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Recharge to the three-aquifer ground-water system in Borrego Valley is due almost entirely to infiltration of intermittent streamflow and is estimated to average about 4,800 acre-feet per year. During 1946-79, net ground-water pumpage (that part of the pumpage actually consumed) ranged from about 1,700 to 13,700 acre-feet per year. The ground water was used mainly for irrigation. Because of the imbalance between recharge and pumpage, ground-water levels declined as much as 100 feet in some areas of the valley during 1945-80. As an aid to analyzing the effects of pumping on the ground-water system, a three-dimensional finite-element ground-water flow model was developed. The model was calibrated for both steady-state (1945) and transient-state (1946-79) conditions. For the steady-state calibration, hydraulic conductivities of the three aquifers were varied within reasonable limits to obtain an acceptable match between measured and computed hydraulic heads. During steady-state conditions, recharge from streamflow infiltration (4,800 acre-feet per year) was balanced by computed evapotranspiration (3,900 acre-feet per year) and computed subsurface outflow from the model area (930 acre-feet per year). The volumes and distribution of net ground-water pumpage were estimated from land-use data and estimates of consumptive use for the various irrigated crops. The pumpage was assigned to the appropriate nodes in the model for each of seventeen 2-year time steps representing the transient-state period (1946-79). For transient-state calibration, the specific yields of the three aquifers were varied within reasonable limits to obtain an acceptable match between measured and computed hydraulic heads. During the transient-state period, ground-water pumpage input to the model was compensated by declines in both the computed evapotranspiration and the amount of ground water in storage.

Water resources of Borrego Valley and vicinity, San Diego County, California: Phase 2 - Development of a ground-water flow model