Central Valley land subsidence

Groundwater in the Central Valley is primarily used for irrigation needs in the agriculture industry and is retrieved using wells. During drought years, groundwater pumping is increased due a shortage of surface water available to farmers. Land subsides most rapidly during drought years due to the increased pumping and lack of sufficient groundwater recharge. Aquifers are not sufficiently recharged due to decreased rainfall and over-utilization of surface water.

As groundwater is pumped, the aquifers are depleted and the water table drops. As the water table falls, competition may occur between farmers to drill deeper underground in order to avoid their well going dry. Agricultural wells may extend as far as 2,000 feet below ground.[4]

Drought years increase the utilization of groundwater which creates a large demand for well drilling businesses. During peak demand, the cost of drilling inflates and clients may be put on waiting lists for lengths of time which exceed growing seasons.[5] There are various irrigation techniques which can reduce water waste and prevent excessive groundwater use. For example, almond orchards require large volumes of water, but due to an increased use of drip tape farmers have dramatically reduced water waste.[6] Using water more effectively saves money and mitigates unnecessary subsidence.

Madera Irrigation District reported that they were able to restore 2,500 acre feet (3.1 million cubic meters) of water in 2017 to aquifers by releasing unused irrigation water onto flood land available to them. They reported that some growers also released excess water allotted to them on flood lands to help restore aquifers. This practice mitigates overutilization of surface water.[7]

Governor Jerry Brown of California also imposed restrictions on unnecessary water usage during the California drought that began in 2012. Measures included drought-tolerant landscaping, rebate programs for purchasing water efficient appliances, and setting a statewide goal of 25% reduction in water usage. Brown also signed a bill allowing for $500 fines to residents who used water unnecessarily during declared drought emergencies. Unnecessary use includes watering sidewalks and exceeding use limits set by urban water suppliers.[8]

Land subsidence in the Central Valley is monitored by various government agencies including NASA, the California Department of Water Resources, the USGS, and various local agencies or businesses within the valley.

The NASA Jet Propulsion Laboratory uses InSAR to remotely measure topographic change over time. In a monitoring study from May 2015 to September 2016, NASA found areas that were subsiding as rapidly as two feet per year. The data also revealed that southern El Nido and Corcoran, California were experiencing the most rapid sinking in a bowl shaped manner.[9]

In the Sacramento Valley, the DWR reports using GPS surveys to study elevation changes in a study known as the Sacramento Valley Height-Modernization Project. Their study includes 339 survey monuments throughout the valley. The DWR also has eleven borehole extensometers to collect detailed information at designated sites. The GPS survey gives a broad picture of change, and the borehole extensometers can focus on key areas.[10]

The USGS collects and synthesizes data from other agencies to make reports on land subsidence in the valley. They use InSAR data from NASA's Jet Propulsion Laboratory in conjunction with interferograms to interpret the topographic change captured by satellite imagery. Continuous Global Positioning Survey station data from UNAVCO, Scripps Orbit and Permanent Array Center, Caltrans, and others are obtained for topography changes over a large region. Spirit Leveling data from the DWR, Delta-Mendota Water Authority, the Central California Irrigation District, and others are used to measure elevation change in a smaller area. The USGS uses the data from borehole extensometers provided by the DWR, and also utilizes information from piezometers to track changes in groundwater.[11]

Key components of studying land subsidence in the Central Valley include large and small scale monitoring, studying topography in relation to groundwater, and a multi-agency approach. Discoveries made by monitoring may allow for better mitigation practices and policies to prevent subsidence.

Infrastructure damage caused by accelerated land subsidence threatens the effectiveness of many dams and canals sourcing surface water to the central valley. Damage has been noted in the California Aqueduct system[12] and is attributed to the rate at which the valley floor is settling. Building foundations, pipelines, and other infrastructure may be at risk of damage as well. Areas with substantial subsidence may be at higher risk of flooding.