The Central Valley in California subsides when groundwater is pumped faster than underground aquifers can be recharged. The Central Valley has been sinking (subsiding) at differing rates since the 1920s and is estimated to have sunk up to 28 feet.[1] During drought years, the valley is prone to accelerated subsidence due to groundwater extraction. California periodically experiences droughts of varying lengths and severity.
The Central Valley is an agriculturally productive region dependent on large volumes of irrigation water. This region is considered arid to semiarid and is reliant on infrastructure to deliver water. The Central Valley is prone to excessive flooding due to snowmelt from the surrounding Sierra Nevada mountain range in the spring. A controlled system of dams and canals have been built by state and federal agencies to ensure a steady flow of water into the valley primarily to support agriculture. The California State Water Project and the Central Valley Project are the two main projects diverting surface water into the valley. This system helps prevent spring flooding and summer and fall water shortages.
Farmers use a combination of surface water and groundwater for irrigation. During drought years there is less surface water being provided to farmers and they rely on pumping more groundwater. In California, there is a hierarchy of water entitlement in which those with water rights have precedence to receive and use surface water.[2] Farmers without water rights may not always receive surface water during drought years. Farmers not receiving surface water may choose to rely on groundwater for irrigation. Drought years accelerate land subsidence in the Central Valley due to a collective effort of increased groundwater pumping without replenishing underground aquifers.[3]