Groundwater Storage Changes in the Sacramento-San Joaquin River Basins, October 2003 – March 2009. NASA

In the 66-month period analyzed, the water stored in the combined Sacramento and San Joaquin Basin decreased by more than 31 cubic kilometers, or nearly the volume of Lake Mead. Nearly two-thirds of this came from changes in groundwater storage, primarily from the Central Valley.

NASA Data Reveal Major Groundwater Loss in California’s Heartland