California Water Science Center

Water Availability and Subsidence in California's Central Valley

By Claudia C. Faunt and Michelle Sneed

Published: September 2015

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California's Central Valley covers about 52,000 square kilometers (km2) and is one of the most productive agricultural regions in the world. More than 250 different crops are grown in the broad alluvial filled structural trough, with an estimated value exceeding $20 billion per year (Faunt 2009). Central Valley agriculture depends on state and federal water systems that divert surface water, predominantly originating from Sierra Nevada snowmelt, to agricultural fields. Because the valley is semi-arid and the availability of surface water varies substantially from year to year, season to season, and from north to south, agriculture, as it grew, developed a reliance on groundwater for irrigation.

The extensive withdrawal of groundwater caused water levels to decline on the west side of the southern two-thirds of the Central Valley, also known as the San Joaquin Valley. Long-term groundwater-level declines resulted in a one-time release of "water of compaction" from compacting fine-grained deposits, which caused land subsidence (Galloway et al. 1999). More than half of the thickness of the Central Valley aquifer system is composed of fine-grained sediments, including clays, silts, and sandy or silty clays (Williamson et al. 1989; Faunt 2009), that are susceptible to compaction if depressurized by groundwater pumping. Land subsidence in the Central Valley from groundwater pumping began in the mid- 1920s, and by 1970 about half of the San Joaquin Valley, or about 13,500 km2, had subsided more than 0.3 meters (m) (Poland et al. 1975). Locally, magnitudes reached 9 m by the early 1980s (Ireland 1986).

Partially in response to these groundwater-level declines and associated subsidence, an extensive surface-water delivery system was developed to redistribute some of the water from north to south and east to west. Surface-water imports from the Delta–Mendota Canal (DMC) since the early 1950s and the California Aqueduct since the early 1970s resulted in decreased groundwater pumping in some parts of the valley, which was accompanied by a steady recovery of water levels and a reduced rate of compaction (Ireland 1986).

This essay describes more recent changes in water availability and competition for water in the Central Valley and evaluates the influence that climate variability and human action has on subsidence, particularly during the most recent drought periods. The hydrology of the present-day Central Valley is driven by surface-water deliveries and associated groundwater pumpage, which in turn reflect the spatial and temporal variability in climate, water availability, and land use. Climate variability has had profound effects on the Central Valley hydrologic system. During droughts, surface water is less available, and groundwater pumpage increases. For example, the diminished availability of surface water during the droughts of 1976–1977 and 1987–1992, reversed the overall trend of groundwater-level recovery and re-initiated land subsidence in the San Joaquin Valley. Following each of these droughts, recovery to pre-drought water levels was rapid and compaction virtually ceased (Swanson 1998; Galloway et al. 1999).

Suggested Citation: Faunt, C.C. and Sneed, M., 2015, Water Availability and Subsidence in California's Central Valley, San Francsico Estuary and Watershed Science, 13(3), 8 p.

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