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Chapter 6 <br />Water Supply Characterization <br />THE CITY OF <br />PL£ASANTON. <br />Zone 7 established historic lows based on the lowest measured groundwater elevations in various wells in <br />the Main Basin. The difference between water surface elevations when the Main Basin is full and water <br />surface elevations when the Main Basin is at historic lows defines Zone 7's operational storage. Of the <br />estimated total storage capacity of 254,000 AF, operational storage is about 126,000 AF based on Zone 7's <br />experience operating the Main Basin, with the remaining 128,000 AF considered emergency reserve storage. <br />6.2.2.2.1 Groundwater Level Monitoring and Storage Estimates <br />Zone 7 routinely monitors groundwater levels within the Main Basin. Some of the data collected is <br />submitted to DWR under the California Statewide Groundwater Elevation Monitoring (CASGEM) program. <br />All the data is reflected in the annual reporting on the Groundwater Management Program. <br />Two independent methods are used to estimate groundwater storage: 1) Hydrologic Inventory, and 2) <br />Nodal Groundwater Elevation. The Hydrologic Inventory method computes storage change each quarter <br />from basin supply and demand data; this method can also be used to forecast future water storage <br />conditions. The Nodal Groundwater Elevation method computes storage from hundreds of water level <br />measurements. Zone 7 continues to refine the calculation methods; the average of the two results is <br />generally used as the estimate of total groundwater storage volume. <br />Figure 6-3 depicts Main Basin storage levels calculated using the average of these two methods in <br />thousand acre-feet (TAF). Note the declines in storage due to drought, particularly between 1987 and <br />1992 and more recently between 2012 and 2015. Stored groundwater at the end of the 2020 water year <br />(October 1, 2019 through September 30, 2020) was approximately 240,000 AF, with 112,000 AF of <br />groundwater available as operational storage. <br />250 - <br />200 -� <br />150 <br />d <br />a� <br />L <br />0 <br />co 100 <br />L <br />r.+ <br />3 <br />50 <br />0 <br />C7 <br />0 <br />WEST YOST <br />7000 <br />0) f�000M70000 000000 NrMV�8)(7 1)000�NC7Vi!7 (O I�ODOO,--N(7 ! t(D I�OD1 <br />0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 <br />N N N N N N N N N N N N N N N N N N NN N <br />N-680-60-20-04- W P -8-680-2020U WMP <br />Figure 6-3. Main Basin Groundwater Storage <br />6-10 <br />City of Pleasanton <br />2020 Urban Water Management Plan <br />May 2021 <br />