New satellite measurements reveal that California’s Central Valley and snow-capped peaks have lost massive amounts of water over the past six years — enough, in fact, to fill up Lake Mead.
More than 30 cubic kilometers of water have been shed from the state’s San Joaquin and Sacramento drainage basins since 2003, according to measurements by twin gravity-detecting satellites known as the Gravity Recovery and Climate Experiment, or GRACE. One cubic kilometer equals about 264 billion gallons.
Drought and excessive pumping of groundwater have combined to cause the water loss, said UC Irvine civil and environmental engineering professor James Famiglietti.
“I think climate change is mixed into it a little bit,” said Famiglietti, who conducted the study with NASA scientists and presented his results at an American Geophysical Union in San Francisco Monday.
But the water loss is a product of “mostly groundwater pumping.”
“It’s a direct result of really three things,” he said. “It’s the drought, which is causing more reliance on groundwater. It’s the cutback on surface water allocations, causing more reliance on groundwater, and it’s the decreasing snowpack,” leading to less groundwater recharge, he said.
Similar measurements by Famiglietti and other scientists a few months ago revealed water loss in India of 17.7 cubic kilometers per year, from October 2002 to August 2008 — almost entirely the result of excess pumping.
Famiglietti says he plans to study more areas of the earth for water loss.
The two satellites are so sensitive to gravity that an area of greater mass on earth’s surface pulls them downward very slightly — in fact, by an amount measured in microns. As the satellites move in tandem, they take turns measuring each other’s slight movements as they pass over features on the surface.
“Water is heavy,” Famiglietti said. “So when there is some storage (of water) over big areas, that’s a huge enough perturbation” to be captured by the satellite measurements.
(Photo of parched orchards and dry fields near Coalinga, Calif., courtesy UC Davis.)