Losses Amount to the Volume of Lake Mead
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 Nevadas -- have lost nearly
enough water combined to fill Lake Mead, America's largest reservoir.
The findings, based on data from the
NASA/German Aerospace Center Gravity Recovery and Climate Experiment (Grace), reflect California's extended
drought and increased rates of groundwater being pumped for human uses, such as irrigation.
Satellite Monitoring of Groundwater
In research being presented at the December, 2009 American Geophysical Union meeting in San Francisco, scientists
from NASA and the University of California, Irvine, detailed California's groundwater changes and outlined
Grace-based research on other global aquifers. The twin Grace satellites monitor tiny month-to-month changes
in Earth's gravity field primarily caused by the movement of water in Earth's land, ocean, ice and atmosphere
reservoirs. Grace's ability to directly 'weigh' changes in water content provides new insights into how Earth's
water cycle may be changing.
Losses are Mainly in the Central Valley
Combined, California's Sacramento and San Joaquin drainage basins have shed more than 30 cubic kilometers of water
since late 2003, said professor Jay Famiglietti of the University of California, Irvine. A cubic kilometer is about
264.2 billion gallons, enough to fill 400,000 Olympic-size pools. The bulk of the loss occurred in California's
agricultural Central Valley. The Central Valley receives its irrigation from a combination of groundwater pumped
from wells and surface water diverted from elsewhere.
Groundwater Pumped at Unsustainable Rates
"Grace data reveal groundwater 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 groundwater depletion rates, Grace can help California water managers make
informed decisions about allocating water resources," said Grace Project Scientist Michael Watkins of NASA's
Jet Propulsion Laboratory, Pasadena, Calif., which manages the mission for NASA's Science Mission Directorate, Washington.
Pumping for Crop Irrigation is Loss Activity
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 suggest the Sacramento River
basin is losing about 2 cubic kilometers of water a year. Surface water losses account for half of this, while
groundwater 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. Of this, more than 75 percent is the result of groundwater pumping in the
southern Central Valley, primarily to irrigate crops.
Potential Negative Impact of Legislation
Famiglietti said recent California legislation decreasing the allocation of surface waters to the San Joaquin Basin is
likely to further increase the region's reliance on groundwater for irrigation. "This suggests the decreasing groundwater
storage trends seen by Grace will continue for the foreseeable future," he said.
The California results come just months after a team of hydrologists led by Matt Rodell of NASA's Goddard Space
Flight Center, Greenbelt, Md., found groundwater levels in northwest India have declined by 17.7 cubic kilometers per
year over the past decade, a loss due almost entirely to pumping and consumption of groundwater by humans.
Other Studies Using Grace Data
"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 to $8 billion annually," said Rodell.
Other regions under study include Australia, the Middle East - North Africa region and the southeastern United States,
where Grace clearly captured the evolution of an extended drought that ended this spring. In the Middle East - North
Africa region, Rodell is leading an effort to use Grace and other data to systematically map water- and weather-related
variables to help assess regional water resources. Rodell added Grace may also 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 the ground.
NASA is working with the National Oceanic and Atmospheric Administration and the University of Nebraska-Lincoln to
incorporate Grace data into NOAA's U.S. and North American Drought Monitors, premier tools used to minimize drought
impacts. The tools rely heavily on precipitation observations, but are limited by inadequate large-scale observations
of soil moisture and groundwater levels.
A Unique Satellite Data System
"Grace is the only satellite system that provides information on these deeper
stores of water that are key indicators of long-term drought," Rodell said.
Grace is a partnership of NASA and the German Aerospace Center (DLR). The University of Texas Center for Space Research,
Austin, has overall mission responsibility. JPL developed the satellites. DLR provided the launch, and GeoForschungsZentrum
Potsdam, Germany, operates the mission.
For more on Grace, see http://www.csr.utexas.edu/grace/ and http://grace.jpl.nasa.gov/ .
Other media contacts: Margaret Baguio, University of Texas Center for Space Research, 512-471-6922; Jennifer Fitzenberger, University
of California, Irvine, 949-824-3969.
JPL is managed for NASA by the California Institute of Technology in Pasadena.
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| Lush agriculture can exist adjacent to semiarid landscapes only because groundwater is being heavily exploited. Landsat GeoCover image by NASA. |
| Groundwater depletion is occurring in all of the groundwater basins of the Great Valley. Map by USGS. |
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