Aquifer systems in the Eloy-Picacho area, Arizona, the Houston-Galveston area, Texas, and the Tulare-Wasco area and Santa Clara Valley, California, appear to have been overconsolidated by an amount that ranged approximately from 1.6 to 6.2 bars (16 to 63 m of water) before man began to withdraw groundwater from them. The relation between land subsidence and water level decline in these areas, consists of two linear segments. In these areas, subsidence per unit water level decline was approximately constant until water levels had declined an amount that ranged from 16 to 63 m. When water levels declined past these values, subsidence per unit water level decline increased to larger constant values. Although slow drainage from aquitards may have contributed to this response, it is interpreted here to be caused primarily by natural overconsolidation of the compacting part of the aquifer system. The water level decline at which the ratio of subsidence to unit water level decline changed indicates approximately the amount by which the preconsolidation stress exceeded the overburden stress on the aquifer system that existed before groundwater withdrawals began.
