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I <br />ii <br />events, a mean value was used for each well based on all estimates of 0.10. The <br />upper limit of 0.10 was chosen because this value represents the lower limit of the specific <br />yield for a sandy soil (Driscoll 1986). The specific yield is the quantity of water that a unit <br />volume of unconfined aquifer gives up by gravity, and can also be thought of as the maximum <br />amount of total porosity that can be filled with air when the soil is drained by gravity (i.e., air - <br />filled porosity). <br />Evapotranspiration data were stored and processed within Lotus spreadsheets (Lotus <br />Development Corporation 1990), and were plotted using PROC GPLOT (SAS Institute Inc. <br />1990b). <br />Groundwater Withdrawal for Irrigation <br />The effect of pumping water from adjacent irrigation wells (within 1 mile ) on the <br />groundwater at each study site was evaluated by examining the response of the study -site wells <br />equipped with recorders during <br />g S' lace groundwater levels were summarized as daily <br />means, the pumping data were also summarized on a daily basis as either off or on. If a pump <br />was on during any portion of the day (midnight to midnight), then the pump was recorded as <br />being on for that day. The groundwater response was evaluated by examining simultaneous <br />plots of groundwater, precipitation, and pumping periods for each irrigation well. <br />Se aration of River Sta a Preci itation and Eva otrans iration <br />Correlation analysis was used to separate the effects of river stage, precipitation, and <br />evapotranspiration on the groundwater level. The analysis was based on mean daily values for <br />stage and groundwater levels, an antecedent precipitation index (API), and total daily <br />ionriver <br />potential evapotranspiration. Potential evapotranspiration was included only in the Crane <br />Meadows analysis, since it was the only site with a weather station equipped to collect data for <br />estimating PET. Turc's method, described in the previous section, was used to estimate PET. <br />An antecedent precipitation index was used to simulate the groundwater recession following <br />each event. Although the water table peaked within hours after a precipitation event, it usually <br />took several days for the water table to return to its previous level when no other inputs were <br />24 <br />l <br />