). However, the
availability of these SWP water supplies will be variable year by year based on hydrologic conditions.
The historical delivery of Annual Allocation from the SWP ranges from 5% to 100% of the contracted
amount. Because current demand for State Water in San Luis Obispo County is only 4,830 AFY out of
the 25,000 AFY Table A amount, in many years there is unused State Water available.
Given the variable availability of SWP supplies, a project to deliver 1,000 AFY of SWP water to Edna
Valley would likely need to be sized to accommodate greater than 1,000 AFY during wet years to
balance out lower delivery amounts during dry years. Alternatively, contracts for the purchase of SWP
could be structured to ensure a minimum delivery of 1,000 AFY of SWP water (e.g., purchasing extra
water to serve as a drought buffer or more Table A Allocation or supply than delivery capacity) to
provide a higher level of reliability for the SWP. However, to incorporate this enhanced reliability would
likely increase the costs of the SWP supplies. For the purposes of the initial project level evaluation
include in this GSP the capacity to deliver and availability of water were assumed to be a constant
1,000 AFY.
9.4.1.3. Project Costs (§ 354.44.8)
The estimated capital cost to construct a turnout off from the Coastal Branch Pipeline and infrastructure
to connect to the existing Edna Valley Growers Mutual Water Company distribution system is
approximately $890,000 equating to an annual payment of $63,000 and a unit cost of $60/AF. These
costs do not include the cost to purchase SWP or the work required to negotiate a contract with the
District or District subcontractors.
9.4.1.4. Project Implementation (§ 354.44.4)
Investigating the use of SWP as a supplemental water source would occur within the first year of
implementation. Following the recommendations of the feasibility study, negotiations to acquire SWP
from the identified sellers could take up to 5 years. The design and construction of the turnout and
pipeline could occur concurrent with the negotiations and occur within 5 years.
9.4.1.5. Basin Uncertainty (§ 354.44.9d)
The benefits from the projects in terms of improved water levels in the Basin are evaluated using the
integrated GSFLOW model. It should be understood that there is uncertainty that is inherent in the
modeling process, including uncertainty with respect to parameters describing the subsurface
environment, historical volumes of pumping, etc. The Integrated Model Calibration TM (Appendix E)
identifies uncertainty and the need for additional data collection in the conceptual model, model
parameters, and calibration.
9.4.1.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water, and privileges. The GSAs have the legal authority to conduct a feasibility study into the use of
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SWP as a supplemental water supply for the SLO Basin. Following the recommendation from the
feasibility study the project could be implemented by the GSAs, GSC members or other parties.
9.4.1.7. Permitting and Regulatory Processes (§ 354.44.3)
No permits or regulatory processes would be necessary for development of the feasibility study.
However, implementation of this project will likely require a California Environmental Quality Act
(CEQA) environmental review process and may require an Environmental Impact Report or a Mitigated
Negative Declaration (the review could also result in a Negative Declaration or Notice of Exemption).
Additionally, permits from a variety of state and federal agencies may be necessary, and any project
that coordinates with federal facilities or agencies may require National Environmental Policy Act
(NEPA) documentation.
A new connection or turnout infrastructure requires coordination and agreements with the District,
CCWA, and DWR.
9.4.1.8. Public Notice and Outreach (§ 354.44B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
9.4.2. City of SLO Recycled Water for Agricultural Irrigation
The City owns and operates a Water Resource Recovery Facility (WRRF) that treats municipal
wastewater from the City, Cal Poly, and the San Luis Obispo County Airport. Tertiary treated and
disinfected effluent is distributed for landscape irrigation and construction uses, or/and dechlorinated
and discharged to San Luis Obispo Creek. The City is required to maintain a minimum daily average
year-round discharge of 2.5 cubic feet per second (cfs) of treated effluent to San Luis Obispo Creek,
which equals approximately 1.6 MGD or 1,800 AFY, for protection of downstream biological resources
as required by the National Oceanic Atmospheric Administration, National Marine Fisheries Service
(NOAA NMFS).
The City of San Luis Obispo has been utilizing recycled water as a component of its multi-source water
supply since 2006. The City’s goal is to use this water source to the highest and most beneficial use.
The City is committed to the expansion of its non-potable recycled water programs and to the
development of a potable reuse program to supplement groundwater and/or surface water supplies.
The delivery of the City’s recycled water to parties within the Edna Valley area has been identified as a
potential short-term augmentation project to offset further lowering of groundwater levels within the
Edna Valley.
With current in-City recycled water demands and influent into the WRRF, it is anticipated that the City
could provide 500-700 acre-feet of recycled water annually with quantities decreasing as new in-City
users come online, as indoor water conservation is increased as a result of statewide water efficiency
mandates, and as the City develops potable reuse projects to supplement its water supplies. In-City
groundwater basin augmentation efforts, new regulations, drought, additional in-City customers, and
the like could reduce the quantity available to outside users by several hundred acre-feet per year in
the foreseeable future.
The project includes the construction of 2,600 feet of 8-inch pipeline, a pumpstation, and a turnout to
connect to the existing Edna Valley Growers Mutual Water Company distribution system. The project
would allow for the delivery of approximately 100 AF/Month in the winter months with minimal amounts
available during summer months and would replace some of the agricultural irrigation demands
currently met by groundwater.
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9.4.2.1. Project Benefits (§ 354.44.5)
This project is considered to be one of the various projects that may provide portions of the water
supply needed to reduce Edna Valley agricultural pumping by 1,000 AFY. As such, it is considered
conceptually to be part of the same model scenario (i.e., Project Scenario 1) as described in Section
9.4.1 State Project Water to Agriculture Irrigation. Because of the uncertainty of the supply, no model
runs were dedicated specifically to this project. It is one of the sources that would provide benefits to
Basin water levels as described in Section 9.4.1.1.
9.4.2.2. Supply Reliability (§ 354.44.6)
The quantity of recycled water available for use to City customers is dependent on the quantity of
untreated wastewater flowing into the City’s WRRF. Unlike most cities that experience relatively uniform
recycled water availability throughout the year, the City of San Luis Obispo’s recycled water availability
is drastically impacted by the students from Cal Poly vacating the community during the summer
months and thus decreasing the wastewater influent into the WRRF. This decrease in wastewater
influent occurs during the summer months when the City’s 50+ recycled water accounts increase
irrigation to combat the warm, dry conditions. This decrease in availability, coupled with a substantial
increase in demand, abnormally limits the recycled water available during the summer months.
Long-Term Versus Short-Term Availability
While there is currently surplus recycled water available year-round, with over 150 acre-feet per month
available in some winter and spring months, it is anticipated that the City will not have a significant
volume of recycled water supply available to sell to any outside-City users from June-October once the
internal City demands increase to support new residential and commercial developments. Recycled
water demands from Avila Ranch, San Luis Ranch, Righetti Ranch, and other future in-City
developments are expected to result in increased recycled water demand of roughly 400-500 acre-feet
per year with most of this demand occurring during the summer. These developments are currently
being constructed with many of the Orcutt Area developments already receiving recycled water
deliveries. The City continues to update its recycled delivery projections, as any amounts obligated for
delivery beyond availability would need to be made up by use of City potable water supplies. This
concern related to availability will continue to increase as both in-City and Cal Poly users continue to
improve in their indoor water use efficiency in alignment with State regulations.
As the City continues to develop its groundwater pumping program, it has been identified that there is
significant recharge potential (upwards of 400 acre-feet per year) within the City’s portion of the SLO
Basin adjacent to the WRRF. Recharge projects in other areas of the City have not yet been studied
but are anticipated to increase the amount of water that could be recharged to the Basin. As the City
resumes its groundwater pumping, additional capacity will likely be created within San Luis Valley
subarea of the Basin, increasing the City’s need for recycled water for recharge projects that may
ultimately be to supplement the basin to ensure compliance with SGMA. As surface water supplies are
adversely impacted by climate change, augmentation of the Basin will be the City’s major water supply
expansion strategy and will limit water availability for outside-City interests as augmentation projects
come online. Potable reuse through storage in the Basin may also address the issues with seasonal
availability by creating a prolonged time lag between highly treated wastewater injection/percolation
and its withdrawal for use.
Physical Delivery Constraints
The City’s recycled water storage and distribution system was designed to provide intermittent in-City
deliveries within the southern half of the City. The City’s storage tank, pumps, telemetry, and pipelines
were not designed to provide recycled water to outside-City customers and may require upgrades in
order to accommodate the continuous 24/7 delivery needed to deliver substantial volumes of water to
the Edna Valley subarea. Additionally, the two potential pipeline alignments that could be utilized to
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deliver water to the Edna Valley area are sized for domestic irrigation delivery and limit the ability to
deliver recycled water during the winter and spring months when it is most abundantly available. One
pipeline located along Broad Street near the Airport is 6-inch diameter C900 pipe and the other,
located along Tank Farm Road, is 8-inch diameter ductile iron pipe. It is estimated that the larger of the
two pipelines could deliver approximately 100 acre-feet of recycled water per month if operated 24-
hours per day for a full month. These undersized pipelines constrain the amount of water that could be
delivered to outside City customers during the winter and spring months when it is available in its
highest quantities.
9.4.2.3. Project Costs (§ 354.44.8)
The estimated capital cost to connect the City’s recycled water distribution to the existing Edna Valley
Growers Mutual Water Company distribution system is approximately $1,004,000 equating to an annual
payment of $153,000 and a unit cost of $260/AF. These costs do not include the cost of the water that
will be purchased from the City. The City’s recycled water is approved to be sold within City limits for
approximately $4,000/AF.
9.4.2.4. Project Implementation (§ 354.44.4)
The circumstance for implementation of this project is driven by the Basin overdraft conditions in the
Edna Valley. The City and representatives from the Edna Valley have been discussing the feasibility of
the project during the development of this GSP. It is estimated that the design and construction of the
pipeline could occur within 1 to 3 years of the GSP Implementation.
9.4.2.5. Basin Uncertainty (§ 354.44.9d)
The addition of recycled water as a supplemental water supply source would help address the
estimated overdraft described in Chapter 6 (Water Budget) in the Edna Valley portion of the Basin. The
benefits from the project in terms of improved water levels in the Basin are evaluated using the
integrated GSFLOW model. It should be understood that there is uncertainty that is inherent in the
modeling process, including uncertainty with respect to parameters describing the subsurface
environment, historical volumes of pumping, etc. The Integrated Model Calibration TM (Appendix E)
identifies the uncertainties and the need for additional data collection in the conceptual model, model
parameters, and calibration.
9.4.2.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water rights, and privileges. The GSAs have the legal authority to conduct a feasibility study into the
use of SWP as a supplemental water supply for the SLO Basin. Following the recommendation from
the feasibility study the project could be implemented by the GSAs, GSC members or other parties.
The City owns its recycled water and has the legal authority to sell its recycled water in alignment with
its policies.
9.4.2.7. Permitting and Regulatory Processes (§ 354.44.3)
This project would require review and approval by the SLO City Council. The project may require a
CEQA environmental review process and may require an Environmental Impact Report or a Mitigated
Negative Declaration (the review could also result in a Negative Declaration or Notice of Exemption).
Additionally, permits from a variety of state and federal agencies may be necessary, and any project
that coordinates with federal facilities or agencies may require NEPA documentation.
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Delivery of recycled water to the Edna Valley may require analysis to confirm that the large-scale,
ongoing application of recycled water does not result in recycled water recharging the groundwater
basin and thus constituting a potable reuse project. Direct application of recycled water at agronomic
rates is allowable under the City’s existing recycled water delivery permit.
While the City has policy language that allows for the sale of recycled water outside of City limits.
Specific findings must be made for this to be permitted. Examples of these findings include
requirements for receiving properties to record a conservation, open space, Williamson Act, or other
easement instrument to maintain the area being served in agriculture and open space, assurance that
recycled water will not be used to increase development potential of the property being served, and that
recycled water will not be further treated to make it potable. Contract negotiations related to the sale
price of recycled water, term of delivery, etc. would require approval of the San Luis Obispo City
Council.
9.4.2.8. Public Notice and Outreach (§ 354.44B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
9.4.3. State Water Project Recharge Basin
To enhance recharge in the Edna Valley, a groundwater recharge basin could be constructed to
percolate SWP water. A groundwater recharge basin is a bermed basin structure designed for the
purpose of efficiently allowing water collected in the basin to infiltrate through the ground surface,
percolate through the vadose zone, and ultimately recharge the underlying aquifer. The concept of this
project is to construct a recharge basin in the Edna Valley and supply it with water obtained from the
SWP to recharge the aquifer.
The conceptual location selected for this project is near the southeast corner of Biddle Ranch Road and
State Highway 227 (aka, Edna Road, Figure 9-3). This area is classified as having high recharge
potential in the Stillwater Percolation zone Study discussed in Chapter 4 (Basin Setting). This land is
currently utilized for agriculture, and it is assumed that a parcel of land adequate to build the recharge
basin could be purchased. Water would be conveyed via a 6,000 foot 6-inch pipeline from the SWP
pipeline, along Biddle Ranch Rd, to a newly constructed recharge basin on approximately 5 acres of
land along Orcutt Road.
9.4.3.1. Project Benefits (§ 354.44.5)
In order to assess this project’s benefits to the aquifer and effect on sustainability of the Basin in terms
of expected water levels, Project Scenario 2 was simulated using the integrated GSFLOW model
developed as part of the GSP effort. The project was defined to represent 500 AFY of supplemental
water provided from the SWP made available to a newly constructed recharge basin to be located in
Edna Valley. Benefits of recharge basins versus direct delivery to offset pumping include the potential
to deliver water during seasonal periods when there is less demand for SWP water supplies and
capacity in the SWP conveyance systems.
A baseline simulation was performed as previously described. The recharge basin is assumed to be
less than 500 feet by 500 feet in area and is simulated in a single cell in the model. Recharge is input
as a flux in MODFLOW (feet/day), so a flux rate equivalent to 500 AFY percolating into a 500 ft by 500
ft cell was input into model cell on a constant basis. The project was defined as beginning in 2026,
allowing five years for project design and implementation.
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Figure 9-3 displays the baseline and Project Scenario 2 hydrographs for this project for the four Edna
Valley wells identified as RMS for the Chronic Lowering of Groundwater Levels Sustainability Indicator.
Data indicate that the increase in water levels over the baseline scenario in year 2042 at these wells
ranges from 2 feet at EV-16 to 52 feet at EV-04, which is the closest RMS to the recharge basin
location. The water level increase in the SWP recharge basin scenario over baseline was 21 feet at EV-
09, and 4 feet at EV-13. These results are summarized in Table 9-6.
9.4.3.2. Supply Reliability (§ 354.44.6)
The supply reliability of the SWP is discussed in detail in Section 9.4.1.2 and is applicable to this
project. This project assumes a total of 500 AFY would be purchased and recharged in the Edna
Valley. If both the SWP for Agricultural Irrigation and the SWP Recharge Basin projects were to be
implemented the total capacity of SWP would be 1,500 AFY and contracts would need to be negotiated
accordingly.
9.4.3.3. Project Costs (§ 354.44.8)
The estimated capital cost to construct a turnout off from the Coastal Branch Pipeline and infrastructure
to connect to a newly constructed recharge basin is approximately $3,624,000 which equates to annual
payment of $337,000 and a unit cost of $670/AF. If multiple SWP groundwater recharge projects are
implemented, the cost of the turnout and other infrastructure can be shared. These costs do not
include the cost to purchase SWP or the work required to negotiate a contract with the District or
District subcontractors.
9.4.3.4. Project Implementation (§ 354.44.4)
The circumstance for implementation of this project is driven by the overdraft conditions in the Edna
Valley. The feasibility study evaluation of the use of the SWP as a supplemental water source to
recharge groundwater within the Edna Valley could occur within the first year of implementation.
Following the recommendations of the feasibility study, negotiations to acquire SWP from the identified
sellers could take up to 5 years. The design and construction of the turnout and pipeline could occur
concurrent with the negotiations and be completed within 5 years.
9.4.3.5. Basin Uncertainty (§ 354.44.9d)
The addition of SWP as a supplemental water supply source would help address the uncertainty of the
estimated overdraft described in Chapter 6 (Water Budget) in the Edna Valley portion of the Basin. The
benefits from the projects in terms of improved water levels in the Basin are evaluated using the
integrated GSFLOW model. It should be understood that there is uncertainty that is inherent in the
modeling process, including uncertainty with respect to parameters describing the subsurface
environment, historical volumes of pumping, etc. The Integrated Model Calibration TM (Appendix E)
identifies uncertainty and the need for additional data collection in the conceptual model, model
parameters, and calibration.
9.4.3.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water, and privileges. The GSAs have the legal authority to conduct a feasibility study into the recharge
of SWP as a supplemental water supply for the SLO Basin. Following the recommendation from the
feasibility study the project could be implemented by the GSAs, GSC members or other parties.
Projects and Management Actions (§354.44) Section 9
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Groundwater Sustainability Agencies 9-21
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9.4.3.7. Permitting and Regulatory Processes (§ 354.44.3)
No permits or regulatory processes would be necessary for development of the feasibility study.
However, implementation of this project will likely require a CEQA environmental review process and
may require an Environmental Impact Report or a Mitigated Negative Declaration (the review could also
result in a Negative Declaration or Notice of Exemption). Additionally, permits from a variety of state
and federal agencies may be necessary, and any project that coordinates with federal facilities or
agencies may require NEPA documentation.
A new connection or turnout infrastructure requires coordination and agreements with the District,
CCWA, and DWR.
9.4.3.8. Public Notice and Outreach (§ 354.44B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
Projects and Management Actions (§354.44) Section 9
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Agencies 9-22 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 9-3. SWP Recharge Basin – 500 AFY – Project Scenario 2
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9.4.4. State Water Project to Golden State Water Company
Golden State Water Company (GSWC) currently provides water to a small service area of County
administered land in the central part of the Basin, near the boundary of Edna Valley and San Luis
Valley. GSWC obtains its supply from groundwater wells within their service area. The recent drought
resulted in significant constraints on GSWC’s groundwater supplies. Because their service area is
relatively small, their ability to site new wells to expand their source locations is limited. For this reason,
the conceptual project of obtaining SWP water to augment GSWC’s current supplies is evaluated.
This project assumes a SWP delivery of 200 AFY to GSWC, representing about 50% of its long-term
demand. To implement this project, a turnout to the SWP pipeline along Orcutt Road will be required.
From the corner of Orcutt Road and Biddle Ranch Road, approximately 8,000 feet of pipeline along
Biddle Ranch Road will be required to convey the water from the SWP pipeline to the edge of the
GSWC service area. Infrastructure improvements internal to GSWC’s system are not included in this
project evaluation.
9.4.4.1. Project Benefits (§ 354.44.5)
In order to assess this project’s benefits to the aquifer and effect on sustainability of the Basin in terms
of expected water levels, Project Scenario 3 was simulated using the integrated GSFLOW model
developed as part of the GSP effort. This project assumes a 200 AFY reduction in pumping by GSWC.
Edna Ranch MWC and Varian Ranch MWC pumping was also reduced, but these water companies are
distant enough that results from one are not expected to have a significant impact on the other. As with
the scenarios for agricultural pumping reduction, the water to offset this pumping reduction may come
from this project or another source; in this case, additional water for GSWC may come from the SWP
or/and City of SLO water (Section 9.4.5).
Modeled pumping for GSWC was reduced by 50% from recent annual pumping volumes at their
operating wells. It is assumed that the remaining demand for GSWC’s service area would be met
through supplemental water from the SWP.
Figure 9-4 displays the baseline and project scenario hydrographs for this project for the four Edna
Valley wells identified as RMS for the Chronic Lowering of Groundwater Levels Sustainability Indicator
(EV-04, EV-09, EV-13, and EV-16). The data indicate that the increase in water levels over the baseline
scenario in year 2042 at these wells ranges from 3 feet at EV-13 to 15 feet at EV-09, which is a GSWC
well. These results are summarized in Table 9-6.
9.4.4.2. Supply Reliability (§ 354.44.6)
The supply reliability of the SWP is discussed in detail in Section 9.4.1.2 and is applicable to this
project. This project assumes a total of 200 AFY would be purchased and delivered to GSWC.
9.4.4.3. Project Costs (§ 354.44.8)
The estimated capital cost to construct a turnout off from the Coastal Branch Pipeline, infrastructure to
connect to the GSWC is approximately $2,685,000 which equates to annual payment of $192,000 and
a unit cost of $960/AF. If multiple projects which require SWP water are implemented, the cost of the
turnout and other infrastructure can be shared. These costs do not include the cost to purchase SWP
or the work required to negotiate a contract with the District or District subcontractors.
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9.4.4.4. Project Implementation (§ 354.44.4)
The circumstance for implementation of this project is driven by the overdraft conditions in the Edna
Valley The feasibility study into the use of the SWP as a supplemental water source to GSWC would
occur within the first year of implementation. Following the recommendations of the feasibility study,
negotiations to acquire SWP from the identified sellers could take up to 5 years. The design and
construction of the turnout and pipeline could occur concurrent with the negotiations and occur within 5
years.
9.4.4.5. Basin Uncertainty (§ 354.44.9d)
The addition of SWP as a supplemental water supply source to GSWC would help address the
uncertainty of the estimated overdraft described in Chapter 6 (Water Budget) in the Edna Valley portion
of the Basin. The benefits from the projects in terms of improved water levels in the Basin are evaluated
using the integrated GSFLOW model. It should be understood that there is uncertainty that is inherent
in the modeling process, including uncertainty with respect to parameters describing the subsurface
environment, historical volumes of pumping, etc. The Integrated Model Calibration TM (Appendix E)
identifies uncertainty and the need for additional data collection in the conceptual model, model
parameters, and calibration.
9.4.4.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water rights, and privileges. The GSAs have the legal authority to conduct a feasibility study into the
obtaining SWP as a supplemental water supply for the SLO Basin. Following the recommendation from
the feasibility study the project could be implemented by the GSAs, GSC members or other parties.
9.4.4.7. Permitting and Regulatory Processes (§ 354.44.3)
No permits or regulatory processes would be necessary for development of the feasibility study.
However, implementation of this project will likely require a CEQA environmental review process and
may require an Environmental Impact Report or a Mitigated Negative Declaration (the review could also
result in a Negative Declaration or Notice of Exemption). Additionally, permits from a variety of state
and federal agencies may be necessary, and any project that coordinates with federal facilities or
agencies may require NEPA documentation.
A new connection or turnout infrastructure requires coordination and agreements with the District,
CCWA, and DWR.
9.4.4.8. Public Notice and Outreach (§ 354.44B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
Projects and Management Actions (§354.44) Section 9
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Agencies 9-25 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Figure 9-4. SWP Purveyor In-Lieu Pumping Reduction – GSWC = 200 AFY, VRMWC & ERMWC = 50 AFY – Project Scenario 3
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9.4.5. Varian Ranch Mutual Water Company Arroyo Grande Subbasin Wells
The Varian Ranch MWC (VRMWC) is located in the southeastern extent of the Basin and currently
supplies its service area from wells within the Basin. However, its service area extends into the
neighboring Arroyo Grande Subbasin of the Santa Maria River Valley Groundwater Basin (SMRVGB).
Twenty-two of their fifty-one parcels are located outside of the Basin in the adjacent Arroyo Grande
Creek watershed. VRMWC owns an existing well, located on its property in the Arroyo Grande
Subbasin that has been tested and found to be suitable for use as a domestic supply source for its
service area.
The concept of this project is to build a conveyance pipeline to deliver approximately 50 AFY of water
from the well that VRMWC owns in the Arroyo Grande Subbasin to an interconnection point within its
current distribution system in the Basin. The project would also evaluate a connection with the adjacent
Edna Ranch MWC (ERMWC). It is estimated that this pipeline will be 6 inches in diameter and
approximately 10,850 feet long. The project also includes well pump and well site improvements.
Utilization of this well to supply a portion of VRMWC and ERMWC’s demand would reduce the pumping
required of their wells in the Basin and would benefit water levels in the area.
9.4.5.1. Project Benefits (§ 354.44.5)
This project is considered to be one of the various projects that may provide supply to reduce pumping
by the small water purveyors in Edna Valley. As such it is considered conceptually to be part of the
same scenario as described in Section 9.4.4, SWP to GSWC.
Modeled pumping for both ERMWC and VRMWC wells in the Edna Valley were reduced by 50 AFY
and is offset by groundwater pumped from the Arroyo Grande Subbasin.
Figure 9-4 displays the baseline and project scenario hydrographs for this project for the four Edna
Valley wells identified as RMS for the Chronic Lowering of Groundwater Levels Sustainability Indicator
(EV-04, EV-09, EV-13, and EV-16). The data indicate that the increase in water levels over the baseline
scenario in year 2042 is about 7 feet at EV-16 (a MWC well).
9.4.5.2. Supply Reliability (§ 354.44.6)
The water source for this project is groundwater from the Arroyo Grande Subbasin. The County and
City of Arroyo Grande are currently developing a GSP for the Arroyo Grande Subbasin and will be
developing a detailed water budget which will provide information regarding the reliability of the
groundwater source.
9.4.5.3. Project Costs (§ 354.44.8)
The estimated capital cost to convey groundwater from the Arroyo Grande Subbasin to the Varian
Ranch distribution system is approximately $2,701,000 equating to an annual payment of $176,000 and
a unit cost of $4,200/AF. These costs do not include any costs to purchase the water since the
VRMWC currently owns the well.
9.4.5.4. Project Implementation (§ 354.44.4)
The circumstance for implementation of this project is driven by the overdraft conditions in the
southeastern portion of Edna Valley. The feasibility study into the use of VRWMC wells in Arroyo
Grande Subbasin as a supplemental water source to both VRMWC and ERMWC would occur within
the first year of implementation. Following the recommendations of the feasibility study the design and
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construction of the turnout and pipeline could occur concurrent with the negotiations and occur within 3
years.
9.4.5.5. Basin Uncertainty (§ 354.44.9d)
The addition of the Arroyo Grande Varian Ranch MWC wells as a supplemental water supply source to
VRMWC and Edna Ranch MWC would help address the uncertainty of the estimated overdraft
described in Chapter 6 (Water Budget) in the Edna Valley portion of the Basin. The benefits from the
projects in terms of improved water levels in the Basin are evaluated using the integrated GSFLOW
model. It should be understood that there is uncertainty that is inherent in the modeling process,
including uncertainty with respect to parameters describing the subsurface environment, historical
volumes of pumping, etc. The Integrated Model Calibration TM (Appendix E) identifies uncertainty and
the need for additional data collection in the conceptual model, model parameters, and calibration.
9.4.5.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water rights, and privileges. The GSAs have the legal authority to conduct a feasibility study into the
utilizing the Arroyo Grande Subbasin as a supplemental water supply for the southeastern portion of
Edna Valley.
San Luis Obispo County Code Chapter 8.95 currently requires that a permit be obtained for any export
of groundwater greater than 0.5 AFY from a Bulletin 118 defined groundwater basin within the County.
The ordinance requires that the export permit only be approved if the Director of Public Works finds that
the proposed export will not cause or contribute to significant detrimental impacts to groundwater
resources, including such impacts to health, safety and welfare of overlying property owners.
9.4.5.7. Permitting and Regulatory Processes (§ 354.44.3)
This project may require a CEQA environmental review process and may require an Environmental
Impact Report or a Mitigated Negative Declaration (the review could also result in a Negative
Declaration or Notice of Exemption). Additionally, permits from a variety of state and federal agencies
may be necessary, and any project that coordinates with federal facilities or agencies may require
NEPA documentation.
9.4.5.8. Public Notice and Outreach (§ 354.44B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
9.4.6. State Water Project to the Mutual Water Companies
The VRMWC and ERMWC located in the southeastern extent of the Basin, currently provides water
supply to their service areas from wells within the Basin. The recent drought resulted in significant
constraints on their supplies.
To implement this project, a turnout to the SWP pipeline along Orcutt Road will be required. From the
corner of Orcutt Road and Biddle Ranch Road, approximately 8,000 feet of pipeline along Biddle Ranch
Road will be required to convey the water from the SWP pipeline to the edge of the ERMWC service
area. Infrastructure internal to ERMWC and VRMWC’s system is not included in this project evaluation.
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9.4.6.1. Project Benefits (§ 354.44.5)
This project is considered to be one of the various projects that may provide water supply to reduce
pumping by the water purveyors in Edna Valley. As such it is considered conceptually to be part of the
same scenario as described in 9.4.4, SWP to GSWC. Because of the uncertainty of the supply, no
model runs were dedicated specifically to this project. It is one of the sources that would provide the
benefits to Basin water levels described in Section 9.4.4.
9.4.6.2. Supply Reliability (§ 354.44.6)
The supply reliability of the SWP is discussed in detail in Section 9.4.1.2 and is applicable to this
project. This project assumes a total of 50 AFY would be purchased and served to ERMWC and
VRMWC.
9.4.6.3. Project Costs (§ 354.44.8)
The estimated capital cost to construct a turnout off from the Coastal Branch Pipeline, infrastructure to
connect to the ERMWC and VRMWC is approximately $835,000 which equates to annual payment of
$59,000 and a unit cost of $1,180/AF. If multiple projects which require SWP water are implemented,
the cost of the turnout and other infrastructure can be shared. These costs do not include the cost to
purchase SWP or the work required to negotiate a contract with the District or District subcontractors.
9.4.6.4. Project Implementation (§ 354.44.4)
The circumstance for implementation of this project is driven by the overdraft conditions in the Edna
Valley The feasibility study into the use of the SWP as a supplemental water source to ERMWC and
VRMWC would occur within the first year of implementation. Following the recommendations of the
feasibility study, negotiations to acquire SWP from the identified sellers could take up to 5 years. The
design and construction of the turnout and pipeline could occur concurrent with the negotiations and
occur within 5 years.
9.4.6.5. Basin Uncertainty (§ 354.44.9d)
The addition of SWP as a supplemental water supply source to ERMWC and VRMWC would help
address the uncertainty of the estimated overdraft described in Chapter 6 (Water Budget) in the Edna
Valley portion of the Basin. The benefits from the projects in terms of improved water levels in the Basin
are evaluated using the integrated GSFLOW model. It should be understood that there is uncertainty
that is inherent in the modeling process, including uncertainty with respect to parameters describing the
subsurface environment, historical volumes of pumping, etc. The Integrated Model Calibration TM
(Appendix E) identifies uncertainty and the need for additional data collection in the conceptual model,
model parameters, and calibration.
9.4.6.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water rights, and privileges. The GSAs have the legal authority to conduct a feasibility study into the
obtaining SWP as a supplemental water supply for the SLO Basin. Following the recommendation from
the feasibility study the project could be implemented by the GSAs, GSC members or other parties.
Projects and Management Actions (§354.44) Section 9
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Groundwater Sustainability Agencies 9-29
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9.4.6.7. Permitting and Regulatory Processes (§ 354.44.3)
No permits or regulatory processes would be necessary for development of the feasibility study.
However, implementation of this project will likely require a CEQA environmental review process and
may require an Environmental Impact Report or a Mitigated Negative Declaration (the review could also
result in a Negative Declaration or Notice of Exemption). Additionally, permits from a variety of state
and federal agencies may be necessary, and any project that coordinates with federal facilities or
agencies may require NEPA documentation.
A new connection or turnout infrastructure requires coordination and agreements with the District,
CCWA, and DWR.
9.4.6.8. Public Notice and Outreach (§ 354.44.B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
9.4.7. Price Canyon Discharge Relocation
Sentinel Peak Resources LLC (Sentinel Peak) is an energy company that operates a well field that
extracts petroleum hydrocarbons from an area approximately 1-2 miles southwest of Edna Valley in
Price Canyon. Sentinel Peak owns and operates a water reclamation facility that treats water to
(CSLRCD, 2014) tertiary standards and has an NPDES permit to discharge into Pismo Creek about 1
mile southwest of Highway 227 near Price Canyon Road. The discharge permit is primarily provided for
increased flow in Pismo Creek and wildlife propagation with a secondary benefit to agriculture.
The proposed project would change the current point of discharge by about 3.5 miles to the upper
portion of West Corral de Piedras Creek in the Edna Valley. The new discharge point would be
approximately 1 mile east of Orcutt Road. The project would provide increased benefit to fisheries from
increased streamflow, and also benefit Edna Valley agriculture by increasing streamflow percolation to
the underlying aquifers. For the purpose of this analysis, it is assumed that 500 AFY of water will be
available to deliver to the new discharge location, resulting in an average of 350 AFY of recharge to the
Basin.
It is anticipated that a 6-inch diameter 17,760 foot long PVC pipeline would convey the water to the new
discharge point. A booster pump would move the water through this pipeline to the new discharge
location. The pipeline would cross approximately 6 agricultural properties, whose owners have already
expressed their willingness to participate in the project, 4 creek crossings and 1 railroad crossing.
9.4.7.1. Project Benefits (§ 354.44.5)
In order to assess this project’s benefits to the aquifer and effects on the sustainability of the Basin,
Project Scenario 4 was simulated using the integrated GSFLOW model developed as part of the GSP
efforts.
This project assumes a transfer of the 500 AFY of tertiary treated water that is currently discharged
from Sentinel Peak’s treatment plant to Pismo Creek downstream of the Basin to a new discharge point
on West Corral de Piedra Creek near the northern edge of the Basin. Therefore, 500 AFY (0.7 cubic
feet per second) was added as inflow to the MODFLOW Stream Flow Routing package in the first
model cell representing West Corral de Piedras Creek that is in the Basin. It should be noted that
adding this inflow to the stream segment is not equivalent to adding recharge directly to the aquifer.
The additional streamflow from the project discharge will be routed downstream in the model, and will
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ultimately result in an increased amount of streamflow percolation to the aquifer. However, this amount
of additional streamflow percolation, which would be additional recharge to the aquifer that will benefit
the groundwater users in the Basin, is not directly defined by the model user. It is calculated by the
model based on the parameters defined in the SFR package. Evaluation of the model water budget
results from the baseline and project scenarios indicates that an average of approximately 350 AFY of
the 500 AFY project stream inflow associated with this project ultimately percolates to the aquifer to
increase storage in the Basin.
Figure 9-5 displays the baseline and project scenario hydrographs for this project for the four Edna
Valley wells identified as RMS for the Chronic Lowering of Groundwater Levels Sustainability Indicator
(EV-04, EV-09, EV-13, and EV-16). The data indicate that the increase in water levels over the baseline
scenario in year 2042 at these wells ranges from 6 feet at EV-16 and EV-13, to 8 feet at EV-04 and EV-
09. Inspection of comparative water levels along West Corral de Piedras Creek indicate a water level
increase of over 30 vertical feet along the creek itself.
9.4.7.2. Supply Reliability (§ 354.44.6)
The supply reliability of the Price Canyon discharge is tied to the operations related to the extraction of
petroleum hydrocarbons from the Price Canyon and the associated permits. The long-term availability
of this water source is uncertain.
9.4.7.3. Project Costs (§ 354.44.8)
The estimated capital cost to relocate the discharge point approximately 3.5 miles to West Corral de
Piedras Creek is $4,909,000 equating to an annual payment of $375,000 and a unit cost of $750/AF.
These costs do not include the cost of the water that will be purchased from Sentinel Peak.
9.4.7.4. Project Implementation (§ 354.44.4)
The circumstance for implementation of this project is driven by the overdraft conditions in the Edna
Valley A mitigated negative declaration/initial study was performed in July 2014 by the Coastal San Luis
Resource Conservation District as the lead agency. The feasibility study into the relocation of the Price
Canyon discharge point would occur within the first year of implementation. Negotiations between
Sentinel Peak and representatives from the Edna Valley Growers MWC have been ongoing throughout
the development of this GSP. The design and construction of the turnout and pipeline could occur
concurrent with the negotiations and occur within 3 years.
9.4.7.5. Basin Uncertainty (§ 354.44.9d)
The increased recharge to the Edna Valley as the result of the relocation of the Price Canyon discharge
point would help address the uncertainty of the estimated overdraft described in Chapter 6 (Water
Budget) in the Edna Valley portion of the Basin. The benefits from the projects in terms of improved
water levels in the Basin are evaluated using the integrated GSFLOW model. It should be understood
that there is uncertainty that is inherent in the modeling process, including uncertainty with respect to
parameters describing the subsurface environment, historical volumes of pumping, etc. The Integrated
Model Calibration TM (Appendix E) identifies uncertainty and the need for additional data collection in
the conceptual model, model parameters, and calibration.
9.4.7.6. Legal Authority (§ 354.44.7)
California Water Code §10726.2 provides GSAs the authority to purchase, among other things, land,
water rights, and privileges.
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9.4.7.7. Permitting and Regulatory Processes (§ 354.44.3)
This project may require a CEQA environmental review process and an Environmental Impact Report
or a Mitigated Negative Declaration (the review could also result in a Negative Declaration or Notice of
Exemption). Additionally, permits from a variety of state and federal agencies may be necessary, and
any project that coordinates with federal facilities or agencies may require NEPA documentation.
In addition, permits from the following government organizations that may be required to
relocate the Price Canyon Discharge Point include:
• United States Army Corps of Engineers (USACE) – A Regional General Permit may be required if
there are impacts to wetlands or connections to waters of the United States.
• California Department of Fish and Wildlife (CDFW) – A Standard Agreement is required if the
project could impact a species of concern.
• Environmental Protection Agency (EPA) Region 9 – National Environmental Policy Act (NEPA)
documentation must be submitted for any project that coordinates with federal facilities or agencies.
Additional permits may be required if there is an outlet or connection to waters of the United States.
• National Marine Fisheries Service (NMFS) – A project may require authorization for incidental take,
or another protected resources permit or authorization from NMFS.
• California Department of Transportation (Caltrans) – An Encroachment Permit is required if any
state highway will be obstructed
9.4.7.8. Public Notice and Outreach (§ 354.44.B)
The public notice and outreach associated with this project would occur through GSA, GSC and/or
future governance structure public meetings. If CEQA is required, the project will follow the public
noticing requirements required by CEQA.
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Figure 9-5. Relocation of Price Canyon Discharge Point – 500 AFY
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9.4.8. Modeling of Multiple Projects
Basin groundwater modeling results for each of the projects previously discussed has represented the
project described exclusively and does not model other projects concurrently. The model results
indicate that it is unlikely that any single project presented will, by itself, maintain water levels above the
defined MTs at the RMSs. Therefore, an additional model scenario was developed in which multiple
projects were represented simultaneously, to demonstrate potential results of a multi-project approach.
Technical details of each of the individual projects are presented in the original chapter sections and
are not represented here.
The projects that are modeled in this multiple-projects scenario are:
• Reduction of agricultural pumping by 1,000 AFY (Sections 9.4.1, 9.4.2)
• Reduction of Edna Valley water purveyor pumping by 250 AFY (Sections 9.4.4, 9.4.5, 9.4.6, 9.4.7)
• State Water Project Recharge Basin – 500 AFY (Section 9.4.3)
• Relocation of Sentinel Peak WRF discharge –350AFY (Section 9.4.8)
As with the individual modeled project scenarios, all projects are represented as beginning in the year
2026.
Figure 9-6 displays the baseline and Project Scenario 5 hydrographs for the combined projects for the
four Edna Valley wells identified as RMS for the Chronic Lowering of Groundwater Levels Sustainability
Indicator (EV-04, EV-09, EV-13, and EV-16). The data indicate that the increase in water levels over
the baseline scenario in year 2042 at these wells ranges from 39 feet at EV-16 to 63 feet at EV-EV-09.
The projected water level increase over baseline was 46 feet at EV-16, and 62 feet at EV-04. These
results are summarized in Table 9-6.
This scenario indicates that with all the projects presented incorporated into the management of the
Basin, the benefit to water levels is more than required to achieve sustainability. So just as it has been
stated previously that no one single project will likely bring the basin into sustainability, this scenario
indicates that all of the projects presented are not required to achieve this goal.
Table 9-6 Summary Results of Modeled Scenarios
Scenario Description Increase Over Baseline Groundwater Elevations (ft) in 2042
EV-04 EV-09 EV-13 EV-16
1 Reduce Agricultural pumping by 1,000 AFY 5 11 24 31
2 500 AFY to Recharge Basin 52 21 4 2
3
Reduce Golden State pumping by 200 AFY.
Reduce ERMWC and VRMWC pumping by
50 AFY (combined).
4 15 3 7
4
Discharge 500 AFY as input into West Corral
de Piedras Creek at its entrance to the SLO
Basin
8 8 6 6
5 Scenarios 1 through 4 62 63 39 46
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Figure 9-6. Model Results from the Combined Modeled Project Scenarios – Project Scenario 5
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9.5. Management Actions
The management actions in this plan include the expansion of the monitoring network, development
and implementation of a groundwater extraction metering and reporting plan, and the development of a
demand management plan.
9.5.1. Expand Monitoring Network
This management action expands the monitoring network from the current County monitoring network
of 12 wells to the new network of 40 monitoring wells as presented in Chapter 7 (Monitoring Network)
within the first two years of the GSP implementation. Chapter 7 describes a proposed monitoring
network that has adequate spatial resolution to properly monitor changes to groundwater and surface
water conditions relative to SMCs within the Basin. The network will provide data with sufficient
temporal resolution to demonstrate short-term, seasonal, and long-term trends in groundwater and
related surface conditions. Included in Chapter 7 are recommendations for additional monitoring sites
to better understand the groundwater and surface water interactions which include five surface water
gages which will be paired with five monitoring wells.
9.5.2. Groundwater Extraction Metering and Reporting Plan
As described in Chapter 6 (Water Budget), groundwater extraction from wells is the primary component
of outflow within the groundwater budget. Estimates for historical pumping were derived from various
sources, including purveyor records, land use data and water duty factors, and daily soil-moisture
budgets. The total estimated groundwater production in the SLO Basin during the water budget period
of 2016 to 2019 was approximately 6,000 AFY. Of the 6,000 AFY, only about 5% or 300 AFY is
metered. Groundwater purveyor meter records were provided by the City of San Luis Obispo, Golden
State Water Company, Edna Ranch MWC, and Varian Ranch MWC. A groundwater extraction
metering and reporting plan is a foundational component of the GSP that will facilitate the reporting of
groundwater extraction data and the development of a groundwater accounting framework. The
collection and reporting of this data will enable the GSAs to adaptively manage the groundwater
resources. The location and quantity of agricultural pumping was identified as a significant data gap
during the development of the water budget and integrated model. The collection of metered
groundwater pumping data will provide a key metric to evaluate the effectiveness of the demand
management strategies that will be included in the Demand Management Plan. The Groundwater
Extraction Metering and Reporting Plan will include a de minimis self-certification and non de minimis
extraction and reporting program.
SGMA provides the authority of a GSA to meter groundwater production:
• 10725.8. MEASUREMENT DEVICES AND REPORTING; INAPPLICABILITY OF SECTION TO
DE MINIMIS EXTRACTORS
− A groundwater sustainability agency may require through its groundwater sustainability plan
that the use of every groundwater extraction facility within the management area of the
groundwater sustainability agency be measured by a water-measuring device satisfactory to
the groundwater sustainability agency
Under California Water Code §10725.8(e) Measurement Devices and Reporting, SGMA exempts de
minimis extractors from metering requirements.
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9.5.2.1. De Minimis Self-Certification
De minimis extractor means a person who extracts, for domestic purposes, two acre-feet or less per
year (CWC 10721). The GSAs will consider developing an approach and process to allow de minimis
basin extractors to self-certify that they extract two (2) acre-feet or less per year for domestic purposes.
§ 1030 g) “Domestic purposes” has the same meaning as “domestic uses” as defined in
section 660 of Division 3 of Title 23 of the California Code of Regulations for the purposes of
identifying if an extractor is a de minimis extractor
§ 660. Domestic Uses. Domestic use means the use of water in homes, resorts, motels,
organization camps, camp grounds, etc., including the incidental watering of domestic stock for
family sustenance or enjoyment and the irrigation of not to exceed one-half acre in lawn,
ornamental shrubbery, or gardens at any single establishments. The use of water at a camp
ground or resort for human consumption, cooking or sanitary purposes is a domestic use.
De-minimis groundwater extractors will not be regulated under this GSP. Growth of de minimis
groundwater extractors could warrant regulated use in this GSP in the future. Growth will be monitored
and reevaluated periodically. Estimated groundwater extractions from de-minimis users will be
documented in the annual reports.
9.5.2.2. Non-De Minimis Extraction and Reporting Program
During the first five years of implementation, the Groundwater Extraction Metering and Reporting Plan
will be developed for non deminimis users to report extractions using metering devices or other suitable
methods. Water Code § 10725.8 provides GSAs the power through their GSPs to measure the use of
groundwater extraction facilities for non de minimis extractions.
9.5.3. Demand Management Plan
A demand management plan will be developed and will include the documentation of water
conservation measures taken by the purveyors, documentation of irrigation efficiencies of the
agricultural fields, water efficient crop conversion, volunteer crop fallowing and pumping reductions. It is
intended that the Demand Management Plan will recognize measures already taken by purveyors to
increase water conservation or water use efficiency prior to the adoption of the GSP.
9.5.3.1. Water Conservation Measures
The purveyors in SLO Basin have implemented significant water conservation measures during the
most recent drought. The following sections summarize the water conservation measures that the
metered purveyors (City of SLO, GSWC, VRMWC, ERMWC) have taken to reduce their water use and
will be described in more detail in the demand management plan.
City of SLO
The City of San Luis Obispo has had a defined water conservation program since the 1970s. As an
original signatory to the California Urban Water Conservation Council, the City has not maintained
effective water conservation programs for several decades. In an effort to preserve groundwater
supplies, the City has made significant investments in three surface water reservoirs and a recycled
water program.
Today the City’s per-capita water use is amongst the lowest in the state and is approximately half of
what it was in the late 1980s. The City’s current GPCD water demand is approximately 92 and has
seen virtually no increase since the end of the 2012-2015 drought. City staff anticipate that GPCD
water use within the City will continue to decrease as the State of California adopts enhanced
conservation and water use efficiency mandates.
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Mutual Water Companies
Edna Ranch East and Varian Ranch MWCs have implemented water conservation measures in
response to Basin conditions and the drought since 2014.
The MWC’s presented a technical memorandum at the December 9, 2020 GSC Meeting which
documented the conservation measures taken by the MWC’s and is summarized below (Wallace
Group, 2020):
• New monitoring technology, combined with conservation policies, have resulted in well water
production of 35% compared to the 2013 baseline year, and 26% compared to the 10 year period of
2005 through 2014.
• The combined groundwater production of the MWC’s (75 AFY on average over the last 5 years)
and represents approximately 2% of the total production in the Edna Valley.
Golden State Water Company
In response to the Governor’s Executive Order (B-29-15) the State Water Resources Control Board
(Water Board) imposed restrictions to achieve a statewide 25% reduction in potable urban water usage
through February 28, 2016.
These restrictions will require water consumers to reduce usage as compared to the amount
they used in 2013. (GSWC, 2015). A Staged Mandatory Conservation and Ration Plan was
developed and implemented in 2015. GSWC’s Edna System is currently in Stage 2 which
includes the following conservation measures:
• Stage 1: Outdoor irrigation limited to two days per week, before 8 AM or after 7 PM; even
addresses on Sunday and Wednesday, odd addresses on Tuesday and Saturday
• Stage 2: Irrigation restrictions from Stage 1; $2.50 emergency surcharge per CCF over allocation
GSWC has reduced the groundwater production from about 318 AFY in 2013 to approximately 210
AFY in 2019.
9.5.3.2. Irrigation Efficiency Improvements
Many of the agricultural users of groundwater in the Basin have implemented efficient irrigation
methods and more is envisioned by agricultural operations to improve the irrigation efficiencies. There
are potential irrigation efficiency benefits to the Basin that can be realized by changing the irrigation
methods for some types of crops. Irrigation efficiency refers to the ratio of the amount of water
consumed by the crop to the amount of water supplied through irrigation. Some irrigation water may be
lost to evaporation, to surface runoff, or to deep percolation past the plant root zone. However, some of
the deep percolation water may return to the underlying aquifer as illustrated later in this section.
Irrigation methods vary in how efficient they utilize water, thus leaving an opportunity for modification in
irrigation methods to result in reductions in water use. For example, flood irrigation is less efficient than
spray irrigation, which is less efficient than drip irrigation applied at the surface, which is less efficient
than drip irrigation applied directly to the root zone. Other on-farm water conservation measures may
be implemented to improve irrigation efficiencies such as irrigation water management practices and
measurement of pump flows. If a large enough area of agricultural fields converts to more efficient
methods of irrigation, there may be a net benefit to the Basin that could offset needs for direct pumping
reductions. A key component to understanding the net benefit (gain) in water savings is the concept of
irrigation return flow, i,e, the amount of water that percolates past the root zone, to ultimately reach and
recharge the underlying aquifer. The following analysis demonstrates an example of this concept.
Figure 9-7 uses data that are approximately representative of conditions in Edna Valley. If it is assumed
that the consumptive demand of a specified area of crops is 3,520 AFY, the amount of required water
and calculated irrigation return flow to the aquifer under varying assumptions of irrigation efficiency may
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be significantly different. Figure 9-7 presents a visual presentation of this analysis and documents how
improvements to irrigation efficiency can result in recovery of groundwater levels.
Figure 9-7. Irrigation Efficiency Comparison
Under the assumption of 80% irrigation efficiency, groundwater pumping of 4,400 AFY is required to
provide the crop consumptive demand of 3,520 AFY (i.e., 3520/4400 = 80%). This results in 880 AFY of
pumped water that is not directly up taken by the crop. For this analysis the assumption used in
Chapter 6 (Water Budget) calculations is that 75% of the unused water reaches to the aquifer as return
flow. (It is assumed the remainder is lost to evaporation or permanent entrapment in the vadose zone
pore space). Therefore, 660 AFY reaches the aquifer as return flow. Thus, the net removal from the
aquifer in this example is 3,740 AFY (4,400 AFY pumped reduced by 660 AFY of return flow).
If it is assumed that conversion to more efficient irrigation methods result in overall irrigation efficiency
of 90%, groundwater pumping of 3,911 AFY is required to provide the crop consumptive demand of
3,520 AFY (i.e., 3520/3911= 90%). This results in 391 AFY of pumped water that is not directly up
taken by the crop. Under the same assumptions as previously discussed, 293 AFY reaches the aquifer
as return flow and 98 AFY is lost. Thus, the net removal from the aquifer in this example is 3,618 AFY
(3,911 AFY pumped reduced by 293 AFY of return flow).
The difference in net removal from the aquifer under the assumptions of improved irrigation efficiency,
displayed on Figure 9-7, is 122 AFY. This, then, is the net benefit to the aquifer of improving irrigation
efficiency from 80% to 90%.
It is acknowledged that this example calculation is conceptual. Although groundwater pumping is easily
measured, it is very difficult to accurately measure irrigation return flow, or the evaporative losses of
applied irrigation. However, the hydrologic assumptions behind this analysis are well founded and
commonly accepted in the industry. Therefore, this analysis demonstrates that conceptually there will
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be a net benefit to the aquifer if irrigation efficiency is improved basin wide. 122 AFY of water is
approximately 10% of the Edna Valley overdraft calculated in Chapter 6 (Water Budget). This indicates
that overall improved irrigation efficiency can be a significant contributor to bringing the Basin into
sustainability.
With the implementation of the Groundwater Extraction and Metering plan, the agricultural entities that
implement improved irrigation methods will be able to document the improvements with reported meter
readings.
9.5.3.3. Volunteer Water Efficient Crop Conversion
Chapter 6 (Water Budget) describes the applied water demand by crops within the SLO Basin. These
crop types included citrus, deciduous (non-vineyard), pasture, vegetable, vineyard, and turfgrass.
Estimates of per-acre annual water demand are shown in the table below:
Table 9-7. Consumptive Use of Applied Water and Total Irrigated Acreage by Land Use/Land Cover Type
LAND USE/ LAND COVER
ACRE-FEET PER ACRE PER YEAR
ACREAGE 2018 LOW MED HIGH
Citrus 1.1 1.6 2.2 256
Deciduous 1.8 2.2 2.5 20
Pasture 2.6 3.1 3.7 41
Vegetables1 1.4 1.6 2 768
Vineyard 0.5 0.6 0.8 2410
Turfgrass2 2 2.6 4.1 164
160 percent of ET applied water to account for fallow fields
2Turfgrass represents irrigated turf i.e. lawns, golf courses, etc
As shown above, crop types use different quantities of water per year and the conversion from a less
efficient crop would reduce the overall groundwater demand. This voluntary water efficient crop
conversion program will be included in the Demand Management Plan.
9.5.3.4. Volunteer Land Fallowing
The Voluntary Fallowing Program will create a process to convert high water use irrigated agricultural
lands to low water use open space or other less water intensive land use on a voluntary basis. The
program would be similar to the volunteer water efficient crop conversion program and the resulting
benefit would depend on the initial crop type. This voluntary fallowing program will be included in the
Demand Management Plan.
9.5.3.5. Pumping Reductions
The projects and management actions described above are developed to maintain groundwater levels
above minimum thresholds through in-lieu pumping reductions or increased recharge. The Demand
Management Plan prioritizes the development of water conservation measures, irrigation efficiencies,
volunteer water efficient crop conversion and the volunteer fallowing of crops to avoid mandatory direct
pumping reductions. Mandatory pumping reductions may be required if the criteria for undesirable
results for the sustainability indicators as described in Chapter 8 (Sustainable Management Criteria) is
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met. The implementation of the mandatory direct pumping reductions will be addressed in the Demand
Management Plan.
9.6. Adaptive Management (§ 354.44A)
Adaptive management allows the GSAs to react to the success or lack of success of actions and
projects implemented in the Basin and to make management decisions to redirect efforts in the Basin to
more effectively achieve sustainability goals. The GSP process under SGMA requires annual reporting
and updates to the GSP at minimum every 5 years. These requirements provide opportunities for the
GSAs to evaluate progress towards meeting its sustainability goals and avoiding undesirable results.
Adaptive management triggers are thresholds that, if reached, initiate the process for
considering implementation of adaptive management actions or projects. For SLO Bain, the
trigger for adaptive management is the following:
• If analytical or modeled projections anticipate that future conditions will exceed the undesirable
result thresholds, then the preparation for implementation of additional projects and management
actions would begin.
• If actual conditions exceed the undesirable result thresholds, then additional projects and
management actions will be implemented
10-1
10
GROUNDWATER SUSTAINABILITY PLAN
Implementation Plan
This chapter is intended to serve as a conceptual roadmap for each
Groundwater Sustainability Agency (GSA) to start implementing the
Groundwater Sustainability Plan (GSP) over the first five years and
discusses implementation effects in accordance with the Sustainable
Groundwater Management Act (SGMA) regulations sections 354.8(f)(2) and
(3).
A general schedule showing the major tasks and estimated
timeline for the GSP implementation is provided in Figure 10-1.
The implementation plan provided in this chapter is based on
current understanding of Basin conditions and includes
consideration of the projects and management actions included in
Chapter 9, as well as other actions that are needed to
successfully implement the GSP including the following:
IN THIS CHAPTER
• GSP Implementation
Schedule and Costs
• Funding
• Reporting
• GSP implementation, administration, and management
• Funding
• Reporting, including annual reports and 5-year evaluations and updates
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10.1. GSP Implementation, Administration, and Management
10.1.1. Administrative Approach/Governance Structure
The City and County (GSAs) and the participating parties will continue to operate under the existing
MOA, including the existing governance structure, until actions are taken amending/revising the existing
MOA or developing new agreements (e.g., joint power agreement). The existing MOA is included in
Appendix A and will automatically terminate upon DWR’s approval of the GSP for the Basin. During
DWR’s GSP review process, the GSAs intend to revisit the governance structure before the GSP is
approved to better serve the implementation of the GSP. For example, the updated governance
structure could be established through a new agreement between the GSAs that supersedes the
existing MOA. The agreement would outline details and responsibilities for GSP administration and
implementation among the participating entities and may include provisions to establish other advisory
bodies to advise the GSAs on GSP implementation, updates, etc.
10.1.2. Implementation Schedule
Figure 10-1 illustrates the GSP implementation schedule. Included in the chart are activities necessary
for ongoing GSP monitoring and updates, as well as tentative schedules for the development of
projects and management actions. Additional details about the activities included in the schedule are
provided in these activities’ respective sections of this GSP. Adaptive management and mandatory
demand management would only be implemented if triggering events are reached, as described in
Chapter 9 (Projects and Management Actions), and are shown as ongoing in the schedule.
10.1.3. Implementation Costs
Implementation of this GSP is estimated to cost approximately $965,000 per year for the first five years
of implementation, excluding the planning and development of the specific projects listed in Chapter 9.
Costs related to the various activities anticipated for the first five years are shown in Table 10-1.
Estimates of future annual implementation costs (Years 6 through 20) will be developed during future
updates of the GSP, which will include the development of the various anticipated projects. The costs of
specific projects and management actions will like vary year by year, based in part on needed adaptive
management activities.
10.1.3.1. Administration and Finance
The Administration and Finance implementation activities include the following: GSP Administration
Development, Ongoing GSP Implementation, Fee Study, Funding Mechanism Implementation,
Demand Management Plan. The total estimated cost during the initial five years of the GSP
implementation is approximately $2,850,000 and is shown in Table 10-1. It is anticipated that the
Administrative and Finance Costs will be paid for by regulatory fees and will be analyzed as part of the
fee study as described in Section 10.2.2.
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Figure 10-1. SLO Basin GSP Implementation Schedule
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10.1.3.2. Monitoring Network Implementation
The Monitoring Network Implementation includes the development of a groundwater metering and
reporting plan, development and implementation of a monitoring program, and conducting annual
monitoring. The Groundwater Metering and Reporting Plan is described in detail in Section 9.5
Management Actions and will provide a key metric to evaluate the effectiveness of the demand
management strategies and enable the GSAs to adaptively manage the Basin. The monitoring program
is described in detail in Chapter 7 (Monitoring Network) and the development and implementation of the
monitoring network is targeted to monitor changes to groundwater and surface water conditions relative
to SMCs within the Basin. The annual monitoring is the execution of the data collection required to
complete the Annual Reports. The total estimated cost during the initial five years of the GSP
implementation is approximately $875,000 as shown in Table 10-1. It is anticipated that the Monitoring
Network Implementation will be paid for by regulatory fees and will be analyzed as part of the fee study
as described in Section 10.2.2.
10.1.3.3. Project Implementation
Project implementation is anticipated to include the following steps: Supplemental Water Feasibility
Study; Planning and Design; Construction and Operation. The initial step for project implementation is
anticipated to include completion a Supplemental Water Feasibility Study to further evaluate the
different supplemental water supply options (e.g. SWP, Recycled Water, Price Canyon Discharge
Water, etc.) described in Chapter 9 (Projects and Management Actions). This evaluation will include a
more granular analysis of the parameters associated with each of the different supplemental supply
options available to address the overdraft in the basin, including assessment of seasonal supply
availability and demand patterns, hydraulic capacity, costs of supplemental water,
environmental/permitting requirements, and updated infrastructure and operation & maintenance costs.
The feasibility study will also include additional groundwater model scenario analysis to further
determine beneficiaries of the individual projects to assist in developing equitable project cost sharing
mechanisms.
The findings from the Supplemental Water Feasibility Study will be utilized to inform agreement
negotiations and planning/design of the preferred supplemental water supply projects for the basin. It is
anticipated that the Projects will be paid for by project proponents/beneficiaries and costs associated
with project implementation is not included in the GSP Implementation Budget estimate shown in Table
10-1. Specific details regarding the cost share mechanisms are anticipated to be determined after the
preferred supplemental water projects are identified and further defined. Additionally, it is anticipated
that grant funding would be available to assist with project implementation, see Section 10.2.3.
10.1.3.4. Reporting
SGMA regulations require the GSAs to submit annual reports to DWR on the status of GSP
implementation. The reporting requirements are presented in Section 10.3.1. SGMA regulations
require the GSAs to evaluate the GSP at least every 5 years and whenever the Plan is amended. The
reporting requirements for the periodic evaluation are presented in Section 10.3.2. The initial 5-year
GSP evaluation is due for submission to DWR in April 2027. The estimated cost to prepare an annual
report is $100,000/year and the cost for the initial Five Year GSP update is estimated to be $500,000,
equating to a total of $1,000,000 over the initial five years of the GSP implementation. It is anticipated
that the Reporting Costs will be paid for by regulatory fees and will be analyzed as part of the fee study
as described in Section 10.2.2.
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10.1.4. Outreach and Communication
To meet the requirements of SGMA, implementation of the GSP will require additional communication
and outreach efforts and coordination among the City and County GSAs and stakeholder groups. The
GSP calls for GSAs to routinely provide information to the public about GSP implementation and
ongoing sustainable management of the Basin. The GSP calls for a website to be maintained as a
communication tool for posting data, reports, and meeting information. The website may also include
forms for on-line reporting of information needed by the GSAs (e.g., annual pumping a shown in
mounts) and an interactive mapping function for viewing Basin features and monitoring information.
10.2. Funding
The budget information included in Section 10.1.3 will be used to conduct a fee study which could
include development of funding mechanisms to cover the costs of implementing the regulatory
programs described in the GSP. This fee could include costs related to monitoring and reporting,
hydrogeologic studies, pumping reduction enforcement (if necessary), public outreach, and other
related costs. Project implementation costs are anticipated to be covered by the project proponents
and the associated beneficiaries. Project implementation costs will be evaluated as part of the
Supplemental Water Feasibility Study.
10.2.1. GSP Implementation Funds
Development of this GSP was partially funded through a Proposition 1 Sustainable Groundwater
Planning Grant from DWR, along with in-kind contributions from the GSAs and GSC members.
Although ongoing implementation of the GSP could include contributions from its member agencies,
which are ultimately funded through customer fees or other public funds, additional funding would be
required to implement the GSP. Included in the GSP implementation is a Fee Study that will evaluate
multiple approaches for funding the ongoing administration and implementation of the GSP.
10.2.2. Fee Study
The GSAs plan to perform a fee study to evaluate and provide recommendations for developing GSP
implementation funding mechanisms. This study will include focused public outreach and meetings to
educate and solicit input on the potential fee structures/funding mechanisms (i.e. pumping fees,
assessments, or a combination of both). California Water Code Sections 10730 and 10730.2 provide
GSAs with the authority to impose certain fees, including fees on groundwater pumping. Any imposition
of fees, taxes or other charges would need to follow the applicable protocols outlined in the above
referenced water code sections and all applicable Constitutional requirements based on the nature of
the fee. It is anticipated that the fee study will cover the costs associated with the Administrative and
Finance, Monitoring Network Implementation, and Reporting. The Fee Study is not anticipated to cover
the costs associated with project implementation.
10.2.3. Grant/Low Interest Financing
The GSAs will pursue grants and low-interest financing to help pay for GSP implementation costs to the
extent possible. If grants or low-interest financing is obtained for GSP implementation it could be
utilized to offset costs for the GSAs and basin pumpers. However, as mentioned previously external
funding/financing may only be eligible for project and management action implementation and not
ongoing GSP administrative expenses.
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10.3. Reporting
As part of GSP implementation, SGMA Regulation §356.2 requires the GSAs to develop annual reports
and more detailed five-year evaluations, which could lead to updates of the GSP. The following
sections describe the reporting requirements for both the annual reports and five-year evaluations.
10.3.1. Annual Reports
Annual reports will be developed to address current needs in the Basin and the legal requirements of
SGMA. As defined by DWR, annual reports must be submitted for DWR review by April 1st of each
year following the GSP adoption, except in years when five-year or periodic assessments are
submitted. Annual reports are anticipated to include three key sections: General Information, Basin
Conditions, and Implementation Progress. The GSAs will compile information relevant to annual reports
and the Basin Point of Contact will coordinate collection of information and submit a single annual
report for the Basin to DWR.
10.3.1.1. General Information
The General Information section will include an executive summary that highlights the key content of
the annual report. This section will include a map of the Basin, a description of the sustainability goals,
a description of GSP projects and their progress, as well as an annual update to the GSP
implementation schedule.
10.3.1.2. Basin Conditions
Basin conditions will describe the current groundwater conditions and monitoring results in the Basin.
This section will include an evaluation of how conditions have changed over the previous year and will
compare groundwater data for the water year to historical groundwater data.
Pumping data, effects of project implementation (if applicable), surface water deliveries, total
water use, and groundwater storage data will be included. Key required components include:
• Groundwater level data from the monitoring network, including contour maps of seasonal high and
seasonal low water level maps
• Hydrographs of groundwater elevation data at RMS
• Groundwater extraction data by water use sector
• Groundwater Quality at RMS
• Surface water supply availability and use data by water use sector and source
• Streamflow
• Total water use data
• Change in groundwater in storage, including maps for the aquifer
• Subsidence rates and associated survey data
10.3.1.3. Implementation Progress
Progress toward GSP implementation will be included in the annual report.
This section of the annual report will describe the progress made toward achieving interim
milestones as well as implementation of projects and management actions. Key required
components include:
• GSP implementation progress, including proposed changes to the GSP
Implementation Plan Section 10
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 10-7
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
• Progress toward achieving the Basin sustainability goals
Development of an annual report will begin following the end of the water year, September 30, and will
include an assessment of the previous water year. The annual report will be submitted to DWR before
April 1st of the following year. The 2021 annual report covering water year 2021 will be submitted by
the GSAs by April 1, 2022. Five annual reports for the Basin will be submitted to DWR between 2022
and 2026, prior to the first five-year assessment of this GSP, which is to be submitted to DWR in
January 2027.
10.3.2. Five-Year Evaluation Reports
As required by SGMA regulations, an evaluation of the GSP and the progress toward meeting the
approved sustainable management criteria and the sustainability goal will occur at least every five
years and with every amendment to the GSP. A written five-year evaluation report (or periodic
evaluation report) will be prepared and submitted to DWR. The information to be included in the
evaluation reports is provided in the sections below.
10.3.2.1. Sustainability Evaluation
A Sustainability Evaluation will contain a description of current groundwater conditions for each
applicable sustainability indicator and will include a discussion of overall sustainability in the Basin.
Progress toward achieving interim milestones and measurable objectives will be included, along with an
evaluation of status relative to minimum thresholds. If any of the adaptive management triggers are
found to be met during this evaluation, a plan for implementing adaptive management as described in
Section 9.6 of this GSP will be included.
10.3.2.2. Plan Implementation Progress
A Plan Implementation Progress section will describe the current status of project and management
action implementation and whether any adaptive management actions have been implemented since
the previous report. An updated project implementation schedule will be included, along with any new
projects identified that support the sustainability goals of the GSP and a description of any projects that
are no longer included in the GSP. The benefits of projects and management actions that have been
implemented will be described and updates on projects and management actions that are underway at
the time of the report will be documented.
10.3.2.3. Reconsideration of GSP Elements
As additional monitoring data are collected, land uses and community characteristics change, and GSP
projects and management actions are implemented, it may become necessary to reconsider elements
of this GSP and revise the GSP as appropriate. GSP elements to be reassessed may include basin
setting, management areas, undesirable results, minimum thresholds, and measurable objectives. If
appropriate, a revised GSP, completed at the end of the five-year assessment period, will include
revisions informed by findings from the monitoring program and changes in the Basin, including
changes to groundwater uses, demands, or supplies, and results of project and management action
implementation.
10.3.2.4. Monitoring Network Description
A description of the monitoring network will be provided. An assessment of the monitoring network’s
function will be included, along with an analysis of data collected to date. If data gaps are identified, the
GSP will be revised to include a method for addressing these data gaps, along with an implementation
schedule for addressing gaps and a description of how the GSA will incorporate updated data into the
GSP.
Implementation Plan Section 10
San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies 10-8 San Luis Obispo Valley Basin Groundwater Sustainability Plan
Table 10-1. GSP Implementation Costs (2022-2027)
GSP IMPLEMENTATION ACTIVITY DESCRIPTION ESTIMATED COST UNIT ANTICIPATED TIMEFRAME ESTIMATED COSTS (2022 -2027)
Administrative and Finance
GSP Administration Development Develop Administrative Approach/Governance Structure for GSP Implementation $100,000 Lump Sum Q1-4, 2022 $100,000
Ongoing GSP Implementation Routine GSP Administration (including staffing, overhead expenses, equipment, outreach and
communication, etc.) $500,000 Annual 2021 - 2025 $2,500,000
Fee Study Prepare a fee study to evaluate and provide recommendations for GSP implementation
funding mechanisms $150,000 Lump Sum Q1-4, 2022 $150,000
Funding Mechanism Implementation Implement and begin collecting GSP Implementation fees $100,000 Lump Sum Q1-4, 2023 $100,000
Demand Management Plan
The demand management plan will include the documentation of water conservation
measures, and develop programs for volunteer water efficient crop conversion, volunteer
fallowing of crops, and pumping reductions, etc. in a stakeholder driven process.
$100,000 Lump Sum 2022 - 2023 $100,000
Monitoring Network Implementation
Groundwater Metering and Reporting Plan Develop a plan to establish and maintain a groundwater pumping, metering, and reporting
plan (does not include meters and installation) $150,000 Lump Sum Q1-4, 2022 $150,000
Monitoring Program
Conduct survey of proposed monitoring well network to verify locations and elevations, and
video logging if applicable $100,000 Lump Sum Q1-4, 2022 $100,000
Construction of 5 new monitoring wells and 5 surface water gages for GW/SW interaction,
transducers and surveying $500,000 Lump Sum Q1-4, 2022 $500,000
Annual Monitoring Complete annual monitoring (Field work) $25,000 Annual Q1-4, 2022 $125,000
Project Implementation
Supplemental Water Feasibility Study
Costs estimates for the Supplemental Water Feasibility Study,
Planning/Design and Construction of Supplemental Water Projects not included in the initial 5-Yr budget.
Planning/Design
Construction
Reporting
Annual Reports Compile data and prepare GSP Annual Report $100,000 Annual 2021 - 2025 $500,000
5-Yr GSP Updates Compile data and prepare 5-yr GSP Updates, including Integrated Model updates $500,000 Lump Sum Q2, 2026 - Q1, 2027 $500,000
TOTAL ESTIMATED COSTS (2022 - 2027) $4,825,000
AVERAGE ANNUAL ESTIMATED COST (2022 - 2027) $965,000
Implementation Plan Section 10
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 10-9
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
10.3.2.5. New Information
New information available since the last five-year evaluation or GSP amendment will be described and
evaluated. If the new information should warrant a change to the GSP, this will also be included, as
described previously in Reconsideration of GSP Elements.
10.3.2.6. Regulations or Ordinances
A summary of the regulations or ordinances related to the GSP that have been implemented by DWR
or others since the previous report will be provided. The report will include a discussion of any required
updates to the GSP.
10.3.2.7. Legal or Enforcement Actions
Legal or enforcement actions taken by the GSA in relation to the GSP will be summarized, including an
explanation of how such actions support sustainability in the Basin.
10.3.2.8. Plan Amendments
A description of amendments to the GSP will be provided in the five-year evaluation report, including
adopted amendments, recommended amendments for future updates, and amendments that are
underway.
10.3.2.9. Coordination
Ongoing coordination will be required among the GSA, members of the GSC, and the public. The five-
year evaluation report will describe coordination activities between these entities such as meetings,
joint projects, data collection and sharing, and groundwater modeling efforts.
10.3.2.10. Reporting to Stakeholders and the Public
Outreach activities associated with the GSP implementation, assessment, and GSP updates will be
documented in the five-year evaluation report.
References Section 11
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 11-2
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
11
GROUNDWATER SUSTAINABILITY PLAN
References
Association for the Advancement of Cost Engineering, I. (2011). ACE International Recommended PracticeNo.
17R‐97; Cost Estimate Classification System; TCM Framework: 7.3 ‐ Cost Estimating and Budgeting.
Balance Hydrologics. (2008). Hydrology and Geology Assessment of the Pismo Creek Watershed, San Luis Obispo
County, California.
Barlow, P. M. (2012). Streamflow depletion by wells: understanding and managing the effects of groundwater
pumping on streamflow. United States Geological Survey.
Bennett, S. (2015). Baseflow Data Compilation and GIS Analysis in San Luis Obispo Creek Watershed.
Blaney. (1933). Ventura County Investigation, Bulletin No. 46, California Deparment of Public Works, Division of
Water Resources.
Blaney. (1963). Utilization of the Water of the Santa Ynez River Basin in Southern Santa Barbara County
California, United Stated Deparment of Agriculture.
Boyle Engineering. (1991). City of San Luis Obispo Groundwater Basin Evaluation. January.
California Department of Forestry and Fire Protection. (2015). Fire and Resource Assessment Program (FRAP)
FVEG [ESRI File Geodatabase]. Sacramento, CA.
Carollo. (2012). San Luis Obispo County Master Water Report.
CDFW, C. D. (2019). California Natural Diversity Database (CNDDB). Retrieved November 2019, from
http://www.dfg.ca.gov/whdab/html/cnddb.html
CDFW, C. D. (2019, December). List of California Sensitive Natural Communities. Vegetation Classification and
Mapping Program. Retrieved December 2019, from
https://nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=153609&inline
CDFW, C. D. (2019). Special Vascular Plants, Bryophytes, and Lichens List. Quarterly publication.
CIMS. (2019). Station 52, San Luis Obispo - Central Coast Valleys.
City of San Luis Obispo. (2000). Water Use Factors.
City of San Luis Obispo. (2015). Water Resources Status Report.
City of San Luis Obispo. (2016). 2015 Urban Water Management Plan.
City of San Luis Obispo. (2018). General Plan.
Cleath & Associates, Inc. (2001). Well Construction and Testing Report for Lewis Lane #4, Edna Valley, San Luis
Obispo County. Prepared for Southern California Water Company. July.
Cleath & Associates, Inc. (2003). Well Construction and Testing Report for Water Supply and Irrigation Wells, City
of San Luis Obispo, Hayashi Irrigation Wells and Highway 101 Water Supply Well. March.
References Section 11
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 11-3
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
Cleath-Harris Geologists. (2010). Edna Valley Water System Groundwater Study. Prepared for Golden State
Water Company. May.
Cleath-Harris Geologists. (2018). Groundwater Flow Analysis, Recycled Water Recharge Project, San Luis Valley
Subarea, San Luis Obispo Valley Groundwater Basin.
Cleath-Harris Geologists. (2019). Geophysical Survey of Bedrock Divide Between San Luis Valley and Edna Valley.
Cleath-Harris Geologists, Inc. (2013). Summary of Drilling,. Testing, and Destruction of the Golden State Water
Company Country Club Test Well, Edna Road System, 6110 Lewis Lane, San Luis Obispo, California.
Prepared for Golden State Water Company. June.
Cleath-Harris Geologists, Inc. (2013). Summary of Exploration and Testing, 5061 Hacienda Avenue, San Luis
Obispo, California. Prepared for Golden State Water Company. February.
Cleath-Harris Geologists, Inc. (2014). Summary of Exploration and Testing, Blodgett parcel, Whiskey Run Lane,
Country Club Area, San Luis Obispo, California. Prepared for Golden State Water Company. July.
County of San Luis Obispo. (2014). San Luis Obispo County Integrated Regional Water Management Plan
(IRWMP).
Creek Lands Conservation. (2019). San Luis Obispo County Low Flow Monitoring Report (2015-2018).
CSLRCD, C. S. (2014). Mitigated Negative Declaration/Initial Study Edna Valley Groundwater Basin Recharge and
Steehead Habitat Enhancement Project.
Dibblee, T. (2004). Geologic Map of the Lopez Mountain Quadrangle, San Luis Obispo County, California. Dibble
Geology Center Map.
Dibblee, T. (2004). Geologic Map of the San Luis Obispo Quadrangle, San Luis Obispo County, California. . Dibble
Geology Center Map.
Dibblee, T. (2006). Geologic Map of the Arroyo Grande NE Quadrangle, San Luis Obispo County, California. Dibble
Geology Center Map.
Dibblee, T. (2006). Geologic Map of the Pismo Beach Quadrangle, San Luis Obispo County, California. Dibblee
Geology Center Map.
DWR. (1958). San Luis Obispo County Investigation. State Water Resources Board Bulletin No. 18. . California
Department of Water Resources (DWR). May.
DWR. (1964). San Luis Obispo and Santa Barbara Counties Land and Water Use Survey, 1959. . California
Department of Water Resources (DWR).
DWR. (1987). Southern Central Coast Land Use Survey, 1985. Morro Bay South 54-30 and San Luis Obispo 54-31
Survey Maps.
DWR. (1996). South Central Coast Land Use Survey.
DWR. (1997). San Luis-Edna Valley Groundwater Basin Study, Draft Report. . California Department of Water
Resources (DWR).
DWR. (2002). Water Resources of the Arroyo Grande - Nipomo Mesa Area.
DWR. (2003). California’s Groundwater: Bulletin 118 – Update 2003, Groundwater Basin Descriptions.
DWR. (2014). DWR Atlas - Aglricultural Lang Use and Irrigated Areas. Retrieved 2019, from gis.water.ca.gov
DWR. (2015). Consumptive Use Program Plus (CUP+) Model, in California Water Plan Update 2013, Volume 4.
Reference Guide, Developed by DWR and UC Davis.
DWR. (2016). Best Management Practices for the Sustainable Management of Groundwater Monitoring
Networks and Identification of Data Gaps.
DWR. (2016). California's Groundwater: Bulletin 118 - Interim Update 2016, Working Towards Sustainability.
References Section 11
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 11-4
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
DWR. (2016). Statewide Crop Mapping Datasets. Retrieved from https://data.cnra.ca.gov/dataset/statewide-
crop-mapping
DWR. (2016). Water Budget Best Management Practices for the Sustainable Management of Groundwater.
DWR. (2018). Natural Communities Commonly Associated with Groundwater (NCCAG) Dataset. Retrieved from
https://gis.water.ca.gov/app/NCDatasetViewer/sitedocs/#
DWR. (2019). Sustainable Groundwater Management Act 2019 Basin Prioritization - Process and Results
Document.
DWR, C. D. (2019). Delivery Capability Report and Studies 2019.
eBird. (2017). eBird: An online database of bird distribution and abundance. Retrieved December 2019, from
http://www.ebird.org
Environmental Protection Agency (EPA). (2018). 2018 Edition of the Drinking Water Standards and Health
Advisory Tables. EPA.
EPA. (2008). Water Sense Factsheet - Indoor Water Use in the United States, EPA-832-F-06-004.
ESA Consultants, Inc. (1994). Hydrologic Investigation, Edna Valley Well Location Study. September.
Fugro West and Cleath & Associates. (2002). Paso Robles Groudnwater Study, Final Report.
GEI Consultants. (2005). San Luis Region Integrated Regional Water Management Plan.
GSI Water Solutions. (2018). San Luis Obispo Valley Basin Characterization and Monitoring Well Installation.
GSWC. (2015). Staged Mandatory Conservation and Rationing Plan.
Hall, C. (1973). Geology of the Arroyo Grande Quadrangle, California. . California Division of Mines and Geology.
Hall, C. (1979). Geologic map of the San Luis Obispo – San Simeon Region, California. U.S. Geological Survey.
ITRC. (2020). Official Cal Poly Precipitation Data, Cal Poly/NOAA Station Rain Gage.
Johnson, A. (1967). Specific Yield - Compilation of Specific Yield for Various Materials, U.S. Geological Survey
Water-Supply Paper 1662-D, prepared in cooperation with the California Deparment of Water Resources.
Klausmeyer, K. J.-W.-F. (2018). Mapping indicators of groundwater dependent ecosystems in California.
Retrieved from https://data.ca.gov/dataset/natural-communities-commonly-associated-groundwater
Lewis, D. C. (1964). The relationship between oak tree roots and groundwater in fractured rock as determined
by tritium testing. Journal of Geophysical Research, 2,579–2,588.
NAIP (National Agricultural Imagery Program). (2018). National Geospatial Data Asset (NGDA) NAIP Imagery.
USDA-FSA-APFO Aerial Photography Field Office. Salt Lake City, UT.
National Marine Fisheries Service, (NMFS). (2013). South-Central California Steelhead Recovery Plan. West Coast
Region, Long Beach, California.
QPS. (2005). DRAFT Background Study South San Luis Obispo Groundwater PCE Plume.
Questa Engineering Corporation. (2007). San Luis Obispo Creek Watershed Calibration Study.
Regional Water Quality Control Board, Central Coast Region. (2017). Water Quality Control Plan for the Central
Coast Basin.
Robinson. (1958). Phreatophytes. U.S. Geological Survey Water-Supply Paper 1423.
Rohde, M. M. (2019). Critical Species LookBook: A compendium of California’s threatened and endangered
species for sustainable groundwater management. San Francisco: The Nature Conservancy.
Rosenberg, L. (2001). Potential Aquifer Recharge Areas: Monterey County, California. Monterey County Planning
Department.
San Luis Obispo County Deparment of Agriculture/Weights and Measures. (2019). Crop Surveys for San Luis
Obispo Valley Groundwater Basin 2013-2018.
References Section 11
San Luis Obispo Valley Groundwater Basin
Groundwater Sustainability Agencies 11-5
San Luis Obispo Valley Basin Groundwater
Sustainability Plan
San Luis Obispo County Department of Public Works. (2005). Water Years 2001-02 and 2002-03 Hydrologic
Report, Final Report. 2005.
San Luis Obispo County Department of Public Works. (2020). Andrews Street Bridge Stream Flow Gage 745.
San Luis Obispo County Department of Public Works. (2020). Gas Company Rain Sensor 3099.
San Luis Obispo County Engineering Department. (1974). Hydrologic & Climatological Data, Seasons of 1970-71
& 1971-72.
San Luis Obispo County Flood Control and Water Conservation District . (2020). 2019 Integrated Regional Water
Management Plan.
SLOCFCWCD. (2014). CASGEM Monitoring Plan for High and Medium Priority Groundwater Basins in the San Luis
Obispo County Flood Control & Water Conservation District. September. San Luis Obispo Flood Control &
Water Conservation District.
Stillwater Sciences. (2014). San Luis Obispo County Regional Instream Flow Assessment.
Stillwater Sciences. (2015). Percolation Zone Study of Pilot-Study Groundwater Basins in San Luis Obispo County,
California. September.
Stillwater Sciences. (2016). Pismo Instream Flow Study.
Stillwater Sciences. (2020). Groundwater-Dependent Ecosystems in the San Luis Obispo Valley Groundwater
Basin.
SWRCB. (1990). Ernest Righetti & Sons Application 28883, Decision 1627.
TEAM Engineering & Management. (2000). Groundwater Yield Analysis. July.
The Nature Conservancy, (TNC). (2019). Freshwater species list for San Luis Obispo Valley Groundwater Basin.
Retrieved from https://groundwaterresourcehub.org/sgma-tools/environmental-surface-water-
beneficiaries
U.C. Davis Cooperative Extension. (2015). Soil Suitability Index Identifies Potential Areas for Groundwater
Banking on Agricultural Lands. California Agriculture, Volume 69, Number 2.
URS. (2013). Final Investigation Report: San Luis Obispo PCE Groundwater Plume.
URS. (2015). Investigation Report: San Luis Obispo PCE Groundwater Plume.
USBR. (1955). Reconnaissance Report San Luis Obispo County Basin, California. . U.S. Bureau of Reclamation,
Region 2, Sacramento.
USDA-NRCS. (2007). Soil Survey Geographic Database (SSURGO). U.S. Department of Agriculture (USDA) Natural
Resources Conservation Service (NRCS).
USFWS. (2018). National Wetlands Inventory website. Retrieved from http://www.fws.gov/wetlands/
USGS. (2004). Quaternary Fault and Fold Database for the Nation.
Valentine, D. W. (1999). Use of InSAR to Identify Land Surface Displacement Caused by Aquifer-System
Compaction in the Paso Robles Area, San Luis Obispo County, California, March to August 1997, Open
File Report 00-447. United States Geological Survey.
Wallace Group. (2020). Policy Considerations for Groundwater Sustainability Plan.
WSC. (2018). Salinas and Whale Rock Reserviors Safe Annual Yield TM.
Yeh and Associates. (2017). Subsidence Evaluation San Luis Obispo Valley Groundwater Basin Characterization
Report.
A
DWR Elements of the Plan Guide
B
City of San Luis Obispo Resolution to Form
GSA
C
County of San Luis Obispo Resolution to
Form GSA
D
Memorandum of Agreement –
Preparation of GSP
E
Notice and Communication
Communication and Engagement Plan
C&E Plan Implementation Workshop
F
Groundwater Dependent Ecosystems in
the San Luis Obispo Valley Groundwater
Basin
G
Surface Water / Groundwater Modeling
Documentation
Selection of appropriate modeling software for development of SLO Basin
integrated SW/GW model
Surface Water/Groundwater Modeling Approach Technical Memorandum
(Modeling TM No.1)
Surface Water/Groundwater Modeling Calibration Technical Memorandum
(Modeling TM No.2
Geophysical Survey TM
H
Data Management
Groundwater Level Measurement Procedures for
the San Luis Obispo Valley Groundwater Basin
GSP
Streamflow Measurement in Natural Channels
Data Management Plan
I
Response to Public Comments