Subsurface Water Flux in California's Central Valley and Its Source Watershed From Space Geodesy

We integrate Global Positioning System displacements, Gravity Recovery and Climate Experiment gravity data, reservoir water volumes, and snowpack to estimate change in subsurface water in California. We find 29% of precipitation infiltrates mountain soil and fractured bedrock each autumn and winter and is lost in the spring and summer by evapotranspiration and lateral subsurface flow either within mountain watersheds or into California's Central Valley. The Central Valley lost groundwater at 2.2 ± 0.7 km³/yr from 2006 to 2021, with 68% of the loss occurring in the southern third of the Valley. Water in Central Valley fluctuates each year by a mean of 10.7 ± 1.1 km³ with maximum water in April (not August). A third of Central Valley groundwater lost during recent severe drought is recharged during subsequent years of heavy precipitation. Of the 50 km³ of water entering Central Valley each year, 28 km³ comes from rivers, 17 km³ from precipitation, and 5 km³ from mountain groundwater.