California Drought

Land Subsidence in the Santa Clara Valley

Throughout the late 1800s and into the 1920s when two thirds of the Santa Clara Valley had been irrigated, water flowed freely from wells. Water-level declines of more than 200 ft occurred in the Santa Clara Valley from the early 1900's to the mid 1960's (Fowler, 1981). Land subsidence was first detected in 1933 (Poland and Ireland, 1988). As the decades passed, groundwater levels continued to decline and spirit-level surveys in 1967 identified subsidence of substantial magnitude—as much as 8 feet—and areal extent.

Arresting Land Subsidence

The Santa Clara Valley Water District began aquifer recharge effort in the mid-1930s by building dams, importing water, and implementing a pumping tax in 1964. These projects proved successful; as the groundwater levels began to recover, some long-dry wells started flowing again, and subsidence was halted.

Santa Clara Valley, CA groundwater Levels.

Between 1992 and 1997 some uplift occurred due to recovering groundwater levels. This interferogram (above) indicates no change for most of the southwestern Santa Clara Valley, and land-surface uplift of up to about 24mm (.94 in) in the northern and eastern parts of the valley. This uplift is correlated to the recovery of groundwater levels that occurred for several years as a result of reduced pumpage and increased recharge.

Shaded relief map and interferogram of the Santa Clara Valley, CA from January to August 1997.

Interferogram of Santa Clara Valley, California, shows patterns of subsidence and uplift occurring over 7 months in 1997, from January 4 to August 2. A central subsidence zone, (red) the result of seasonal groundwater pumping, is fully recoverable.

Seasonal Recoverable Subsidence

InSAR imagery reveals seasonal and long-term land-surface-elevation changes influenced by groundwater levels and fault alignment in the Santa Clara Valley.

Between January and August 1997, seasonal, recoverable subsidence occurred due to seasonal groundwater-level declines. Extensometer data indicate about 0.2 ft of elastic deformation in the San Jose area and about 0.06 ft of elastic deformation in the Sunnyvale area (F.S. Riley, USGS, oral commun., 1998). Similarly, interferograms from InSAR images indicate about 0.09 ft of seasonal deformation occurred in the San Jose area for the period October, 1995 to May, 1996 (Galloway, 1997a, b). These data, along with recovered water levels, collectively suggest that the aquifer systems have predominantly experienced elastic deformation that is typical of regional alluvial aquifer systems (Hanson, 1987, 1989; Epstein, 1987; Anderson and Hanson, 1987; Ireland and others, 1984, Galloway, 1998).