Simulation of Ground-Water Flow and Land Subsidence in the Antelope Valley Ground-Water Basin, California
By David A. Leigton, and
Steven P. Phillips
U.S. GEOLOGICAL SURVEY
Water–Resources Investigations Report 03-4016
Sacramento, California 2003
Prepared in cooperation with the Antelope Valley Water Group
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Abstract Antelope Valley, California, is a topographically closed basin in the western part of the Mojave Desert, about 50 miles northeast of Los Angeles. The Antelope Valley ground-water basin is about 940 square miles and is separated from the northern part of Antelope Valley by faults and low-lying hills. Prior to 1972, ground water provided more than 90 percent of the total water supply in the valley; since 1972, it has provided between 50 and 90 percent. Most ground-water pumping in the valley occurs in the Antelope Valley ground-water basin, which includes the rapidly growing cities of Lancaster and Palmdale. Ground-water-level declines of more than 200 feet in some parts of the ground-water basin have resulted in an increase in pumping lifts, reduced well efficiency, and land subsidence of more than 6 feet in some areas. Future urban growth and limits on the supply of imported water may continue to increase reliance on ground water. To better understand the ground-water flow system and to develop a tool to aid in effectively managing the water resources, a numerical model of ground-water flow and land subsidence in the Antelope Valley ground-water basin was developed using old and new geohydrologic information.
The ground-water flow system consists of three aquifers: the upper, middle, and lower aquifers. The aquifers, which were identified on the basis of the hydrologic properties, age, and depth of the unconsolidated deposits, consist of gravel, sand, silt, and clay alluvial deposits and clay and silty clay lacustrine deposits. Prior to ground-water development in the valley, recharge was primarily the infiltration of runoff from the surrounding mountains. Ground water flowed from the recharge areas to discharge areas around the playas where it discharged either from the aquifer system as evapotranspiration or from springs. Partial barriers to horizontal ground-water flow, such as faults, have been identified in the ground-water basin. Water-level declines owing to ground-water development have eliminated the natural sources of discharge, and pumping for agricultural and urban uses have become the primary source of discharge from the ground-water system. Infiltration of return flows from agricultural irrigation has become an important source of recharge to the aquifer system.
The ground-water flow model of the basin was discretized horizontally into a grid of 43 rows and 60 columns of square cells 1 mile on a side, and vertically into three layers representing the upper, middle, and lower aquifers. Faults that were thought to act as horizontal-flow barriers were simulated in the model. The model was calibrated to simulate steady-state conditions, represented by 1915 water levels and transient-state conditions during 1915-95 using water-level and subsidence data. Initial estimates of the aquifer-system properties and stresses were obtained from a previously published numerical model of the Antelope Valley ground-water basin; estimates also were obtained from recently collected hydrologic data and from results of simulations of ground-water flow and land subsidence models of the Edwards Air Force Base area. Some of these initial estimates were modified during model calibration. Ground-water pumpage for agriculture was estimated on the basis of irrigated crop acreage and crop consumptive-use data. Pumpage for public supply, which is metered, was compiled and entered into a database used for this study. Estimated annual pumpage peaked at 395,000 acre-feet (acre-ft) in 1952 and then declined because of declining agricultural production. Recharge from irrigation-return flows was estimated to be 30 percent of agricultural pumpage; the irrigation-return flows were simulated as recharge to the regional water table 10 years following application at land surface. The annual quantity of natural recharge initially was based on estimates from previous studies. During model calibration, natural recharge was reduced from the initial estimate of 40,700 acre-ft per year (acre-ft/yr) to 30,300 acre-ft/yr.
Results of the model simulations indicate that ground-water storage declined more than 8.5 million acre-ft from 1915 to 1995. During the period of peak agricultural pumping (1949-53), pumpage averaged 363,000 acre-ft/yr, and 79 percent of the ground water withdrawn came from storage primarily from layer 1 (the upper aquifer). Water released from compaction of the aquitards accounted for about 21,600 acre-ft/yr of the ground water removed from storage. Downward leakage from layer 1 into layer 2 (the middle aquifer) accounted for most (86 percent) of the pumpage from layer 2. For the simulation period 1991-95 (a period representing current conditions when pumpage for public supply exceeded agricultural pumpage), pumpage averaged 81,700 acre-ft/yr, and most of the ground water withdrawn from layer 2 came from downward leakage from layer 1. During this period, ground water removed from storage accounted for 17 percent of the total pumpage and recharge from irrigation return accounted for about 39 percent of the total pumpage. Ground water removed from storage as a result of compaction of aquitards was reduced to about 3,800 acre-ft/yr.
The calibrated model was used to simulate the response of the aquifer to future pumping scenarios. Results of the simulation of scenario 1, for which total annual pumpage for 1996-2025 remained at the level specified for 1995, showed that water levels continued to rise (as much as 36 feet) in agricultural areas, continuing the long-term recovery from drawdown caused by historical agricultural pumpage. In the areas where pumping for public supply is concentrated, water levels continued to decline and subsidence continued in the central part of the ground-water basin. Water-level declines were largest (more than 100 feet) in the south-central part of the ground-water basin; most of the public-supply pumpage occurs in this area. As much as 1.9 feet of additional subsidence was simulated in the central part of the ground-water basin from 1996 to 2025. For scenario 2, public-supply pumpage was increased by 3.3 percent annually, and annual agricultural pumpage was increased by 75 percent more than that specified for 1995. Pumpage increases for scenario 2 resulted in significant water-level declines in the southern and northeastern part of the Lancaster subbasin; most pumping for public supply occurs in these areas. Results of this simulation showed that water levels declined more than 150 feet in the south-central part of the ground-water basin and that an additional 5 feet of subsidence was simulated in the central part of the basin.
Purpose and Scope
Description of Study Area
Aquifer System and Boundaries
Ground-Water Levels and Movement
Land Subsidence And Aquifer-system Compaction
Simulation Of Ground-Water Flow
Model Discretization and Boundaries
Hydraulic Conductivity and Transmissivity
Model Sensitivity Analysis
Limitations of the Model
Simulation of Aquifer-System Response to Pumping Scenarios
Appendix: Water use 1992-95
Local Surface Water
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Water Resources of California