Groundwater is found beneath the Earth’s surface in the spaces between soil, sand, gravel, or rocks. These underground reservoirs, called aquifers, range in size from small pockets of water to vast underground formations. Groundwater is the world’s largest source of freshwater and supports 40 to 60% of California's freshwater needs. 

Groundwater provides essential support to agriculture, industries, and communities while playing a crucial role in sustaining rivers, wetlands, and habitats. Furthermore, California's groundwater is critical in mitigating the impacts of drought because it is available even during dry years; however, long periods of drought or overextraction can lead to groundwater depletion.