Key farming regions in the US are drawing water from underground sources at unsustainable rates, with slightly more than one-third of the southern Great Plains at risk of tapping out its sources within the next 30 years.

Those are among the conclusions of a study of the nation's two major aquifers – one underlying the high plains, the other beneath California's Central Valley – published this week in the Proceedings of the National Academy of Sciences.

Concerns over the loss of groundwater in these areas aren't new. But the researchers say the tools they've used build a detailed picture of these critical water sources – how the amount of water they contain varies with time, location, and regional climate patterns – could allow for more nuanced approached to local water management.

Other water specialists suggest that the ability to combine detailed well measurements with satellite data could open the door to developing regionwide, even multistate planning for groundwater use – an approach that currently is applied to surface water.

Moreover, careful tracking of aquifers is likely to become more critical as global warming's effects become more pronounced, particularly during the second half of this century, the team suggests.

A number of recent studies have highlighted the problem of groundwater depletion globally for irrigation, notes Bridget Scanlon, who heads the Sustainable Water Resources Program at the University of Texas at Austin and was the study's lead author.

“We wanted to look in more detail at the two areas where there has been the most groundwater depletion in the US and try to better understand what is going on so that we could see if its possible to manage them more sustainably or reduce the depletion,” she says.

Ogallala aquifer

Some of the biggest surprises involved the Ogallala aquifer, a resource that stretches north along the Texas-New Mexico border through the Oklahoma panhandle and western Kansas to extend through virtually all of Nebraska and into eastern Wyoming.

Farming in the high plains contributes about $35 billion a year to the economy. Conventional wisdom has held that from north to south, the aquifer represents “fossil” water from the melting of the continent's glaciers at the end of the last Ice Age. And in the central and southern high plains, that remains true. This is where groundwater losses have been most pronounced.

About 4 percent of the land area above the aquifer, which falls in parts of Kansas and Texas, is responsible for about a third of its water losses, the team estimated.

But the researchers found that in the northern high plains, groundwater levels either have been holding relatively steady or have increased between 1950 and 2007 – fed by rain and seepage from lakes that come and go with the rainy season and snow melt. Some water also finds its way underground from the Platte, Republican, and Arkansas Rivers. The water percolates through soils that are coarser than soils in the central and southern high plains.

Overall, however, the Ogallala Aquifer has lost an average of 5.3 cubic kilometers of water a year between the 1950s and 2007 – a rate that increased to an average of 7 cubic kilometers of water between 1987 and 2007.

For the southern high plains in particular, if consumption continues into the future as it did between 1997 and 2007, the aquifer there will be unable to support irrigation for about 35 percent of the region within the next 30 years, the researchers estimate.

California's Central Valley

In California, the aquifer that runs the length of the Central Valley is fed more by runoff from melting mountain snow than directly from rainfall over the valley. Like the Ogallala, the Central Valley's aquifer shows a north-south divide, with most of the aquifer's depletion centering on its southern end, the Tulare Basin.

Unlike the Ogallala aquifer, however, the Central Valley undergoes wide swings in water loss because growers draw most heavily on groundwater during droughts. Otherwise, they rely largely on water from the valley's rivers.

The draw on water from the Central Valley can range from 24.6 cubic kilometers during a severe one-year drought to nearly 50 cubic kilometers a year for multiyear droughts.

Although the aquifer recovers during periods of heavy snows or winter storms bringing rain into the valley, long-term depletion remains its story as well.

The team derived its estimates from water-level logs kept at thousands of wells in each of the two regions, as well as from data gathered by NASA's GRACE satellite. GRACE makes high-precision measurements of changes in Earth's gravity as the satellite orbits overhead. Gravity depends on the amount of mass beneath the craft at any point in its orbit. Among the features that can change the amount of mass Grace detects are increases or decreases in groundwater.

Where GRACE can provide a basinwide narrative of groundwater changes, Dr. Scanlon says, the well data allow for the detailed assessments of how depletion and recharge vary with location.

Patchwork policies

For Juliet Christian-Smith, who studies water issues at the Pacific Institute in San Francisco, the new study highlights a growing gap between scientific observations of the vanishing resource and policies that foster wiser use.

“We have the science,” she says, “but we don't have a real conversation yet about the management and policy framework” needed to strive for sustainable use of the water.

California has a statewide management plan for surface water, she says, but not for groundwater. It has voluntary guidelines.

In the high plains, where the aquifer spans several states, individual water districts in each state may have different approaches to management, notes Ken Rainwater, director of the Water Resources Center at Texas Tech University in Lubbock. Some districts have set up mandatory conservation schemes; others take voluntary guidelines.

These agencies are straddling the divide between private ownership of land, and by extension the water under it, and a growing need to manage what often turns out to be a shared resource.

Dr. Christian-Smith cites the Central Valley's King Basin as an example. The first step in any management system is to gather information on water depletion and map the aquifer's distribution. When the agency did, it found that the aquifer was connected to another aquifer that the city of Fresno draws on for some of its water. The city's drawdown was pulling water away from farmlands elsewhere in the basin.

For the high plains, efforts may depend on how willing landowners are to cede decisions to an outside regulatory agency.

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Just as with oil producers do with petroleum, landowners growing irrigated crops can get at a tax break from the federal government to help offset the effect lower groundwater levels have on their property value, Dr. Rainwater explains.

If multistate compacts, similar to those governing some surface waters, are to emerge, these are some of the issues with which the compact's architects will have to grapple.