Land subsidence in the San Joaquin Valley, California, 1926–70

The extensive withdrawal of groundwater from the unconsolidated deposits of the San Joaquin Valley has caused widespread land subsidence—locally exceeding 8.5 meters (m) between 1926 and 1970 (Poland and others, 1975; fig. 2), and reaching 9 m by 1981 (Ireland, 1986). Long-term groundwater-level declines can result in a vast one-time release of “water of compaction” from compacting silt and clay layers (aquitards), which causes land subsidence (Galloway and others, 1999). Land subsidence from groundwater pumping began in the mid-1920s (Poland and others, 1975; Bertoldi and others, 1991; Galloway and Riley, 1999), and by 1970, about half of the San Joaquin Valley, or about 13,500 km2 had land subsidence of more than 0.3 m (Poland and others, 1975; fig. 2).