LOS ANGELES (Reuters) — California's two main river basins and the aquifers beneath its agricultural heartland have lost nearly enough water since 2003 to fill Lake Mead, America's largest reservoir, new satellite data showed on Monday.

Depleted aquifers account for two-thirds of the loss measured, most of it attributed to increased groundwater pumping for irrigation of drought-parched farmland in California's fertile but arid Central Valley, scientists said.

The findings have major implications for the economy as the Central Valley is home to one-sixth of all irrigated US cropland, said Jay Famiglietti, a hydrologist at the University of California, Irvine, and member of the research team.

The Central Valley, stretching 500 miles from Bakersfield to Redding, has traditionally produced over half the US harvest of fruits and vegetables. California as a whole ranks as the nation's No. 1 farm state in terms of crop value — more than $36 billion a year.

Central Valley farms have increasingly tapped into aquifers during the past few years to help offset drastic cuts in their regular allocations of irrigation water pumped in by the state and federal government from farther north.

How much water remains in California's aquifers is unknown, but satellite studies show that groundwater is being used up faster than nature can restore it.

"I don't think people realize how quickly groundwater is being depleted," Famiglietti said. "It does point to the fact that the pumping is occurring at an unsustainable rate."

Results of the satellite-imaging study, conducted by NASA and the German space agency, were presented by researchers at the American Geophysical Union meeting in San Francisco.

The data are based on subtle month-to-month fluctuations in the Earth's gravitational field used to gauge changes in the presence of groundwater, surface water, ice and precipitation.