According to GRACE (two satellites called the Gravity Recovery and Climate Experiment), within six years drought and irrigation in central California and the Sierra Nevada mountains has lost enough water to equal Lake Meade, the largest reservoir in the United States. GRACE is (are?) satellites that monitor the miniscule differences in Earth’s field of gravity. The gravity field changes because of moving water in the land, ocean, ice, and atmosphere. The recordings are sensitive enough to note changes in the water content (in effect by weighing) of specific regions. In this case, central California, in another case it was northern India.

GRACE was designed from the beginning to provide data on water volume changes that can be measured by comparing the strength of the gravity field over specific regions on a month to month basis. These changes, such as the rate of water loss in California, can be used to spot potential droughts or provide a regional profile of the water loss.

Combined, California’s Sacramento and San Joaquin drainage basins have shed more than 30 cubic kilometers of water since late 2003, says Jay Famiglietti, UCI Earth system science professor and director of the UC Center for Hydrologic Modeling.

“GRACE data reveal groundwater in these basins is being pumped for irrigation at rates that are not sustainable if current trends continue,” Famiglietti says. “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.”

[Source: Futurity]

Unfortunately water shortage problems are global, and GRACE can only cover small portions – typically those most exposed to drought and over-irrigation. Nonetheless, the GRACE data are invaluable to developing a picture of the water supply and usage. GRACE is a joint project of NASA and the German Aerospace Center.