California's groundwater resources are nearly as important as its rivers in terms of supply, and have been perhaps even more important economically.
Current supplies of useable groundwater in California are estimated to be six times the volume of all the state's surface water reservoirs combined. About half the population in California relies on groundwater for drinking.
Groundwater is water that resides beneath the ground surface, in small spaces around soil or rock particles. The water got there by draining through surface layers of soil and rock, and the groundwater may in turn seep out to join streams and rivers. Thus groundwater and surface water are linked in the hydrologic cycle. Typically, streams contribute to the recharging of groundwater supplies in the spring, when runoff is high. In the fall, stream flow may be fed by the groundwater.
Groundwater sources may be found below the surface in most of the state, particularly along the Central Valley and underlying the Colorado Desert. In some cases the groundwater is under pressure, so that it rises to the surface without the aid of pumps when wells are dug. Such wells are called flowing artesian wells. In other cases, electric or wind-driven pumps may be used to bring the water to the surface.
Typically, about 30 percent of urban and agricultural water use is groundwater, but in dry years and in some areas, it may be 60 percent.