New space observations reveal that since October 2003, the aquifers for California’s primary agricultural region – the Central Valley – and its major mountain water source – the Sierra Nevada – have lost nearly enough water combined to fill Lake Mead, America’s largest reservoir. The findings, based on satellite data, reflect California’s extended drought and increased pumping of ground water for human uses, such as irrigation.
At the recent American Geophysical Union meeting in San
Francisco, University of California Irvine (UCI) and NASA scientists detailed
the state’s ground water changes, and outlined research on other global
aquifers conducted via twin satellites, called the Gravity Recovery and Climate
Experiment (GRACE). GRACE monitors tiny month-to-month differences in Earth’s
gravity field primarily caused by the movement of water in the planet’s land,
ocean, ice and atmosphere. Its ability to “weigh” changes in water content
provides new insights into how climate change is affecting Earth’s water cycle.
Combined, California’s Sacramento and San Joaquin drainage
basins have shed more than 30 cubic kilometers of water since late 2003, noted
Jay Famiglietti, UCI Earth system science professor and director of the UC
Center for Hydrologic Modeling. A cubic kilometer is about 264.2 billion
gallons, enough to fill 400,000 Olympic-size pools. The bulk of the loss
occurred in the state’s agricultural Central Valley. The Central Valley depends
on irrigation from both ground water wells and diverted surface water.
“GRACE data reveal ground water in these basins is being
pumped for irrigation at rates that are not sustainable if current trends
continue,” Famiglietti said. “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.”
“By providing data on large-scale ground water depletion
rates, GRACE can help California water managers make informed decisions about
allocating water resources,” said project scientist Michael Watkins of NASA’s
Jet Propulsion Laboratory.
Preliminary studies show most of the water loss is coming
from the more southerly located San Joaquin basin, which gets less
precipitation than the Sacramento River basin farther north. Initial results
indicate the Sacramento River basin is losing about 2 cubic kilometers of water
a year. Surface water losses account for half of this, while ground water
losses in the northern Central Valley add another 0.6 cubic kilometers
annually. The San Joaquin basin is losing 3.5 cubic kilometers a year. More than
75 percent of this is due to ground water pumping in the southern Central
Valley, primarily to irrigate crops.
Famiglietti said recent California legislation decreasing
the allocation of surface water to the San Joaquin basin is likely to further
increase the region’s reliance on ground water for irrigation. “This suggests
the decreasing ground water storage trends seen by GRACE will continue for the
foreseeable future,” he said.
The California results come just months after Matt Rodell of
NASA’s Goddard Space Flight Center, Isabella Velicogna of UCI, and Famiglietti
found ground water levels in northwest India declining by 17.7 cubic kilometers
per year between October 2002 and August 2008, a loss attributed almost
entirely to pumping and consumption of ground water by humans.
“California and India are just two of many regions around
the world where GRACE data are being used to study droughts, which can have
devastating impacts on societies, and cost the U.S. economy $6 billion to $8
billion annually,” said Rodell.
regions under study include the southeastern U.S., where GRACE clearly captured
the evolution of an extended drought that ended this spring; Australia; and the
Middle East-North Africa region. There, Rodell is leading an effort to assess
regional water resources by using GRACE and other data to systematically map
water and weather-related variables. He said that GRACE also may help predict
droughts, since it can identify pre-existing conditions favorable to the start
of a drought, such as a deficit of water deep below ground.
Satellites Reveal Significant Ground Water Loss in California
December 17, 2009