Irrigation draining California groundwater at ‘unsustainable’ pace

The GRACE satellites have tracked water movement from the Central Valley since 2003

SAN FRANCISCO — In the past six years, the irrigation of crops in California’s Central Valley has pulled groundwater from aquifers there at rates that are unsustainable if current trends continue, scientists say.

The Central Valley, which covers about 52,000 square kilometers, is one of the world’s most productive agricultural regions, says Jay Famiglietti, director of the University of California Center for Hydrologic Modeling in Irvine. In 2002, farmers there produced more than 250 different crops worth a total of around $17 billion — an amount that adds up to around one-twelfth of the nation’s agricultural production, he notes.

But the productivity of those fertile fields is increasingly at risk: Satellite data suggest that more than 20 cubic kilometers of groundwater has been pumped from the valley’s aquifers since October 2003, Famiglietti reported December 14 at the fall meeting of the American Geophysical Union. That’s roughly 4 percent the volume of Lake Erie.

Famiglietti and his colleagues analyzed data gathered by the twin satellites of the GRACE mission, which can discern and measure the movements of water both above and below the ground, on a month-to-month basis (SN: 1/4/03, p. 6). Between October 2003 and March 2009, the San Joaquin and Sacramento River basins — the watersheds that include the Central Valley — together lost more than 31 cubic kilometers of water, the data suggest. About one-third of that net loss evaporated from the soil or flowed out to sea after melting from the region’s snowpack or being pulled from surface reservoirs in those watersheds.

The rest, about 20.3 cubic kilometers, drained away after being pulled from underground aquifers for irrigation, the researchers speculate.

On average, water tables across the region dropped about 24 centimeters per year during the 66-month period the researchers studied. But most of the water loss occurred in the San Joaquin River basin, so water tables there probably dropped an average of about 50 centimeters each year.

Because central California has been afflicted by drought conditions since 2006, state and local governments have imposed restrictions on how much water can be withdrawn from surface reservoirs. Those restrictions, in turn, have triggered an even greater reliance on groundwater withdrawals, just at a time when the precipitation needed to recharge the region’s aquifers is in short supply, says Famiglietti.

The satellites can detect changes in the amount of water in a region but not how much is left. Regardless of how much water remains in the aquifer, the researchers note that a declining water table will degrade water quality and will eventually force Californians to drill deeper wells. In the long term, continued depletions of groundwater in the region could pose a significant threat to U.S. food production and to the California economy, the researchers contend.

“By providing data on large-scale groundwater depletion rates, GRACE can help California water managers make informed decisions about allocating water resources,” says Michael Watkins, a project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

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