Snow runoff in a Sierra Nevada mountain stream in Placer County.

Snow runoff in a Sierra Nevada mountain stream in Placer County. DWR/2012.

Groundwater is an important source of water stored in the earth beneath our feet, in spaces between sand, soils, and fractured rock known as an aquifer. Layers of aquifers make up a groundwater basin. During an average year, California's 515 alluvial groundwater basins and subbasins contribute approximately 38 percent toward the State's total water supply. During dry years, groundwater contributes up to 46 percent (or more) of the statewide annual supply, and serves as a critical buffer against the impacts of drought and climate change. Many municipal, agricultural, and disadvantaged communities rely on groundwater for up to 100 percent of their water supply needs.

We mostly access groundwater through wells and pumps, and it is a crucial buffer against drought when surface water levels, like that inlakes and reservoirs, are running low. When groundwater is extracted in excess of what nature or manmade recharge efforts can replenish, groundwater elevations drop. Low groundwater elevations can cause the ground to gradually sink, a phenomenon known as subsidence. Subsidence, which is particularly bad in parts of the Central Valley, puts state and federal aqueducts and flood control structures at risk of damage.

In order to balance levels of groundwater pumping and recharge, California passed the Sustainable Groundwater Management Act (SGMA). Signed into law by Governor Edmund G. Brown, Jr. in September of 2014, the law requires groundwater-dependent regions to halt overdraft and develop plans to bring basins into balanced levels of pumping and recharge through local planning efforts. SGMA tasked DWR to implement the law and provide ongoing support to local agencies around the state.

Image showing the hydrologic cycle. Contact DWR if you need more information about this image.
The hydrologic cycle.

Groundwater and surface water are essentially one resource, physically connected by the hydrologic cycle in which water evaporates, forms clouds, and falls to the ground as rain or snow. Some of this precipitation seeps into the ground and moves slowly into an underground aquifer, eventually becoming groundwater.  Water law and water policy often consider groundwater and surface water as separate resources, though they are functionally inter-dependent.

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