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 km 3/ 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 km 3 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 km 3 of water entering Central Valley each year, 28 km 3 comes from rivers, 17 km 3 from precipitation, and 5 km 3 from mountain groundwater. Plain Language Summary We combine measurements from Global Positioning System positioning and Gravity Recovery and Climate Experiment gravity to infer change in water components in Central Valley and its source watershed, the Sacramento-San Joaquin-Tulare River basin better than possible with either technique separately. We find that the Central Valley has lost groundwater from 2006 to 2021 at 2.2 ± 0.7 km 3/ yr (95% confidence limits follow the “±” sign), with 2/3 of the groundwater loss occurring in the southern part of the Valley. We estimate the seasonal recharge and loss of subsurface water in the Central Valley. Comparison to a model accounting for precipitation, evapotranspiration, and river water entering and leaving the Central Valley suggests that deep groundwater may be flowing from the Sierra Nevada mountains into the Central Valley.