Character and evolution of the ground-water flow system in the central part of the western San Joaquin Valley, California

Calculation of a water budget and delineation of contributing sources to drainflows in the western San Joaquin Valley, California

“…The study area generally coincides with the northern part of regional-scale studies of lithology (Laudon and Belitz, 1991), a water budget for the central part of the valley , the character and evolution of the ground-water flow system (Belitz and Heimes, 1990), and ground-water flow modeling (Phillips and Belitz, 1991;Belitz and others, 1992;. Therefore, methods and results of model construction and parameter estimation in this study generally are consistent with those used in regional studies.…”

“…This general decrease in drainage system age from the north to the south parallels the historical rise and areal expansion of the regional water table (Belitz and Heimes, 1990).…”

“…This approach is reasonable for drained areas because water-level measurements from a regional network of shallow wells indicate that drainage systems have maintained the altitude of the water table at a fairly uniform level for 10 to 20 years (Belitz and Heimes, 1990). In areas where hydraulic heads change with time, ground-water flow is nonsteady and the model is intended only to reproduce average conditions represented by the data set.…”

“…After 1958, and upon completion of the California Aqueduct and Central Valley Projects, the principal source of irrigation water became surface water imported from northern California. The availability of surface water reduced the need for pumping, and recharge from percolating irrigation water has resulted in a rise in the potentiometric surface and areal expansion of a shallow water table (Belitz and Heimes, 1990). In 1988, about 460 km2 of the central part of the western San Joaquin Valley was underlain by a water table within 1.8 m of land surface (Belitz and others, 1992).…”

“…Prior to 1958, ground-water pumping was the primary source of irrigation water, and historic water demands resulted in drawdowns of the potentiometric surface as much as 30 to 60 m below predevelopment conditions (Belitz and Heimes, 1990). After 1958, and upon completion of the California Aqueduct and Central Valley Projects, the principal source of irrigation water became surface water imported from northern California.…”