Water storage in the Sacramento and San Joaquin river basins measured "11 trillion gallons below normal seasonal levels" earlier this year, according to data presented by NASA scientists at the American Geophysical Union meeting in San Francisco. The new information shows precisely how severe California's drought is. Since 2011, those basins have decreased in volume by 4 trillion gallons of water per year — a rate that outpaces the amount used yearly by California residents.

The agency attributes two-thirds of the loss to the depletion of groundwater in California's Central Valley. They also discovered that the groundwater levels in the entire Southwest United States "are in the lowest 2 to 10 percent since 1949."

NASA scientists were able to view the impact of drought on the groundwater resources — something that was "typically ignored in standard drought indices" according to Matt Rodell, chief of the Hydrological Sciences Laboratory at Goddard — by integrating data collected from the Gravity Recovery and Climate Experiment (GRACE) with other satellite measurements. According to Jay Famiglietti — who led the team of NASA Jet Propulsion Laboratory scientists — these airborne measurements are the first calculations of their kind, and the analyses "would be impossible using only ground-based observations."

In addition to the groundwater loss, NASA also found that the snowpack in California's Sierra Nevada range was half of the levels estimated for 2014, according data from the agency's Airborne Snow Observatory. The snowpack was "one of the three lowest on record and the worst since 1977, when California's population was half what it is now," according to Tom Painter, the principal investigator for the Observatory. That reduction will make it very hard for the state's reservoirs to replenish quickly. "It takes years to get into a drought of this severity, and it will likely take many more big storms, and years, to crawl out of it," Famiglietti said.