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LOGE 1847: Jhamat SS/NLS Addendum Page 3 March 6, 2019 Dames and Moore discuss three methods of calculating the rate of recharge from percolating waters, two of which are computer models (SWQCB and HELP) that consider measured water levels, soil types, irrigation, stream flow, and other factors, and the third of which is a monthly consideration of precipitation versus evapotranspiration. The report concludes that estimates using precipitation versus evapotranspiration "appear to be too low" when compared to those values calculated by the two computer models. Dames and Moore eventually determine to use a slight modification of the result of the computer modeling. This method of evaluating ground- water recharge, which includes recharge from a variety of sources, was found to be more applicable to the City of Chico's needs than the simplified rainfall recharge-only method. The value Dames and Moore eventually select (18 inches per year), although derived by the modeling methods discussed rather than from rainfall alone, is equivalent 83 percent of the total rainfall (21.52 inches per year). Methods in use in San Joaquin County Within San Joaquin County, four methods have been identified to derive a value for the rate of recharge from percolating waters: • 100 Percent of Rainfall: This method involves identifying rainfall levels from the closest data station to the project site, and assuming that 100 percent of rainfall deep-percolates to the first saturated zone. This method likely over-estimates the rate of recharge from percolating waters. Rainfall data for the north Stockton area was obtained from the Western Regional Climate Center. The Lodi station (045032), the closest active station to the Site, reports the average total annual precipitation from January 1893 through January 2015 to be 17.24 inches (Table 1). Based on this approach, the rate of recharge from percolating waters is calculated to be an average of 17.24 inches per year. • Evapotranspiration Reduction: This method involves using average monthly precipitation of rainfall and evapotranspiration rates for the area. The calculation first determines whether the average monthly precipitation exceeds average monthly potential evapotranspiration. If so, then the difference is the percolating recharge water into the aquifer. Runoff and stream infiltration drainage are not factored into the calculation. We believe that this approach under-estimates recharge to the aquifer; indeed, a current assessment of the Chico area by this method yields a value of 11.1 inches per year as compared with the 16.8 inches utilized in the Hantzsche and Finnemore paper. For the project Site, the local percolation and evapotranspiration rates are presented in Table 1. Based on the calculation shown, the estimated rate of recharge from percolating waters for the north Stockton, California area is found to be 5.29 inches per year by this method.