SAN FRANCISCO -- California's Central Valley – one of the most productive agricultural regions in the U.S. – has lost enough water since late 2003 to fill up Lake Mead, a dire trend for the state and the country.

Between October 2003 and March 2009, the total amount of water in the Sacramento and San Joaquin basins, which support a 154,000 square kilometer region, dropped by 31.3 cubic kilometers, or about enough to fill in a basin the size of Lake Mead, according to Jay Famiglietti, a professor at the University of California Irvine speaking at the American Geophysical Union taking place this week in San Francisco. The AGU is an annual gathering of earth/space scientists.

The total loss of 20.3 cubic kilometers comes from groundwater depletion, caused largely by a drought that started in 2006 and restrictions on allocation of surface water. Most of the losses are coming from the southern part of the region.

"Continued reliance on groundwater will deplete critical reserves and eliminate a buffer," he said. "The numbers we are getting are pointing to groundwater use at unsustainable rates."

The trend poses "significant threats to food production in the U.S. and the state's economy," he added. 250 different crops are grown in the Central Valley. The harvest comes to around $17 billion a year, about 8 percent of U.S. food production by revenue. As a whole, the Central Valley constitutes one-sixth of the irrigated land in the U.S. and one-fifth of the demand for groundwater.

California isn't alone. India, Australia and the southeast of the U.S. has also seen groundwater losses. Three states in northern India have lost 17.7 cubic kilometers a year over the past decade. (A cubic kilometer holds 264.2 billion gallons – enough for 400,000 Olympic-sized pools.)

The data comes from the Gravity Recovery and Climate Experiment (GRACE) conducted by NASA. GRACE takes space observations and cross checks it against data on the Earth's gravitational field.

Although various startups have launched products for conserving or desalinating water, the market is largely dominated by large companies. General Electric and Siemens have bought several water companies in recent years while IBM is working in China, Iowa, the Netherlands, Ireland and other regions on water management strategies.

Photo of Lake Mead via Flickr/Creative Commons.