The National Aeronautics and Space Administration, using data from its Gravity Recovery and Climate Experiment (GRACE) mission and satellite observations, has calculated California needs 11 trillion gallons of water to recover from its ongoing severe drought.

That information was released Tuesday by NASA and its scientists, speaking in San Francisco.

The need — 42 cubic kilometers of water, or nearly 10.08 cubic miles — is 1.6 times the maximum volume of the largest United States reservoir, according to NASA’s statement.

The findings, made possible by space and airborne measurements, were part of a presentation on California’s drought given at a meeting of the American Geophysical Union.

The data gathered by the satellites are letting scientists identify key features of droughts in unprecedented ways, the agency statement said, and can be used in making informed water management decisions.

Jay Famiglietti, of NASA’s Jet Propulsion Laboratory in Pasadena, led a team that used data from GRACE satellites to develop the unprecedented calculation of the volume of water required to end an episode of drought.

Earlier this year, when California’s three-year drought was at its peak, the team determined that water storage in the Sacramento and San Joaquin river basins was 11 trillion gallons below normal seasonal levels.

Data collected since the launch of GRACE in 2002 shows this deficit has been increasing steadily, NASA said. Only satellites can give scientists this information, the scientists reported.

“Spaceborne and airborne measurements of Earth’s changing shape, surface height and gravity field now allow us to measure and analyze key features of droughts better than ever before, including determining precisely when they begin and end and what their magnitude is at any moment in time,” Famiglietti said.

“That’s an incredible advance and something that would be impossible using only ground-based observations.”

Since 2011, the Sacramento and San Joaquin river basins decreased in volume by 4 trillion gallons of water each year, according to the data obtained through GRACE satellites.

By comparison, that’s more water than California’s 38 million residents use each year for both community and municipal purposes.

About two-thirds of the loss is because of depletion of groundwater beneath California’s Central Valley, NASA said.

Early 2014 data from NASA’s Airborne Snow Observatory, an aerial imaging spectrometer and scanning lidar system flown to heights up to 20,000 feet, indicate that snowpack in California’s Sierra Nevada range was only half of previous estimates, the agency said.

For the first time, the observatory is providing high-resolution examinations of snow water volume in the Tuolumne River, Merced, Kings and Lakes basins of the Sierra Nevada and Uncompahgre watershed in the Upper Colorado River Basin.

The observatory obtained that information through measuring how much water is in the snowpack and how much sunlight the snow absorbs. That influences how fast the snow melts, NASA explained.

The information enables NASA to make accurate estimates of how much water will flow out of a basin when the snow melts. In turn, that helps guide decisions about reservoir filling and water allocation.

“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 Tom Painter, another Jet Propulsion Laboratory employee who is the Airborne Snow Observatory’s principal investigator.

“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,” he said. “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 American Southwest are in the lowest 2 to 10 percent since 1949, the agency noted.

These maps, developed at NASA’s Goddard Space Flight Center in Greenbelt, Md., are drawn using information from GRACE and other satellites, the agency report said.

“Integrating GRACE data with other satellite measurements provides a more holistic view of the impact of drought on water availability, including on groundwater resources, which are typically ignored in standard drought indices,” said Matt Rodell, chief of the Hydrological Sciences Laboratory at Goddard Flight Center.
Recent storms have helped replenish some of California’s water resources, the scientists said, but they aren’t coming close to ending the multi-year drought.

“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.

Those interested in learning more about GRACE can visit the websites www.nasa.gov/grace and www.csr.utexas.edu/grace.

The Airborne Snow Observatory has been developed in a partnership between the California Department of Water Resources and NASA Jet Propulsion Laboratory. Its website is aso.jpl.nasa.gov.

NASA’s earth-science activities information is available at www.nasa.gov/earthrightnow.