Land subsidence, the gradual settling or sinking of the Earth’s surface, impacts various regions globally, with California experiencing significant effects. California’s landscape is undergoing such changes, leading to concerns across agricultural, urban, and environmental sectors. This geological shift involves examining the land’s descent, its causes, and impacts on natural systems and infrastructure.
Measuring the Descent
California’s land is sinking at varying rates across the state, with some areas experiencing substantial annual declines. In the Central Valley, the most dramatic sinking can be as much as 8 inches (20 centimeters) per year in specific locations. A recent study indicated that the San Joaquin Valley, a major agricultural region, experienced an average sinking rate of nearly an inch per year between 2006 and 2022. During drought periods, parts of the Central Valley have shown subsidence rates up to 60 centimeters per year.
Scientists use several methods to measure these changes in land elevation. Interferometric Synthetic Aperture Radar (InSAR) uses satellite radar data to track surface deformation with centimeter-level accuracy over wide areas. Continuous Global Positioning System (CGPS) stations, along with traditional GPS surveying, provide consistent data on elevation changes. Borehole extensometers offer site-specific measurements of compaction within subsurface clay layers.
Primary Drivers of Land Subsidence
Extensive groundwater pumping from underground aquifers is the primary cause of land subsidence in California. As water is extracted, particularly from fine-grained sediments like clay, the reduction in fluid pressure causes these layers to compact. This compaction is often irreversible, permanently reducing the aquifer’s capacity to store water even if water levels later recover.
Groundwater pumping intensifies during droughts when surface water supplies are scarce, as farmers and urban areas rely more heavily on it, accelerating subsidence. While groundwater depletion is the dominant cause, other factors contribute, such as oil and natural gas extraction, which removes subsurface support, and the decomposition of organic soils. Natural geological processes like tectonic plate movement and sedimentary settling also play a role, though their influence is less pronounced than groundwater withdrawal.
Regions Experiencing Significant Subsidence
The Central Valley, particularly the San Joaquin Valley, is the area most significantly affected by land subsidence in California. This region has experienced historical sinking of up to 28 feet in some sections since the 1920s. Major subsidence “bowls” have formed around towns like Corcoran and El Nido, covering hundreds of square kilometers and continuing to deepen.
Beyond the Central Valley, other areas of California also show signs of subsidence. Parts of the Sacramento Valley have experienced sinking, including areas in Yolo County where land has declined by up to 1.1 feet, and near Arbuckle in Colusa County, with a recorded sink of 2.14 feet. Coastal regions, including parts of the San Francisco Bay Area and Los Angeles, have also seen land drops due to sediment compaction and, in some cases, slow-moving landslides.
Consequences for Infrastructure and Environment
Land subsidence challenges California’s infrastructure. It can damage water conveyance systems, such as canals and aqueducts, by altering their gradients and reducing flow capacity. The California Aqueduct, for instance, has lost over 20 percent of its capacity in some sections due to sinking land. Roads, bridges, and building foundations can also crack or become misaligned, requiring costly repairs.
Environmentally, subsidence increases flood risk in low-lying areas, as the land surface drops closer to sea level or river levels. It can also lead to saltwater intrusion into freshwater aquifers, particularly in coastal areas, compromising drinking water supplies and agricultural irrigation. The permanent compaction of aquifer systems results in a loss of groundwater storage capacity, impacting long-term water availability.
Strategies for Managing Subsidence
Managing land subsidence in California centers on sustainable groundwater management. The Sustainable Groundwater Management Act (SGMA), enacted in 2014, mandates local agencies to develop plans to achieve groundwater sustainability. These plans aim to balance groundwater withdrawals with recharge, reducing reliance on excessive pumping that causes subsidence.
Key strategies include increasing groundwater recharge, directing surface water to seep into aquifers. This involves using recharge basins or diverting floodwaters to agricultural lands for infiltration. Water conservation and alternative water sources also reduce groundwater demand. These approaches stabilize groundwater levels and prevent further land compaction.