Impacts of Subsidence on California’s Water Supply
What Is Subsidence?Land subsidence is a gradual or sudden sinking of the land from changes that take place underground, primarily when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. Land subsidence may not be immediately noticeable because it can occur over large areas rather than in a small spot, like a sinkhole, and is often overlooked as an environmental consequence of certain water-use practices. Subsidence is a global problem, but in the United States more than 17,000 square miles in 45 states — an area roughly the size of New Hampshire and Vermont combined — have been directly affected by subsidence.
California ImpactsIn California, subsidence is caused by past groundwater pumping practices. For example, between 1926 and 1970, several locations in the San Joaquin Valley have experienced nearly 30 feet of subsidence. Since then, over six feet of additional subsidence has occurred.
Subsidence has impacted important infrastructure throughout the San Joaquin Valley, including roads, bridges and water conveyance systems. For example, subsidence along the California Aqueduct, the cornerstone of the State Water Project (SWP), has caused the canal to slump, putting reliable water delivery at risk. The damage has resulted in higher operational and power costs and increased water delivery outages and major repairs. In fact, the SWP has lost more than 20 percent of its capacity due to subsidence. In wet years, when the full capacity of the California Aqueduct is needed to convey water to groundwater recharge, storage and use throughout the Valley and Southern California, this reduced capacity for conveyance hindered climate change adaptation efforts that deliver and store water when conditions are wet. The impacts of this subsidence are felt far beyond the Central Valley.
What Can Be Done?When the Sustainable Groundwater Management Act (SGMA) became law in 2014, California took action to help prevent future subsidence by stopping the over-pumping of groundwater, but previous damage had already been done.
Significant repairs must be made to effectively address the existing impacts of subsidence to California’s water supply infrastructure.
There are four subsidence repair projects that affect significant arterial water supply canals. One project is a state-owned SWP facility, another is a state-federal joint use facility that is federally owned, and two others are federally owned features of the Central Valley Project (CVP). Specifically, the four projects are:
- California Aqueduct (downstream of Kettleman City) -- SWP
- Joint Use Facility (between Dos Amigos and Kettleman City) -- CVP
- Friant-Kern Canal – CVP
- Delta-Mendota Canal – CVP
Subsidence also is a key focus of the California Water Commission as it assesses a potential state role in financing conveyance projects that help meet the needs of the changing climate.
ConclusionCalifornians depend on the water that flows through the SWP and CVP facilities to maintain the state’s economy, support its thriving agricultural industry, develop additional sources of water supplies, reduce reliance on the Sacramento-San Joaquin Bay Delta, and serve affordable and safe drinking water to disadvantaged communities throughout the state.
CMUA supports the creation of a statewide program to repair and restore this critical water infrastructure. Restoring this conveyance capacity would benefit the state through reduced energy consumption, improved climate change resiliency, job creation and a secure water supply. These repairs will require the involvement and contribution of all levels of government — local, state, and federal.