Satellites peering down on California's great Central Valley have
discovered evidence that the nation's prime food source is fast losing precious
reserves of water from the valley's underground aquifers.
Loss of water from beneath the surface, combined with increasing shortages of surface irrigation caused by climate change, is quietly adding up to a crisis that threatens one of the state's major industries, scientists say.
"We don't recognize the dire water situation we face," said Jay Famiglietti, a UC Irvine hydrologist who has monitored those stored water levels every month since the spacecraft began measuring the sources 11 years ago.
Even as those reserves are being depleted, diminished surface water sources are forcing many farm districts in the region to pump their water from ever deeper levels of the underground aquifers, farm experts say.
Pumps that once brought water from 500 feet deep are now reaching as deep as 900 or even 1,500 feet, said Chase Hurley, general manager of the San Luis Canal Co., a 100-year-old water agency in Los Banos (Merced County) that represents more than 300 landowners farming 45,000 acres of irrigated crops -- mostly with water from the San Joaquin River.
"As you suck that water out of deep clay layers," Hurley said, "you not only get subsidence but changes in water quality. It's salty, and acidic, and that's not good for crops."
Additionally, groundwater pumping is creating difficult subsidence problems as the land surface in many areas is sinking -- by 4 or 5 feet or even more.
Chronic water shortages
Irrigation districts serving Central Valley farmers obtain most of their water from the Central Valley Project, whose dams and reservoirs release the water from the annual thaw of the Sierra snowpack.
But those farmers have long faced chronic water shortages, and this year the mountain snowpack has been at barely half its normal water yield. Managers of the project have warned that the irrigation districts south of the delta will be allocated only 20 percent of the water they have contracted for.
That could force farmers to pump more water out of the aquifers, which are filled by long, sustained drainage from above.
"We're losing those groundwater reserves every month, and with climate change affecting snowmelt, the risks and uncertainties are changing faster than ever," Famiglietti said. "We don't see that there'll be any replenishment in the future, so there's a critical need to improve the way we monitor and regulate groundwater systems now."
Famiglietti, who directs UC Irvine's Center for Hydrologic Monitoring, and Matthew Rodell, of NASA's Goddard Space Flight Center in Greenbelt, Md., reported on the spacecraft's underground water findings in this week's issue of the journal Science.
Satellites in action
The twin NASA satellites, named Grace, for Gravity Recovery and Climate Experiment, work together like a pair of scales in space, and their sensitive instruments detect with unprecedented accuracy, among other things, extremely small variations in Earth's gravity fields caused by changes in the mass of land underground.
So subtle are those changes that the satellites can map changes in the density of ocean water as currents shift, and the melting of the world's glaciers as their waters flow into the oceans.
The spacecraft have now found that in the past 10 years, aquifers beneath the Central Valley have lost more than 30 cubic kilometers of water a year -- nearly 10 times the volume of all the water in Lake Mead on the Colorado River, the largest reservoir in America, Famiglietti said.
The satellites have far outlived their planned five-year life span, and a follow-up Grace mission is scheduled for launch in 2017. Its planners hope a new, more advanced Grace mission can report changes in crucial groundwater levels at least weekly, instead of monthly, so water planners and regulators can manage the underground aquifers more rapidly and effectively, Famiglietti said.
David Perlman is the San Francisco Chronicle's science editor. E-mail: email@example.com
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