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LOGE 1907: Bavaro SS/NLS Addendum 3 Page 3 <br /> March 15, 2019 <br /> For the Chico area, Hantzsche and Finnemore present an average rainfall of 22.5 <br /> inches per year and an estimated recharge of 16.8 inches per year; the recharge value <br /> is 75 percent of the average rainfall. <br /> Methods in use in San Joaquin County <br /> Within San Joaquin County, four methods have been identified to derive a value for the <br /> rate of recharge from percolating waters: <br /> • 100 Percent of Rainfall: This method involves identifying rainfall levels from the <br /> closest data station to the project site, and assuming that 100 percent of rainfall <br /> deep-percolates to the first saturated zone. This method likely over-estimates the <br /> rate of recharge from percolating waters. <br /> Rainfall data for the Escalon area were obtained from the Western Regional <br /> Climate Center. The Site is located virtually equidistant from the Manteca, <br /> Oakdale Woodward Dam, and Modesto City County Airport stations. Rainfall <br /> data from these stations is presented, and an average is calculated, in the <br /> following table: <br /> Table 1: Estimated Average Rainfall at Site <br /> Station Annual Rainfall <br /> (inches) <br /> Manteca (045303), 4/1/1965-6/30/1977 11.84 <br /> Oakdale Woodward Dam (046305), 3/1/1906-12/31/1967 13.33 <br /> Modesto City County Airport (045738), 3/1/1906-6/9/2016 12.21 <br /> Average 12.46 <br /> Based on this approach, the rate of recharge from percolating waters is <br /> calculated to be an average of 12.46 inches per year. <br /> • Evapotranspiration Reduction: This method involves using average monthly <br /> precipitation of rainfall and evapotranspiration rates for the area. The calculation <br /> first determines whether the average monthly precipitation exceeds average <br /> monthly potential evapotranspiration. If so, then the difference is the percolating <br /> recharge water into the aquifer. Runoff and stream infiltration drainage are not <br /> factored into the calculation. This approach likely under-estimates recharge to <br /> the aquifer. <br /> The San Joaquin County Environmental Health Department is in the process of <br /> developing a modified version of the evapotranspiration reduction method which <br /> may be recommended for use in future nitrate loading calculations. For this <br />