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GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A23 specific capacities ranged from about 20 to 100 (gal/ min)/ft of drawdown, and the saturated thickness tapped ranged from about 100 to 400 ft. Well depths in the San Joaquin Valley range from about 100 to 3,500 ft. The deepest wells are in the west-central and south-central parts of the valley, where the primary source of water is the lower zone. Elsewhere in the San Joaquin Valley, most wells tap the upper zone. For example, in the eastern part of the Los Banos- Kettleman City area, Bull and Miller (1975) noted that wells tapping the highly permeable upper-zone sands may be only 150 to 200 ft deep and yield 1,500 gal/min. In the western part of that area, however, where the upper zone has low permeability, wells must be 2,500 to 3,500 ft deep to obtain adequate yields (900 to 1,200 gal/min). Davis and others (1964) summarized data from 15,000 well-performance tests in the San Joaquin Valley. They noted that most wells yielded 500 to 1,500 gal/min with specific capacities ranging from 10 to 100 (gal/min)/ft of drawdown. EFFECTS OF GROUND-WATER WITHDRAWAL ON THE CENTRAL VALLEY AQUIFER SYSTEM The effects of ground-water withdrawal on the Central Valley aquifer system were investigated by computer simulations of ground-water flow prior to and following development as described in chapter D (Williamson and others, 1989). As noted earlier, the valley deposits (clay, silt, sand, and gravel) were simulated as one aquifer system characterized by variations in vertical leakance properties. The leakance depends not only on the vertical permeability of the sediments but also on the density of wells and their construction. Many of the wells are constructed with long intervals of perforated casing that connect several aquifer layers and thus greatly increase the vertical hydraulic connection through the aquifer system. CHANGES TO THE GROUND-WATER FLOW SYSTEM Before ground-water development, the flow system of the Central Valley was in a state of dynamic equilib- rium natural recharge was balanced by natural dis- charge (fig. 16). As described earlier, ground water flowed toward the axial part of the valley and discharged primarily as evapotranspiration from marshes that ex- isted prior to development. Some discharge also occurred along stream channels where aquifer heads were higher than stream stages. The total flow through the aquifer system was small (about 2 million acre-ft/yr) compared to the surface-water inflow (about 32 million acre-ft/yr). Total precipitation in the valley was estimated to be 12.4 million acre-ft/yr, and total evapotranspiration directly from precipitation was estimated to be 10.9 million acre-ft/yr, leaving 1.5 million acre-ft/yr of water to recharge the aquifer system. The construction of about 100,000 irrigation wells and annual ground-water withdrawals of about 11 million acre-ft during the 1960's and 1970's, together with greatly increased recharge from irrigation returns (de- rived from imported surface water and recirculated pumped water), have significantly altered the ground- water flow pattern of the Central Valley aquifer system. Because ground-water pumpage and recharge from irri- gation water since the 1960's has greatly exceeded the estimated predevelopment recharge rate (fig. 16), flow is largely from areas recharged by imported irrigation water toward areas of irrigation pumpage. Flow through the aquifer system increased nearly sixfold from about 2 to nearly 12 million acre-ft/yr. The values shown in figure 16 do not include water that recharged the aquifer system only to be discharged a short distance away. Thus, total ground-water flow during the 1960's and 1970's, which represents both regional and local flow systems, was greater than that presented in figure 16. Simulation suggests that downward flow from the shal- low deposits and from the upper part of the lower pumped zone has increased by an order of magnitude (Williamson and others, 1989). Water during the 1960's and 1970's was supplied principally by irrigation returns and, to a lesser extent, by natural recharge and by continuing depletion of aquifer storage. However, during the early 1980's, ground-water pumpage decreased slightly and was about equal to the combined rate of natural recharge and irrigation returns. Direct evapotranspiration from the ground-water reservoir was almost completely elimi- nated owing to lowering of the water table. The aquifer system's ability to transmit water verti- cally has changed in direct response to the construction of about 100,000 irrigation wells (fig. 17). Most of the wells in the Central Valley contain perforated casing through- out their lower two-thirds (Diamond and Williamson, 1983). Where the Corcoran Clay Member is present, the perforated sections of many wells fall above and below this confining unit to provide direct hydraulic connection vertically through the perforated zone. Vertical flow is substantial inside many unpumped wells. On the basis of current-meter traverses in 16 wells, Davis and others (1964) concluded that vertical flow through about 3,000 wells that pierced confining beds was about 100,000 acre-ft/yr in the western part of the San Joaquin Valley. Probably an even greater amount of flow occurs through wells in the rest of the Central Valley (Williamson and others, 1989). Conversely, decreased vertical flow through the con- fining beds probably resulted from the inelastic compac-