Source: http://www.nasa.gov/press/2014/december/...

GRACE data reveal that, since 2011, the Sacramento and San Joaquin river basins decreased in volume by four trillion gallons of water each year (15 cubic kilometers). That's more water than California's 38 million residents use each year for domestic and municipal purposes. About two-thirds of the loss is due to depletion of groundwater beneath California's Central Valley.

In related results, early 2014 data from NASA's Airborne Snow Observatory indicate that snowpack in California's Sierra Nevada range was only half of previous estimates.

"The 2014 snowpack was one of the three lowest on record and the worst since 1977, when California's population was half what it is now," said Airborne Snow Observatory principal investigator Tom Painter of JPL. "Besides resulting in less snow water, the dramatic reduction in snow extent contributes to warming our climate by allowing the ground to absorb more sunlight. This reduces soil moisture, which makes it harder to get water from the snow into reservoirs once it does start snowing again."

New drought maps show groundwater levels across the U.S. Southwest are in the lowest two to 10 percent since 1949.