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Home My WebLink AboutItem 06 - COUNCIL READING FILE_f_Draft GSP Chapter 6 Draft Groundwater Sustainability Plan Chapter 6 – Water Budget for the San Luis Obispo Valley Groundwater Basin Groundwater Sustainability Agencies Prepared by 6/25/2020 SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO i TABLE OF CONTENTS Table of Contents List of Figures ................................................................................................................................................. i Tables ........................................................................................................................................................... iv Appendices .................................................................................................................................................... v List of Terms Used ........................................................................................................................................ vi Executive Summary ....................................................................................................................................... 1 Introduction to the SLO Basin GSP ......................................................................................................... 1.1 Purpose of the Groundwater Sustainability Plan ............................................................................ 1.2 Description of SLO Basin ................................................................................................................. 1.3 Basin Prioritization .......................................................................................................................... 2 Agency Information (§ 354.6) ................................................................................................................ 2.1 Agencies Names and Mailing Addresses ......................................................................................... 2.2 Agencies Organization and Management Structures ..................................................................... 2.2.1 County of San Luis Obispo ....................................................................................................... 2.2.2 City of San Luis Obispo ............................................................................................................ 2.2.3 Other Participating Parties in the MOA .................................................................................. 2.2.3.1 Edna Valley Growers Mutual Water Company ............................................................... 2.2.3.2 Varian Ranch Mutual Water Company ........................................................................... 2.2.3.3 Edna Ranch Mutual Water Company .............................................................................. 2.2.3.4 Golden State Water Company ........................................................................................ 2.3 Authority of Agencies ...................................................................................................................... 2.3.1 Groundwater Sustainability Agencies ..................................................................................... 2.3.1.1 County of San Luis Obispo ............................................................................................... 2.3.1.2 City of San Luis Obispo .................................................................................................... 2.3.2 Memorandum of Agreement .................................................................................................. 2.3.3 Coordination Agreements ....................................................................................................... 2.4 Contact information for Plan Manager ........................................................................................... 3 Description of Plan Area (§ 354.8) ......................................................................................................... 3.1 SLO Basin Introduction .................................................................................................................... 3.2 Adjudicated Areas ........................................................................................................................... 3.3 Jurisdictional Areas ......................................................................................................................... 3.3.1 Federal Jurisdictions ................................................................................................................ 3.3.2 Tribal Jurisdiction .................................................................................................................... SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO ii 3.3.3 State Jurisdictions ................................................................................................................... 3.3.4 County Jurisdictions ................................................................................................................ 3.3.5 City and Local Jurisdictions ..................................................................................................... 3.3.6 Special Districts ....................................................................................................................... 3.4 Land Use .......................................................................................................................................... 3.4.1 Water Source Types ................................................................................................................ 3.4.2 Water Use Sectors ................................................................................................................... 3.5 Density of Wells .............................................................................................................................. 3.6 Existing Monitoring and Management Programs ........................................................................... 3.6.1 Groundwater Monitoring ........................................................................................................ 3.6.1.1 Groundwater Level Monitoring ..................................................................................... 3.6.1.2 Groundwater Quality Monitoring .................................................................................. 3.6.1.3 Surface Water Monitoring .............................................................................................. 3.6.1.4 Climate Monitoring ......................................................................................................... 3.6.2 Existing Management Plans .................................................................................................... 3.6.2.1 SLO Basin Characterization and Monitoring Well Installation ........................................ 3.6.2.2 San Luis Obispo County Master Water Report (2012) .................................................... 3.6.2.3 San Luis Obispo County Integrated Regional Water Management Plan (2014) ............. 3.6.2.4 City of San Luis Obispo 2015 Urban Water Management Plan (2016) ........................... 3.6.3 Existing Groundwater Regulatory Programs ............................................................................ 3.6.3.1 Groundwater Export Ordinance (2015) .......................................................................... 3.6.3.2 Well Ordinances, County and City .................................................................................. 3.6.3.3 Countywide Water Conservation Program Resolution 2015-288 (2015) ....................... 3.6.3.4 Agricultural Order R3-2017-002 (2017) .......................................................................... 3.6.3.5 Water Quality Control Plan for the Central Coast Basins (2017) .................................... 3.6.3.6 California DWR Well Standards (1991) ........................................................................... 3.6.3.7 Requirements for New Wells (2017) ............................................................................... 3.6.3.8 Title 22 Drinking Water Program (2018) ......................................................................... 3.6.3.9 Waterway Management Plan – San Luis Obispo Creek Watershed (2003) .................... 3.6.3.10 Incorporation Into GSP .................................................................................................... 3.6.3.11 Limits to Operational Flexibility ...................................................................................... 3.7 Conjunctive Use Programs .............................................................................................................. 3.8 Land Use Plans ................................................................................................................................ 3.8.1 City of San Luis Obispo General Plan ...................................................................................... 3.8.2 County of San Luis Obispo General Plan ................................................................................. SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO iii 3.8.3 Los Ranchos/Edna Village Plan ................................................................................................ 3.8.4 Plan Implementation Effects on Existing Land Use ................................................................. 3.8.5 Plan Implementation Effects on Water Supply ....................................................................... 3.8.6 Well Permitting ....................................................................................................................... 3.8.7 Land Use Plans Outside of Basin ............................................................................................. 3.9 Management Areas ......................................................................................................................... 3.9.1 Reason for Creation ................................................................................................................ 3.10 Additional GSP Elements, if Applicable ........................................................................................... 4 Basin Setting (§ 354.14) ......................................................................................................................... 4.1 Basin Topography and Boundaries ................................................................................................. 4.2 Primary Users of Groundwater ....................................................................................................... 4.3 Soils Infiltration Potential................................................................................................................ 4.4 Regional Geology ............................................................................................................................ 4.4.1 Regional Geologic Structures .................................................................................................. 4.4.2 Geologic Formations within the Basin .................................................................................... 4.4.2.1 Alluvium .......................................................................................................................... 4.4.2.2 Paso Robles Formation ................................................................................................... 4.4.2.3 Pismo Formation ............................................................................................................. 4.4.3 Geologic Formations Surrounding the Basin .......................................................................... 4.4.3.1 Monterey Formation ....................................................................................................... 4.4.3.2 Obispo Formation ........................................................................................................... 4.4.3.3 Franciscan Assemblage ................................................................................................... 4.5 Principal Aquifers and Aquitards .................................................................................................... 4.5.1 Cross Sections ......................................................................................................................... 4.5.2 Aquifer Characteristics ............................................................................................................ 4.5.3 Aquitards ................................................................................................................................. 4.6 Surface Water Bodies ...................................................................................................................... 4.7 Subsidence Potential ....................................................................................................................... 5 Groundwater Conditions (§ 354.16) ...................................................................................................... 5.1 Groundwater Elevations and Intepretation .................................................................................... 5.1.1 Fall 1954 Groundwater Elevations .......................................................................................... 5.1.2 Spring 1990 Groundwater Elevations ..................................................................................... 5.1.3 Modeled 1990s Groundwater Elevations ............................................................................... 5.1.4 Spring 1997 Groundwater Elevations ..................................................................................... 5.1.5 Spring 2011 Groundwater Elevations ..................................................................................... SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO iv 5.1.6 Spring 2015 Groundwater Elevations ..................................................................................... 5.1.7 Spring 2019 Groundwater Elevations ..................................................................................... 5.1.8 Fall 2019 Groundwater Elevations .......................................................................................... 5.1.9 Changes in Groundwater Elevation ........................................................................................ 5.1.10 Vertical Groundwater Gradients ............................................................................................. 5.2 Groundwater Elevation Hydrographs ............................................................................................. 5.3 Groundwater Recharge and Discharge Areas ................................................................................. 5.3.1 Groundwater Recharge Areas ................................................................................................. 5.3.1.1 Infiltration of Precipitation ............................................................................................. 5.3.1.2 Subsurface Inflow ............................................................................................................ 5.3.1.3 Percolation of Streamflow .............................................................................................. 5.3.1.4 Anthropogenic Recharge ................................................................................................ 5.3.2 Groundwater Discharge Areas ................................................................................................ 5.4 Change in Groundwater Storage..................................................................................................... 5.5 Seawater Intrusion .......................................................................................................................... 5.6 Subsidence ...................................................................................................................................... 5.7 Interconnected Surface Water ........................................................................................................ 5.7.1 Depletion of Interconnected Surface Water ........................................................................... 5.8 Potential groundwater dependent ecosystems .............................................................................. 5.8.1 Hydrology ................................................................................................................................ 5.8.1.1 Overview of GDE Relevant Surface and Groundwater Hydrology .................................. 5.8.1.2 Losing and Gaining Reaches ............................................................................................ 5.8.2 Vegetation and Wetland Groundwater Dependent Ecosystem Identification ....................... 5.8.3 Identification of Special-Status Species and Sensitive Natural Communities Associates with GDE’s .............................................................................................................................................. 5.9 Groundwater Quality Distribution and Trends ............................................................................... 5.9.1 Groundwater Quality Suitability for Drinking Water .............................................................. 5.9.2 Distribution and Concentrations of Point Sources of Groundwater Constituents ................. 5.9.3 Distribution and Concentrations of Diffuse or Natural Groundwater Constituents ............... 5.9.3.1 Total Dissolved Solids ...................................................................................................... 5.9.3.2 Nitrate ............................................................................................................................. 5.9.3.3 Arsenic ............................................................................................................................. 5.9.3.4 Boron ............................................................................................................................... 5.9.3.5 Other Constituents .......................................................................................................... SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO v 6 Water Budget (§ 354.18) ...................................................................................................................... 2 6.1 Climate ........................................................................................................................................ 17 6.1.1 Historical Climate/Base Period ........................................................................................... 17 6.2 Water Budget Data Sources ........................................................................................................ 22 6.3 Historical Water Budget .............................................................................................................. 22 6.3.1 Historical Time Period ......................................................................................................... 22 6.3.2 Historical Land Use .............................................................................................................. 23 6.3.3 Historical Surface Water Budget ......................................................................................... 26 6.3.4 Historical Groundwater Budget .......................................................................................... 33 6.3.5 Total Groundwater in Storage ............................................................................................ 44 6.3.6 Change in Storage ............................................................................................................... 49 6.3.7 Sustainable Yield ................................................................................................................. 52 6.3.8 Quantification of Overdraft (Historical) .............................................................................. 53 6.4 Current Water Budget ................................................................................................................. 53 6.5 Projected Water Budget ............................................................................................................. 64 6.5.1 Assumptions ........................................................................................................................ 64 6.5.2 Inflows ................................................................................................................................. 64 6.5.3 Outflows .............................................................................................................................. 64 6.5.4 Change In Storage ............................................................................................................... 64 6.5 Projected Water Budget ................................................................................................................. 6.5.1 Assumptions ............................................................................................................................ 6.5.2 Inflows ..................................................................................................................................... 6.5.3 Outflows .................................................................................................................................. 6.5.4 Change In Storage ................................................................................................................... 7 Sustainable Management Criteria (§ 354.22-30) ................................................................................... 7.1 Sustainability Goal........................................................................................................................... 7.2 Process for Establishing Sustainable Management Criteria ........................................................... 7.2.1 Minimum Thresholds .............................................................................................................. 7.2.2 Measurable Objectives ........................................................................................................... 7.2.3 Undesirable Results................................................................................................................. 7.3 Chronic Lowering of Groundwater Levels Sustainability Indicator ................................................. 7.3.1 Locally Defined Undesirable Results ....................................................................................... 7.3.2 Minimum Thresholds and Measurable Objectives ................................................................. 7.3.3 Relation to Other Sustainability Indicators ............................................................................. SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO vi 7.4 Change in Storage Sustainability Indicator ..................................................................................... 7.4.1 Locally Defined Undesirable Results ....................................................................................... 7.4.2 Minimum Thresholds .............................................................................................................. 7.4.3 Measurable Objectives ........................................................................................................... 7.4.4 Relation to Other Sustainability Indicators ............................................................................. 7.5 Seawater Intrusion Sustainability Indicator .................................................................................... 7.5.1 Locally Defined Undesirable Results ....................................................................................... 7.5.2 Minimum Thresholds .............................................................................................................. 7.5.3 Measurable Objectives ........................................................................................................... 7.5.4 Relation to Other Sustainability Indicators ............................................................................. 7.6 Degraded Water Quality Sustainability Indicator ........................................................................... 7.6.1 Locally Defined Undesirable Results ....................................................................................... 7.6.2 Minimum Thresholds .............................................................................................................. 7.6.3 Measurable Objectives ........................................................................................................... 7.6.4 Relation to Other Sustainability Indicators ............................................................................. 7.7 Subsidence Sustainability Indicator ................................................................................................ 7.7.1 Locally Defined Undesirable Results ....................................................................................... 7.7.2 Minimum Thresholds .............................................................................................................. 7.7.3 Measurable Objectives ........................................................................................................... 7.7.4 Relation to Other Sustainability Indicators ............................................................................. 7.8 Depletion of Interconnected Surface Water Sustainability Indicator ............................................. 7.8.1 Locally Defined Undesirable Results ....................................................................................... 7.8.2 Minimum Thresholds .............................................................................................................. 7.8.3 Measurable Objectives ........................................................................................................... 7.8.4 Relation to Other Sustainability Indicators ............................................................................. 7.9 Management Areas ......................................................................................................................... 7.9.1 Minimum Thresholds and Measurable Objectives ................................................................. 7.9.2 Monitoring and Analysis ......................................................................................................... 7.9.3 Explanation of How Operation of Management Area Will Avoid Undesirable Results .......... 8 Monitoring Networks (§ 354.34) ............................................................................................................ 8.1 Monitoring Objectives .................................................................................................................... 8.2 Monitoring Network ....................................................................................................................... 8.2.1 Chronic Lowering of Groundwater Levels ............................................................................... 8.2.2 Reduction of Groundwater Storage ........................................................................................ 8.2.3 Seawater Intrusion .................................................................................................................. SLO Basin Groundwater Sustainability Plan Table of Contents County of SLO and City of SLO vii 8.2.4 Groundwater Quality .............................................................................................................. 8.2.5 Land Subsidence...................................................................................................................... 8.2.6 Depletion of Interconnected Surface Water ........................................................................... 8.3 Groundwater Monitoring Protocol ................................................................................................. 8.4 Data Management System .............................................................................................................. 8.5 Assessment and Improvement of Monitoring Network ................................................................. 8.6 Annual Reports ................................................................................................................................ 8.7 Periodic Evaluation by Agency ........................................................................................................ 9 Projects and Management Actions (§ 354.44) ....................................................................................... 9.1 Projects ........................................................................................................................................... 9.1.1 Project A .................................................................................................................................. 9.2 Management Actions ...................................................................................................................... 9.2.1 Management Action A ............................................................................................................ 9.3 Projects Needed to Mitigate Overdraft .......................................................................................... 10 Implementation Plan .............................................................................................................................. 10.1 Cost of Implementation .................................................................................................................. 10.2 Funding Alternatives ....................................................................................................................... 10.3 Implementation Schedule ............................................................................................................... 10.4 GSP Annual Reporting ..................................................................................................................... 10.5 Periodic Evaluations of GSP ............................................................................................................ 11 Notice and Communications (§ 354.10) ................................................................................................. 11.1 Communications and Engagement Plan ......................................................................................... 11.2 Nature of Consultations .................................................................................................................. 11.3 Public Meetings ............................................................................................................................... 11.4 Incorporation of Feedback in Decision-Making Process ................................................................. 11.5 Comments Received ....................................................................................................................... 11.6 Responses to Comments ................................................................................................................. 12 Interagency Agreements (§ 357.2-4) ..................................................................................................... 12.1 Coordination Agreements ............................................................................................................... 13 References .............................................................................................................................................. 14 Appendices ............................................................................................................................................. The grey highlighted sections in the Table of Contents (TOC) indicate that the section has been previously released (Chapters 1 through 5) or will be released in the future (Chapters 7 through 14). The complete list of the anticipated TOC is presented to give the reader context as to how Chapter 6 – Water Budget, connects with the complete Groundwater Sustainability Plan. SLO Basin Groundwater Sustainability Plan List of Figures County of SLO and City of SLO viii LIST OF FIGURES Figure 6-1: The Hydrologic Cycle. Source: Department of Water Resources (Water Budget BMP, 2016) ... 3 Figure 6-2: Components of the Water Budget. Source: Modified from Department of Water Resources (Water Budget BMP, 2016) ........................................................................................................................... 4 Figure 6-3: Water Budget Subareas. ............................................................................................................. 5 Figure 6-4: Surface Water Budget – San Luis Valley Subarea. .................................................................... 11 Figure 6-5: Surface Water Budget – Edna Valley Subarea. ......................................................................... 12 Figure 6-6: Surface Water Budget – Basin Total. ........................................................................................ 13 Figure 6-7: Groundwater Budget – San Luis Valley Subarea. ..................................................................... 14 Figure 6-8: Groundwater Budget – Edna Valley Subarea. .......................................................................... 15 Figure 6-9: Groundwater Budget – Basin Total. ......................................................................................... 16 Figure 6-10: 1987-2019 Historical Base Period Climate. ............................................................................ 18 Figure 6-11: Rainfall Correlation Cal Poly vs. Gas Company. ...................................................................... 20 Figure 6-12: San Luis Obispo Valley Basin Irrigated Crops 2016. ................................................................ 24 Figure 6-13: Basin Sub-watershed Areas and Isohyetals. ........................................................................... 28 Figure 6-14: Runoff vs Rainfall Correlation for Subareas. ........................................................................... 29 Figure 6-15: Rainfall vs Infiltration. ............................................................................................................. 35 Figure 6-16: Bedrock Subsurface Inflow Reaches. ...................................................................................... 38 Figure 6-17: Consumptive Use of Applied Water. ...................................................................................... 42 Figure 6-18: Groundwater Elevation Contours Spring 1986. ...................................................................... 46 Figure 6-19: Groundwater Elevation Contours Spring 2019. ...................................................................... 47 Figure 6-20: Storage Volume Grids. ............................................................................................................ 48 Figure 6-21: Groundwater Storage Estimate Comparison for Basin Subareas. .......................................... 51 Figure 6-22: Historical and Current Average Annual Surface Water Budget – San Luis Valley Subarea. ... 58 Figure 6-23: Historical and Current Average Annual Surface Water Budget – Edna Valley Subarea. ........ 59 Figure 6-24: Historical and Current Average Annual Surface Water Budget – Basin Total. ....................... 60 Figure 6-25: Historical and Current Average Annual Groundwater Budget – San Luis Valley Subarea. ..... 61 Figure 6-26: Historical and Current Average Annual Groundwater Budget – Edna Valley Subarea. ......... 62 Figure 6-27: Historical and Current Average Annual Groundwater Budget – Basin Total. ......................... 63 SLO Basin Groundwater Sustainability Plan Tables County of SLO and City of SLO ix TABLES Table 6-1: Historical Water Budget - San Luis Valley Subarea. ..................................................................... 8 Table 6-2: Historical Water Budget - Edna Valley Subarea. .......................................................................... 9 Table 6-3: Historical Water Budget - San Luis Obispo Valley Groundwater Basin...................................... 10 Table 6-4: Historical Base Period Rainfall. .................................................................................................. 21 Table 6-5: Irrigated Agriculture Acreages. .................................................................................................. 25 Table 6-6: Land Cover Acreages. ................................................................................................................. 26 Table 6-7: Stream Outflow Comparison. .................................................................................................... 33 Table 6-8: Minimum Rainfall for Infiltration. .............................................................................................. 36 Table 6-9: Subsurface Inflow Estimates. ..................................................................................................... 37 Table 6-10: Rural Residential Water Use. ................................................................................................... 41 Table 6-11: Consumptive Use of Applied Water. ........................................................................................ 43 Table 6-12: Subsurface Outflow Estimates. ................................................................................................ 44 Table 6-13: Specific Yield Averages. ............................................................................................................ 45 Table 6-14: Spring Groundwater Storage Estimates. .................................................................................. 49 Table 6-15: Change in Storage Comparison – Historical Base Period 1987 – 2019. ................................... 50 Table 6-16: Preliminary Sustainable Yield (AFY). ........................................................................................ 52 Table 6-17: Estimated Overdraft (AFY). ...................................................................................................... 53 Table 6-18: Current Water Budget - San Luis Valley Subarea. .................................................................... 55 Table 6-19: Current Water Budget - Edna Valley Subarea. ......................................................................... 56 Table 6-20: Current Water Budget - Basin Total. ........................................................................................ 57 SLO Basin Groundwater Sustainability Plan Appendices County of SLO and City of SLO x APPENDICES SLO Basin Groundwater Sustainability Plan List of Terms Used County of SLO and City of SLO xi LIST OF TERMS USED Abbreviation Definition AB Assembly Bill ADD Average Day Demand AF Acre Feet AFY Acre Feet per Year AMSL Above Mean Sea Level Basin Plan Water Quality Control Plan for the Central Coast Basin Cal Poly California Polytechnic State University CASGEM California State Groundwater Elevation Monitoring program CCR California Code of Regulations CCRWQCB Central Coast Regional Water Quality Control Board CCGC Central Coast Groundwater Coalition CDFM Cumulative departure from the mean CDPH California Department of Public Health CIMIS California Irrigation Management Information System City City of San Luis Obispo County County of San Luis Obispo CPUC California Public Utilities Commission CPWS-52 Cal Poly Weather Station 52 CRWQCB California Regional Water Quality Control Board CWC California Water Code DDW Division of Drinking Water Du/ac Dwelling Units per Acre DWR Department of Water Resources EPA Environmental Protection Agency ERMWC Edna Ranch Mutual Water Company ET0 Evapotranspiration EVGMWC Edna Valley Growers Ranch Mutual Water Company °F Degrees Fahrenheit FAR Floor Area Ratio FY Fiscal Year GAMA Groundwater Ambient Monitoring and Assessment program GHG Greenhouse Gas GMP Groundwater Management Plan GPM Gallons per Minute GSA Groundwater Sustainability Agency GSC Groundwater Sustainability Commission GSP Groundwater Sustainability Plan GSWC Golden State Water Company IRWMP San Luis Obispo County Integrated Regional Water Management Plan kWh Kilowatt-Hour LUCE Land Use and Circulation Element LUFTs Leaky Underground Fuel Tanks MAF Million Acre Feet MCL Maximum Contaminant Level SLO Basin Groundwater Sustainability Plan List of Terms Used County of SLO and City of SLO xii Abbreviation Definition MG Million Gallons MGD Million Gallons per Day Mg/L Milligrams per Liter MOA Memorandum of Agreement MOU Memorandum of Understanding MWR Master Water Report NCDC National Climate Data Center NOAA National Oceanic and Atmospheric Administration NWIS National Water Information System RW Recycled Water RWQCB Regional Water Quality Control Board SB Senate Bill SGMA Sustainable Groundwater Management Act SGMP Sustainable Groundwater Management Planning SGWP Sustainable Groundwater Planning SLO Basin San Luis Obispo Valley Groundwater Basin SLOFCWCD San Luis Obispo Flood Control and Water Conservation District SCML Secondary Maximum Contaminant Level SOI Sphere of Influence SNMP Salt and Nutrient Management Plan SWRCB California State Water Resources Control Board TDS Total Dissolved Solids TMDL Total Maximum Daily Load USGS United States Geological Survey USFW United States Fish and Wildlife Service USTs Underground Storage Tanks UWMP Urban Water Management Plan UWMP Act Urban Water Management Planning Act UWMP Guidebook Department of Water Resources 2015 Urban Water Management Plan Guidebook VRMWC Varian Ranch Mutual Water Company WCS Water Code Section WMP Water Master Plan WPA Water Planning Areas WRF Water Reclamation Facility WRCC Western Regional Climate Center WRRF Water Resource Recovery Facility WSA Water Supply Assessment WTP Water Treatment Plant WWTP Wastewater Treatment Plant SLO Basin Groundwater Sustainability Plan Executive Summary County of SLO and City of SLO 1 EXECUTIVE SUMMARY This section to be completed after GSP is complete. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 2 6 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 as used in this document 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 will be used as part of the Basin conceptual model to prepare input estimates and provide a check for the numerical model, from which the projected water budget will be produced. This chapter presents the analytical water budget for the historical and current periods and the numerical model water budget for the projected future period. Once the numerical model water budget is calibrated, the results will be presented as comparisons to the analytical 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. 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 of 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, 1991; DWR, 1997). SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 3 Figure 6-1: The Hydrologic Cycle. Source: Department of Water Resources (Water Budget BMP, 2016) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 4 Figure 6-2: Components of the Water Budget. Source: Modified from Department of Water Resources (Water Budget BMP, 2016) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 5 Figure 6-3: Water Budget Subareas. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 6 As presented in Chapter 4, 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; Chapter 3). 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 7 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 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 other wet years when the subarea was effectively full. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 8 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 6-3 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 9 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) Change in GW Storage (AF) 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 10 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 11 Figure 6-4: Surface Water Budget – San Luis Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 12 Figure 6-5: Surface Water Budget – Edna Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 13 Figure 6-6: Surface Water Budget – Basin Total. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 14 Figure 6-7: Groundwater Budget – San Luis Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 15 Figure 6-8: Groundwater Budget – Edna Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 16 Figure 6-9: Groundwater Budget – Basin Total. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 17 6.1 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 (see Section 6.3.4.2). Cal Poly, the San Luis Valle, 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 (CIMIS, 1999). 6.1.1 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 (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; Chapter 3. 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, 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 18 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 19 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; Chapter 3), and an isohyetal map of average annual rainfall is shown in Figure 4-3 (Chapter 4). The average annual precipitation across the Basin between 1981 and 2010 was approximately 19 inches (Figure 4-3; Chapter 4), 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 Cal Poly and the Basin. A correlation between the Gas Company and Cal Poly NOAA 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 20 Figure 6-11: Rainfall Correlation Cal Poly 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) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 21 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; Chapter 4). Table 6-4: Historical Base Period Rainfall. Year Type Cal Poly NOAA Gas Company 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 22 6.2 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 6.3 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 6.3.8). (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 6.3.7). 6.3.1 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 23 6.3.2 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 (Section 6.3.4.2), and acreages of various land covers are multiplied by empirical correlations to estimate their respective evapotranspiration and percolation of precipitation (Section 6.3.4.1). The land uses/land covers including the following: • Irrigated Agriculture o Citrus o Deciduous o Pasture o Vegetable o Vineyard • Native Vegetation o Brush, trees, native grasses o Wetlands/open water • Urban/Suburban o Developed (City, subdivisions) o Open space (parks, empty lots) o 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 24 Figure 6-12: San Luis Obispo Valley Basin Irrigated Crops 2016. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 25 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. 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 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 26 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 6.3.3 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. 6.3.3.1 Components of Surface Water Inflow The surface water budget includes the following sources of inflow: • Local Supplies o Precipitation o Groundwater extractions o Stream inflow at Basin boundary o Groundwater-Surface Water Interactions o 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 27 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 Historical Groundwater Budget section (Section 6.3.3). 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 are 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 28 Figure 6-13: Basin Sub-watershed Areas and Isohyetals. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 29 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) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 30 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 0.71) is. 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. Groundwater-Surface Water Interaction (Net) Groundwater-surface water interactions take place primarily along stream channels. 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. This 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 (Section 6.3.4.1). 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 (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. 6.3.3.2 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 31 • Surface Water Deliveries Offset • Wetland/Lake ET • Groundwater-Surface Water Interaction • Stream outflow (runoff) 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, 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 (1933, 1963), and which has been used for other analytical water budgets in major studies of central coast Basins (DWR, 2002; Fugro, 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 (2002) 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, 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 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 6.3.4.1 (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 6.3.4.1 (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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 32 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 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, 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; Chapter 5). 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 6.3.4.1 (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 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; Chapter 3). 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, 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 Table 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. The net interaction is an outflow component for the surface water budget and an inflow component for the groundwater budget (losing streams. Details of the methodology used to develop the groundwater-surface water interaction are presented in the Sections 6.3.4.1 and 6.3.6. Stream Outflow from Basin SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 33 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. 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, 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 6.3.4 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 34 The groundwater budget includes the following sources of outflow: • Groundwater Extractions • Subsurface Outflow • Groundwater-Surface Water Interaction 6.3.4.1 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, 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, 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 35 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) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 36 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. 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 6.3.6). 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) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 37 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 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. 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, 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 38 Figure 6-16: Bedrock Subsurface Inflow Reaches. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 39 of the Basin, while depositional boundaries are on the north side (see geologic-cross sections in Chapter 4). Thicknesses at the Basin boundary are estimated from Basin cross-sections (Chapter 4). 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 (CHG, 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 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,2018). The resulting estimated average subsurface cross-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, 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 are 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 40 6.3.4.2 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 (SLO City, 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 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 41 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. 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 42 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) SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 43 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. 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 6.3.2 (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 Obispo subsurface (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 (see Subsurface Inflow in Section 6.3.4.2). Table 6-12 presents the parameters used for subsurface outflow estimates. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 44 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 (Figure 4-2; Chapter 4) 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, 2018; CHG, 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 cross- flow through the bedrock high; see Subsurface Inflow section above). 6.3.5 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, 2018). Groundwater in storage calculations were performed for the Spring conditions of 1986, 1990, 1995, 19 98, 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 5-8 and 5-9 of Chapter 5. Of note, however, is that Spring 2019 water levels shown in Figure 6-18 are lower near the intersection of SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 45 Edna and Biddle Ranch Road than for the same period shown in Figure 5-6 (Chapter 5). 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. 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, 2018) Qal = alluvium; QTp = Paso Robles Formation; Pismo = Pismo Formation Weighted averages based on penetrated thicknesses of aquifer type. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 46 Figure 6-18: Groundwater Elevation Contours Spring 1986. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 47 Figure 6-19: Groundwater Elevation Contours Spring 2019. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 48 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 6-14. Figure 6-20: Storage Volume Grids. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 49 Table 6-14: Spring Groundwater Storage Estimates. Year SLO Subarea Edna Subarea 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 are much 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 (Groundwater Basin Evaluation, 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, 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, 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. 6.3.6 Change in Storage Balancing the water budget 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 50 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; Chapter 5). 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 51 Figure 6-21: Groundwater Storage Estimate Comparison for Basin Subareas. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 52 6.3.7 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 wasted and unavailable for use. Undesirable results will be defined for six sustainable management criteria in Chapter 7. 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 San Luis Obispo groundwater 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 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, because 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 addition 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. Table 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 53 The above values are lower overall than historical estimates by Boyle (1991) and DWR (1997 Draft). 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. 6.3.8 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 reuse 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 by Boyle (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. 6.4 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 54 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 are shown in Figure 6-22 through Figure 6-27. 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. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 55 Table 6-18: Current Water Budget - San Luis Valley Subarea. 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 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 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 56 Table 6-19: Current Water Budget - Edna Valley Subarea. 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 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 Groundwater extractions (Urban) 880 820 Groundwater extractions (Ag) 3,210 3,440 Subsurface outflow 100 100 TOTAL OUT 4,190 4,360 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 57 Table 6-20: Current Water Budget - Basin Total. 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 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 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 SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 58 Figure 6-22: Historical and Current Average Annual Surface Water Budget – San Luis Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 59 Figure 6-23: Historical and Current Average Annual Surface Water Budget – Edna Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 60 Figure 6-24: Historical and Current Average Annual Surface Water Budget – Basin Total. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 61 Figure 6-25: Historical and Current Average Annual Groundwater Budget – San Luis Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 62 Figure 6-26: Historical and Current Average Annual Groundwater Budget – Edna Valley Subarea. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 63 Figure 6-27: Historical and Current Average Annual Groundwater Budget – Basin Total. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 64 6.5 PROJECTED WATER BUDGET 6.5.1 Assumptions 6.5.2 Inflows 6.5.3 Outflows 6.5.4 Change In Storage SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 65 REFERENCES Balance Hydrologics. 2008. Hydrology and Geology Assessment of the Pismo Creek Watershed, San Luis Obispo County, California. 2008. Blaney. 1963. Utilization of the Water of the Santa Ynez River Basin in Southern Santa Barbara County California, United Stated Deparment of Agriculture. 1963. —. 1933. Ventura County Investigation, Bulletin No. 46, California Deparment of Public Works, Division of Water Resources. 1933. Boyle Engineering. 1991. City of San Luis Obispo Groundwater Basin Evaluation. January. 1991. Carollo. 2012. San Luis Obispo County Master Water Report. 2012. CIMS. 2019. Station 52, San Luis Obispo - Central Coast Valleys. 2019. City of San Luis Obispo. 2016. 2015 Urban Water Management Plan. 2016. —. 2018. General Plan. 2018. —. 2015. Water Resources Status Report. 2015. —. 2000. Water Use Factors. 2000. 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. 2001. —. 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. 2003. Cleath-Harris Geologists. 2018. Groundwater Flow Analysis, Recycled Water Recharge Project, San Luis Valley Subarea, San Luis Obispo Valley Groundwater Basin. 2018. —. 2019. Optional Task 2.4B Geophysical Survey. 2019. Cleath-Harris Geologists, Inc. 2010. Edna Valley Water System Groundwater Study. Prepared for Golden State Water Company. May. 2010. —. 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. 2013. —. 2013. Summary of Exploration and Testing, 5061 Hacienda Avenue, San Luis Obispo, California. Prepared for Golden State Water Company. February. 2013. —. 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. 2014. County of San Luis Obispo. 2014. San Luis Obispo County Integrated Regional Water Management Plan (IRWMP). 2014. Cuesta Engineering Corporation. 2007. San Luis Obispo Creek Watershed Calibration Study. 2007. Dibble, T.W. 2004. Geologic Map of the Lopez Mountain Quadrangle, San Luis Obispo County, California. s.l. : Dibble Geology Center Map, 2004. #DF-130. Dibblee, T.W. 2006. Geologic Map of the Arroyo Grande NE Quadrangle, San Luis Obispo County, California. s.l. : Dibble Geology Center Map, 2006. #DF-211. —. 2006. Geologic Map of the Pismo Beach Quadrangle, San Luis Obispo County, California. s.l. : Dibblee Geology Center Map, 2006. #DF-212. —. 2004. Geologic Map of the San Luis Obispo Quadrangle, San Luis Obispo County, California. . s.l. : Dibble Geology Center Map, 2004. #DF-129. DWR. 2003. California’s Groundwater: Bulletin 118 – Update 2003, Groundwater Basin Descriptions. 2003. —. 2016. California's Groundwater: Bulletin 118 - Interim Update 2016, Working Towards Sustainability. 2016. —. 2003. California's Groundwater: Bulletin 118 - Update 2003, Groundwater Basin Descriptions. 2003. —. 2015. Consumptive Use Program Plus (CUP+) Model, in California Water Plan Update 2013, Volume 4. Reference Guide, Developed by DWR and UC Davis. 2015. —. 2014. DWR Atlas - Aglricultural Lang Use and Irrigated Areas. [Online] 2014. gis.water.ca.gov. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 66 —. 1964. San Luis Obispo and Santa Barbara Counties Land and Water Use Survey, 1959. . s.l. : California Department of Water Resources (DWR), 1964. —. 1958. San Luis Obispo County Investigation. State Water Resources Board Bulletin No. 18. . s.l. : California Department of Water Resources (DWR). May., 1958. —. 1997. San Luis-Edna Valley Groundwater Basin Study, Draft Report. . s.l. : California Department of Water Resources (DWR)., 1997. —. 1996. South Central Coast Land Use Survey. 1996. —. 1987. Southern Central Coast Land Use Survey, 1985. Morro Bay South 54-30 and San Luis Obispo 54-31 Survey Maps. 1987. —. 2019. Sustainable Groundwater Management Act 2019 Basin Prioritization - Process and Results Document. 2019. —. 2016. Water Budget Best Management Practices for the Sustainable Management of Groundwater. 2016. —. 2002. Water Resources of the Arroyo Grande - Nipomo Mesa Area. 2002. EPA. 2008. Water Sense Factsheet - Indoor Water Use in the United States, EPA-832-F-06-004. 2008. ESA Consultants, Inc. 1994. Hydrologic Investigation, Edna Valley Well Location Study. September. 1994. Fugro West and Cleath & Associates. 2002. Paso Robles Groudnwater Study, Final Report. 2002. GSI Water Solutions. 2018. San Luis Obispo Valley Basin Characterization and Monitoring Well Installation. 2018. Hall, C.A. 1979. Geologic map of the San Luis Obispo – San Simeon Region, California. s.l. : U.S. Geological Survey, 1979. Map I-1097. —. 1973. Geology of the Arroyo Grande Quadrangle, California. . s.l. : California Division of Mines and Geology, 1973. Map Sheet 24. ITRC. 2020. Official Cal Poly Precipitation Data, Cal Poly/NOAA Station Rain Gage. 2020. Johnson, A.I. 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. 1967. National Land Cover Database. Multi-Resolution Land Characteristics Consortium. 2016. Multi-Resolution Land Characteristics Consortium. Rosenberg, L.I. 2001. Potential Aquifer Recharge Areas: Monterey County, California. s.l. : Monterey County Planning Department, 2001. San Luis Obispo County Deparment of Agriculture/Weights and Measures. 2019. Crop Surveys for San Luis Obispo Valley Groundwater Basin 2013-2018. 2019. San Luis Obispo County Department of Public Works. 2020. Andrews Street Bridge Stream Flow Gage 745. 2020. —. 2020. Gas Company Rain Sensor 3099. 2020. San Luis Obispo County Engineering Department. 1974. Hydrologic & Climatological Data, Seasons of 1970-71 & 1971-72. 1974. SLO-FCWCD. 2014. CASGEM Monitoring Plan for High and Medium Priority Groundwater Basins in the San Luis Obispo County Flood Control & Water Conservation District. September. s.l. : San Luis Obispo Flood Control & Water Conservation District, 2014. Stillwater Sciences. 2015. Percolation Zone Study of Pilot-Study Groundwater Basins in San Luis Obispo County, California. September. 2015. SWRCB. 1990. Ernest Righetti & Sons Application 28883, Decision 1627. 1990. TEAM Engineering & Management. 2000. Groundwater Yield Analysis. July. 2000. USBR. 1955. Reconnaissance Report San Luis Obispo County Basin, California. . s.l. : U.S. Bureau of Reclamation, Region 2, Sacramento., 1955. USDA-NRCS. 2007. Soil Survey Geographic Database (SSURGO). s.l. : U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS), 2007. WSC. 2018. Salinas and Whale Rock Reserviors Safe Annual Yield TM. 2018. SLO Basin Groundwater Sustainability Plan Water Budget (§ 354.18) County of SLO and City of SLO 67