HomeMy WebLinkAbout03/20/1989, 1-B - WATER STATUS - EFFECT OF DALIDIO OFFER ON CITY WATER SUPPLY WONG DATE:
city Of San LUIS OBISPO March 14 198
COUNCIL AGENDA REPORT MM Nu
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FROM: William T. Hetland, Utilities Director p
SUBJECT: Water Status - Effect of Dalidio Offer on City Water Supply
CAO Recommendation: Receive and file report
At the City's emergency council meeting of March 3, 1989 the Dalidio family offered the
City of San Luis Obispo 2000 acre feet of groundwater to help it through the current dry
spell. This report will try to quantify the current water supply status and the impact
that this additional 2000 acre feet of Dalidio groundwater plus the City groundwater will
have on the City's supply.
The City uses its Annual Water Operational Plan (AWOP) curve to identify the water supply
situtaion and to trigger key policy actions. The curve shows the City reservoir water in
storage and is based on a six year dry cycle which is the worst dry period that we have
experienced during over a hundred years of record. The AWOP also identifies four key
action levels based on the reservoir storage. Specific levels of conservation are spelled
out at each of the action levels. The lowest two levels are the severe and critical. At
the severe level a reduction of 25% in consumption is required, and at the critical level
a reduction of 50% is required. The City's reservoir storage currently is within the
severe level and therefore the city should be experiencing a reduction of 25% in water
use. The average reduction in use since last spring has been around 7-10 percent.
The existing AWOP curve (see attachement #1) was calculated based on annual average data
_ at six month increments. Because we are getting very close to the critical level, it was
felt that staff needed to expand the curve to a monthly basis. This better shows when the
City would cross into the critical level. Staff developed an AWOP curve for the next 24
months (see attachment #2). The curve includes the influences of groundwater
development. Listed below is a summary of groundwater projects, their estimated yield
and the effective on-line date.
1. Groundwater Phase I (500 Acre Feet/Year) 42 AF/Month March 1989
2. Dalidio Wells (2000 Acre Feet/Year) 167 AF/Month May 1989
3. Groundwater Phase II (500 Acre Feet/Year) 42 AF/Month June 1989
Total (3000 Acre Feet/Year) 251 AF/Month
2000 to 3000 acre feet per year is believed to be the maximum amount of groundwater that
can be consumed within the Laguna Lake and South Higuera areas.
The curves were developed by starting with the reservoir storage amounts at the beginning
of the month. This includes all of Salinas Reservoir storage and the City's share of the
Whale Rock Reservoir. Reservoir inflow data based on the average inflow during the worst
case six year dry cycle is added. Then evaporation, based on historical data, releases
based on average releases during the worst case six year dry cycle, and consumer demand
based on 1988 demand, is subtracted each month. The result is the net storage at the end
of the month. The equation is:
Beginning Storage + Inflow - Evaporation - Releases - Demand = Ending Storage.
The demand amounts were adjusted to take into account groundwater usage. A slight
variation may exist because of the assumptions made on captured storage. Separate curves
were then developed using 0%, 15%, 20%, and 50% conservation to show the relative impacts
of different levels of mandatory water conservation.
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RUNNUMMMEW COUNCIL AGENDA REPORT
Page 2
Water Status
The important point is when does the reservoir storage curves cross into the critical
level and the required 50% in water conservation take effect. At 0% conservation and
taking into account groundwater development, the curve (dots) crosses into the critical
area in August of 1989. The 15% and 20% curves (crosses and stars) cross into the
critical area sometime in October or November 1989, while the 50% curve (squares) does not
cross the critical level during the next 24 months based on these assumptions.
This clearly shows that if the City is to avoid implementing 50% mandatory water
conservation, it is important that at least 15-20% in overall conservation go into effect
at this time, in addition to the groundwater development. It is also important to note
that the impact of groundwater and conservation on the reservoir storage curve for the
short term is relatively minor. The significant impact shows when we are able to conserve
over the long term. ;
RECOMMENDATION
Receive and file report.
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