SAN FRANCISCO, CA, DECEMBER 16, 2014 – The epic California drought has left its bountiful Central Valley short of water — and NASA now knows by how much: about 11 trillion gallons.

The number comes from a recent satellite survey, “something that would be impossible using only ground-based observations” said NASA Jet Propulsion Laboratory scientist Jay Famiglietti at a press conference today at the American Geophysical Union’s Fall Meeting.

California’s Central Valley (source: Wikipedia.org).

Famiglietti led a team of scientists who were able to make this calculation thanks to a pair of NASA satellites that map minute changes in the Earth’s gravitational field. Launched in 2002, the GRACE (Gravity Recovery and Climate Experiment) mission is providing insights in several scientific fields, from monitoring how ice sheets thin to movements of magma inside the Earth to groundwater storage deficits in the Central Valley, the continental U.S., and other regions around the world, such as India.

GRACE is revolutionizing how droughts can be tracked, quantifying their beginning, their ends, and what Famiglietti called the drought’s “instantaneous magnitude.”

Unsustainable Trend

California’s current drought is its worst in 1,200 years. It seems to have been caused by natural factors, but warmer temperatures in California, as around the Earth from manmade climate change, intensify droughts by amping up water evaporation rates from lakes and soils.

Now entering its fourth winter, NASA’s measurements show the Sacramento and San Joaquin river basins have lost four trillion gallons of water each year — more water than California’s 38 million residents use annually. About two-thirds of this loss comes from the depletion of subsurface groundwater.

An 11 trillion gallon deficit isn’t easy to envision. It’s one and a half times the maximum volume of Lake Mead, the largest reservoir in the U.S. It would be a ball of water 2.7 miles in diameter. It would cover the entire Central Valley in water 29 inches deep — in a region whose annual rainfall can be as low as 5 inches in some places.

NASA’s GRACE results begin in 2003, and when combined with earlier USGS data show the Central Valley has lost about 100 cubic kilometers since 1962, or 26 trillion gallons.

Source: NASA, Jet Propulsion Laboratory.

That’s a sobering and unsustainable trend for one of the world’s most agriculturally productive regions, producing 11 percent of U.S. agriculture with a total crop value in 2012 of $45 billion.

Record-low Snowpack

GRACE’s twin satellites orbit about 310 miles above Earth. One satellite trails the other by 140 miles, and a microwave ranging system measures changes in their distance down to 400-millionths of an inch and their speed as each reacts to the changes in gravity below it due to variations in mass.

GRACE has also been used to measure snowpack in California’s Sierra Nevada mountain range, showing it was only half of previous estimates. Such data will help reservoir managers and water allocators to make decisions as the crucial snowpack melts each spring — 2014’s snowpack was the worst on record, deeper even than a very low snowpack in 1977 that occurred when California’s population was half what it is today.

Results from GRACE have also been combined with other satellite measurements to give a broader view of the impacts of drought nation- and world-wide.

Despite recent heavy rains in the Central Valley region from winter storms barreling across the Pacific Ocean, California’s multi-year drought is nowhere close to being over. The water will help re-moisturize the soil, which must happen before rainwater can percolate into groundwater basins.

“It takes years to get into a drought of this severity,” said Famiglietti, “and it will take many more big storms and years to crawl out of it.”