California's troubled waters: Satellite-based findings reveal major groundwater loss in Central Valley

PhysOrg: New space observations reveal that since October 2003, the aquifers for California's primary agricultural region - the Central Valley - and its major mountain water source - the Sierra Nevada - have lost nearly enough water combined to fill Lake Mead, America's largest reservoir. The findings, based on satellite data, reflect California's extended drought and increased pumping of groundwater for human uses such as irrigation.

At the American Geophysical Union meeting this week in San Francisco, UC Irvine and NASA scientists detailed the state's groundwater changes and outlined research on other global aquifers conducted via twin satellites called the Gravity Recovery and Climate Experiment. GRACE monitors tiny month-to-month differences in Earth's gravity field primarily caused by the movement of water in the planet's land, ocean, ice and atmosphere. Its ability to "weigh" changes in water content provides new insights into how climate change is affecting Earth's water cycle.

Combined, California's Sacramento and San Joaquin drainage basins have shed more than 30 cubic kilometers of water since late 2003, said Jay Famiglietti, UCI Earth system science professor and director of the UC Center for Hydrologic Modeling. A cubic kilometer is about 264.2 billion gallons, enough to fill 400,000 Olympic-size pools. The bulk of the loss occurred in the state's agricultural Central Valley. The Central Valley depends on irrigation from both groundwater wells and diverted surface water.

"GRACE data reveal groundwater in these basins is being pumped for irrigation at rates that are not sustainable if current trends continue," Famiglietti said. "This is leading to declining water tables, water shortages, decreasing crop sizes and continued land subsidence. The findings have major implications for the U.S. economy, as California's Central Valley is home to one-sixth of all U.S. irrigated land and the state leads the nation in agricultural production and exports."

"By providing data on large-scale groundwater depletion rates, GRACE can help California water managers make informed decisions about allocating water resources," said project scientist Michael Watkins of NASA's Jet Propulsion Laboratory...

A NASA artist's concept of Gravity Recovery and Climate Experiment satellites