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Eastern San Joaquin Groundwater Basin Groundwater Management Plan Northeastern San Joaquin County Section 2 Groundwater Banking Authority 66 Hydrogeology Figures 2-9, 11, 12, 14, 15, 17, 18, and 21 illustrate groundwater levels for selected wells located in and around the principal cone of depression in eastern San Joaquin County. The groundwater levels in these wells clearly illustrate the significant decline in water levels since the 1960s, an average drop of 60 feet. The hydrographs of these wells illustrate average groundwater level drops of around 1.3 feet per year. In general, the lowest groundwater levels were reached in the late 1970s, recovering 10 to 20 feet, but then declined again in the mid- 1990s. Wells in this area have a significant seasonal variation of 10 to 20 feet. Figures 2-7, 8, 10, 13, 16, 19, and 20 illustrate groundwater levels for wells located further away from the main cone of depression, primarily further west and north. These wells show a less dramatic drop than the other wells, and more noticeable increase due to the wet years of 1981 through 1983 (total rainfall in 1983 was more than double the long-term average). The seasonal variation in these wells is distinct but not as pronounced as shown on the other hydrographs. In summary, the hydrographs reviewed illustrate the following general patterns: 1. In the central part of the County the groundwater table dropped continuously from the 1950s and possibly earlier to the mid 1980s. The decline was temporarily reversed due to climatic events. 2. In the northern part of the County groundwater table decline continued into the early 1990s. 3. Starting in the early 1980s a distinct drawdown and recovery cycle appears to have developed. The cycle covers a 10 to 15 year time period, and appears to be driven by climatic conditions more than long-term changes in groundwater use. This recovery and drawdown cycle may indicate that groundwater levels are beginning to equilibrate under current groundwater/surface water use patterns. 2.3.4 Groundwater Discharge and Recharge The estimates of groundwater discharge and recharge presented in these sections are based on the modeling conducted by CDM for the San Joaquin County Water Management Plan, and the modeling originally conducted for the American River Water Resources Investigation (AWRI, 1996), and updated in 1999 for the Bureau of Reclamation by CH2MHill (CH2MHill, 1999). The results are for the Basin only. 2.3.4.1 Groundwater Pumping Groundwater pumping records are not typically available for all wells within the study area. The approach adopted by DWR and other agencies to estimate groundwater withdrawals is based on land use. Figure 2-23 illustrates the ‘simulated’ total agricultural and municipal groundwater pumping for the model domain. Average annual groundwater withdrawal for the period from 1970 to 1993 for the Eastern San Joaquin portion of the model was 850,000 af. 2.3.4.2 Lateral Outflow Under predevelopment conditions, lateral outflow from the Basin discharged to the San Joaquin River and the Delta area. For the period from 1970 to 1993, the net flow was positive, indicating no net groundwater outflow from study area. 2.3.4.3 Deep Percolation The amount of water from natural and human activities that reaches the groundwater table is referred to as deep percolation. Deep percolation is the net of rainfall, applied irrigation water,