University of California Water Resources Center

Land subsidence caused by the excessive use of groundwater resources has traditionally caused serious and costly damage to the Los Banos-Kettleman City area of California's San Joaquin Valley. Although the arrival of surface water from the Central Valley Project has reduced subsidence in recent decades, the growing instability of surface water supplies has refocused attention on the future of land subsidence in the region. This report develops a three-dimenslonal, numerical simulation model for both groundwater flow and land subsidence. The simulation model is calibrated using observed data from 1972 to 1998. A probable future drought scenario is used to consider the effect on land subsidence of three management alternatives over the next thirty years. Maintaining present practices virtually eliminates unrecoverable land subsidence, but with a growing urban population to the south and concern over the ecological implications of water exportation from the north, it does not appear that the delivery of surface water can be sustained at current levels. The two other proposed management alternatives reduce the dependency on surface water by increasing groundwater withdrawl. Land subsidence is confined to tolerable levels in the more moderate of these proposals, while the more aggressive produces significant long-term subsidence. Finally, an optimization model is formulated to determine maximum groundwater withdrawl from nine water sub-basins without causing irrecoverable subsidence over the forecasted period. The optimization reveals that withdrawl of groundwater supplies can be increased in certain areas in the eastern side of the study area without causing significant subsidence.