InfrastructureCalifornia's San Joaquin Valley still sinking

Published 3 March 2017

Since the 1920s, excessive pumping of groundwater at thousands of wells in California’s San Joaquin Valley has caused land in sections of the valley to subside, or sink, by as much as 28 feet (8.5 meters). This subsidence is exacerbated during droughts, when farmers rely heavily on groundwater to sustain one of the most productive agricultural regions in the nation. Long-term subsidence is a serious and challenging concern for California’s water managers, putting state and federal aqueducts, levees, bridges, and roads at risk of damage. Already, land subsidence has damaged thousands of public and private groundwater wells throughout the San Joaquin Valley. Furthermore, the subsidence can permanently reduce the storage capacity of underground aquifers, threatening future water supplies.

Since the 1920s, excessive pumping of groundwater at thousands of wells in California’s San Joaquin Valley has caused land in sections of the valley to subside, or sink, by as much as 28 feet (8.5 meters). This subsidence is exacerbated during droughts, when farmers rely heavily on groundwater to sustain one of the most productive agricultural regions in the nation.

Long-term subsidence is a serious and challenging concern for California’s water managers, putting state and federal aqueducts, levees, bridges, and roads at risk of damage. Already, land subsidence has damaged thousands of public and private groundwater wells throughout the San Joaquin Valley. Furthermore, the subsidence can permanently reduce the storage capacity of underground aquifers, threatening future water supplies. It’s also expensive. While there is no comprehensive estimate of damage costs associated with subsidence, state and federal water agencies have spent an estimated $100 million on subsidence-related repairs since the 1960s.

JPL says that todetermine the extent to which additional groundwater pumping associated with California’s current historic drought, which began in 2012, has affected land subsidence in the Central Valley, California’s Department of Water Resources (DWR) commissioned NASA’s Jet Propulsion Laboratory, Pasadena, California, to use its expertise in collecting and analyzing airborne and satellite radar data. An initial report of the JPL findings (August 2015) analyzed radar data from several different sensors between 2006 and early 2015. Due to the continuing drought, DWR subsequently commissioned JPL to collect and analyze new radar images from 2015 and 2016 to update DWR on the land subsidence.

How much sinking?
Several trouble spots identified in the first report continue to subside at rates as high as 2 feet (0.6 meters) a year. Significant subsidence was measured in two subsidence bowls located near the towns of Chowchilla, south of Merced; and Corcoran, north of Bakersfield. These bowls cover hundreds of square miles and continued to grow wider and deeper between May 2015 and Sept. 2016. Maximum subsidence during this time period was almost 2 feet (0.6 meters) in the Corcoran area and about 16 inches (41 centimeters) near Chowchilla. Subsidence also intensified near Tranquility in Fresno County during the past year, where the land surface has settled up to 20 inches (51 centimeters) in an area that extends 7 miles (11 kilometers). Subsidence in these areas affects aqueducts and flood control structures.