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GROUND WATER IN THE CENTRAL VALLEY,CALIFORNIA 023
<br /> specific capacities ranged from about 20 to 100 (gall from precipitation was estimated to be 10.9 million
<br /> min)/ft of drawdown, and the saturated thickness tapped aere-ft/yr, leaving 1.5 million acre-ft/yr of water to
<br /> ranged from about 100 to 400 ft. recharge the aquifer system.
<br /> Well depths in the San Joaquin Valley range from The construction of about 100,000 irrigation wells and
<br /> about 100 to 3,500 ft. The deepest wells are in the annual ground-water withdrawals of about 11 million
<br /> west-central and south-central parts of the valley, where acre-ft during the 1960's and 1970's, together with
<br /> the primary source of water is the lower zone. Elsewhere greatly increased recharge from irrigation returns (de-
<br /> in the San Joaquin Valley, most wells tap the upper zone. rived from imported surface water and recirculated
<br /> For example, in the eastern part of the Los Banos- pumped water), have significantly altered the ground-
<br /> Kettleman City area, Bull and Miller (1975) noted that water flow pattern of the Central Valley aquifer system.
<br /> wells tapping the highly permeable upper-zone sands Because ground-water pumpage and recharge from irri-
<br /> may be only 150 to 200 ft deep and yield 1,500 gal/min. In gation water since the 196(]'s has greatly exceeded the
<br /> the western part of that area, however,where the upper estimated predevelopment recharge rate(fig. 16),flow is
<br /> none has low permeability,wells must be 2,500 to 3,500 ft largely from areas recharged by imported irrigation
<br /> deep to obtain adequate yields(900 to 1,200 gal/min). water toward areas of irrigation pumpage. Flow through
<br /> Davis and others (1964) summarized data from 15,000 the aquifer system increased nearly sixfold—from about
<br /> well-performance tests in the San Joaquin Valley. They 2 to nearly 12 million acre-ft/yr. The values shown in
<br /> noted that most wells yielded 500 to 1,500 gal/min with figure 16 do not include water that recharged the aquifer
<br /> specific capacities ranging from 10 to 100 (gal/min)/ft of system only to be discharged a short distance away.
<br /> drawdown. Thus, total ground-water flow during the 1960's and
<br /> 1970's, which represents both regional and local flow
<br /> EFFECTS OF GROUND-WATER WITHDRAWAL ON systems, was greater than that presented in figure 16.
<br /> THE CENTRAL VALLEY AQUIFER SYS'T'EM Simulation suggests that downward flow from the shal-
<br /> low deposits and from the upper part of the lower
<br /> The effects of ground-water withdrawal on the Central pumped zone has increased by an order of magnitude
<br /> Valley aquifer system were investigated by computer (Williamson and others, 1989).
<br /> simulations of ground-water flow prior to and following Water during the 1960's and 1970's was supplied
<br /> development as described in chapter D (Williamson and principally by irrigation returns and, to a lesser extent,
<br /> others, 1989).As noted earlier, the valley deposits(clay, by natural recharge and by continuing depletion of
<br /> silt, sand, and gravel) were simulated as one aquifer aquifer storage. However, during the early 1980'x,
<br /> system characterized by variations in vertical leakance ground-water pumpage decreased slightly and was about
<br /> properties. The leakance depends not only on the vertical equal to the combined rate of natural recharge and
<br /> permeability of the sediments but also on the density of irrigation returns. Direct evapotranspiration from the
<br /> wells and their construction. Many of the wells are ground-water reservoir was almost completely elimi-
<br /> constr•ucted with long intervals of perforated casing that nated owing to lowering of the water table.
<br /> connect several aquifer layers and thus greatly increase The aquifer system's ability to transmit water verti-
<br /> the vertical hydraulic connection through the aquifer cally has changed in direct response to the construction of
<br /> system. about 100,000 irrigation wells (fig. 17). Most of the wells
<br /> in the Central Valley contain perforated casing through-
<br /> CHANGES T01 HE GROUND-WA]ER F1.0W SN STEM out their lower two-thirds (Diamond and Williamson,
<br /> 1983). Where the Corcoran Clay Member is present, the
<br /> Before ground-water development, the flow system of perforated sections of many wells fall above and below
<br /> the Central Valley was in a state of dynamic equilib- this confining unit to provide direct hydraulic connection
<br /> rium—natural recharge was balanced by natural dis- vertically through the perforated vane_ Vertical flow is
<br /> charge (fig. 16), As described earlier, ground water substantial inside many unpumped wells. On the basis of
<br /> Rowed toward the axial part of the valley and discharged current-meter traverses in 16 wells, Davis and others
<br /> primarily as evapotranspiration from marshes that ex- (19414) concluded that vertical flow through about 3,000
<br /> isted prior to development. Some discharge also occurred wells that pierced confining beds was about 100,000
<br /> along stream channels where aquifer heads were higher acre-ft/,yr in the western part of the San Joaquin Valley.
<br /> than stream stages. The total flow through the aquifer Probably an even greater amount of flow occurs through
<br /> system was small(about 2 million acre-ft/yr)compared to wells in the rest of the Central Valley (Williamson and
<br /> the surface-water inflow (about 32 million acre-ftlyr). others, 1989).
<br /> Total precipitation in the valley was estimated to be 12.4 Conversely, decreased vertical flow through the con-
<br /> million acre-ft/yr, and total evapotranspiration directly fining beds probably resulted from the inelastic compac-
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