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City of Pleasanton 4.0 Water Agency Background Final Water Supply Assessment Stoneridge Drive Specific Plan Amendment and Staples Ranch Project years. Basically, this additional storage capacity acts as drought protection by allowing Zone 7 the option of storing excess imported surface water during wet or normal hydrologic years and then withdrawing this water during dry years.t8 Zone 7 will pumpback at least 10,000 AFA or up to 50 percent of the banked supplies;19 the contract extends to 2035. This CWSD water can be transferred back to Zone 7 via SWP conveyance facilities when the SWP pumping and conveyance facilities are operational. 5.5 Zone 7 Supply Reliability in Single and Multiple Dry Years Sustainable water supply is the aggregated quantities of the aforementioned sources; briefly, these include: the sustainable groundwater yield from the Main Basin, the State Water Project average annual delivery, Lake Del Valle average yield, Byron-Bethany Irrigation District water transfer, "Chain of Lakes" supplies, recycled water, and the additional supplies through Zone 7's groundwater Out-of- Basin banking program. In April of each year, Zone 7 staff prepares a Sustainable Water Supply Report based on best available hydrologic data and current contract status; the current sustainable supply is 87,500 AFA.20 These sustainable water supply quantities are long-term average annual quantities; it is anticipated that in dry years surface water supply sources will be greatly reduced, therefore, as a preventive measure, Zone 7 stores excess surface water in wet years when SWP and Lake Del Valle supplies are above average, then in dry years, Zone 7 pumps the stored water out of the Main Basin or from STWSD or CWSD. 21 The Zone 7 UWMP states, "Zone 7 is capable of providing 100 percent of its contracted water obligations even in the event of the worst case multi-year drought."22 It should be noted, that consumer conservation efforts could reduce demand during droughts, and increase of groundwater extraction coupled with supplies transferred back through either the STWSD or CWSD out-of-basin supplies could sustain the Tri-Valley customers for even longer periods than under a 100 percent (normal year) demand scenario.23 In other words, Zone 7 plans as if "no conservation" can be achieved. If consumers do reduce demands, Zone 7 could potentially have more water supplies available during drought years. Tables 5-3 and 5-4 forecast Zone 7's supply reliability over the next 25 years during single and multiple-dry year conditions. Table 5-3: Zone 7 Single Driest Year Water Supply (AFA) Water Su I Source Single Driest Year 2005-2012 Single Driest Year 2013-2030 State Water Pro'ecta 4,030 4,030 SWP Car over Su I b 20,000 20,000 STWSD Pum back 8,170 8,170 Cawelo Pum back ~ 10,000 Arro o del Valle Watershed 20 20 Groundwater Main Basin 28,200 28,200 B ron Bethan Irri ation District 2,000 2,000 Su I Total 62,420 72,420 Notes: a. DWR calculated approximately 5 percent of annual supplies would be available in the Single Driest Year. b. SWP Carryover Supply is surplus Zone 7 water stored in DWR facilities in wet or normal years can be utilized to meet demands in dry years. Source: 2005 Zone 7 Urban Water Mana ement Plan, ada ted b PBSBJ/EIP Se tember 2007. 18 2005 Zone 7 Water Agency, Urban Water Management Plan, page 30. 19 2005 Zone 7 Water Agency, Urban Water Management Plan, page 33. 20 2007 Zone 7 Water Agency, Sustainable Water Supply Report page 1. 21 2005 Zone 7 Water Agency, Urban Water Management Plan, page 33. 22 2005 Zone 7 Water Agency, Urban Water Management Plan, page 22. 23 2005 Zone 7 Water Agency, Urban Water Management Plan, page 22. P:\Projects - WP OnIy111162.01 Staples Ranch WSA\FineOFinal WSA.doc 5-16