Simulation of Groundwater Flow and Land Subsidence in the Antelope Valley Groundwater Basin

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Prior to 1972, groundwater provided more than 90 percent of the total water supply in Antelope Valley, California (~50 miles northeast of Los Angeles); since 1972, it has provided between 50 and 90 percent (the balance provided by imported surface water). Most groundwater pumping in the valley occurs in the Antelope Valley groundwater basin, which includes the rapidly growing cities of Lancaster and Palmdale.

Groundwater was primarily used for agriculture prior to the late 1960s, the maximum demand was about 400,000 acre-ft/yr in 1951 and declined to 34,000 acre-ft/yr in 1995. Recently, agricultural demand has increased with the introduction of carrot farming. Public-supply demand increased from less than 1,000 acre-ft/yr in 1951 to more than 40,000 acre-ft/yr in 1995. By 1995, groundwater pumpage resulted in water-level declines of more than 200 ft in some parts of the groundwater basin and land subsidence of more than 6 ft in some areas. Future urban growth, increased agricultural demand, and limits on the supply of imported water will continue to increase the demand for ground water.

A previous USGS study (Leighton and Phillips, 2003) estimated that for the period 1915-1995 the total natural recharge equaled 30,000 acre-ft/yr and agricultural return flows ranged from 0 to about 120,000 acre-ft/yr with an average of about 50,000 acre-ft/yr. Leighton and Phillips (2003) estimated that the return flows require 10 years to travel through the unsaturated zone; therefore, the simulated return flows were delayed by 10 years after the water was applied. Geohydrologic data collected since 1995 provide an opportunity to refine and improve the understanding of the quantity and distribution of recharge in the Antelope Valley. An improved understanding of recharge will allow local decision makers to better manage their water resources.

The objectives of this study are to: (1) improve the understanding of the quantity, spatial distribution, and temporal distribution of natural recharge and irrigation return flow and (2) incorporate this improved understanding into a ground-water flow model that will help to better manage the Antelope Valley groundwater basin.