Even under modest climate warming scenarios, the continental United States faces a "significant loss of groundwater," a new study says.

The estimated groundwater loss on the low side could be about 119 million cubic meters, or roughly enough to fill Lake Powell in Arizona four times or one quarter of Lake Erie's volume.

Estimated groundwater depletion in the United States during 1900 through 2008 totals approximately 1,000 cubic kilometers (km3), according to the U.S. Geological Survey. The rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000 to 2008) when the depletion rate averaged almost 25 km3 per year (compared to 9.2 km3 per year averaged over the 1900 to 2008 timeframe). Source: USGSThe results were published in Nature Communications, and show that as warming temperatures shift the balance between water supply and demand, shallow groundwater storage will increasingly be unable to buffer plant water stress.

As warming persists, underground water storage can be depleted at the expense of connections between surface rivers and streams and underground water reservoirs.

Losing groundwater

“Even with a 1.5 degrees Celsius warming case, we’re likely to lose a lot of groundwater,” said Reed Maxwell, director of the Integrated GroundWater Modeling Center at the Colorado School of Mines. He was co-author of the paper, along with Laura Condon of the University of Arizona and Adam Atchley of the Energy Department's Los Alamos National Laboratory.

Estimated groundwater depletion in the United States during 1900 through 2008 totals approximately 1,000 cubic kilometers (km³), according to the U.S. Geological Survey. Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000 to 2008) when the depletion rate averaged almost 25 km³ per year (compared to 9.2 km³ per year averaged over the 1900 to 2008 timeframe).

Laura Condon, University of ArizonaThe trio of researchers writing in Nature Communications said that most global water circulation models do not take into account the lateral movement of water in the subsurface. Typically, those circulation models include only a limited up-and-down movement, such as rain percolating from vegetation into the soil and roots pulling up water from the ground. In addition, these models tend to limit their scope to a few meters above or below ground.

The new study simulates how water moves in the subsurface and connects with the land surface. The calculations revealed a direct response of shallow groundwater storage to warming. The researchers said that this response demonstrates the "strong and early effect" that low to moderate warming may have on groundwater storage and evapotranspiration.

In the western U.S., changes in groundwater storage may remain masked for a long time. That is because the groundwater there is already deep and falling levels would not have as great an effect on surface waters. In addition, the region's vegetation is already largely water limited and adapted to being disconnected from deep groundwater sources.

Greater sensitivity

By contrast, the eastern U.S. will likely be much more sensitive to a lowering of the water table, the researchers said. Groundwater and surface water in the east are more closely linked, and depleting

Reed Maxwell, Colorado School of Minesthe groundwater will be more disruptive to vegetation, streams and rivers. What's more, many of the systems that have been put in place in the western U.S. for handling and managing water shortage are lacking in the eastern part of the country.

The study revealed that regions in the eastern U.S. may reach a tipping point sooner rather than later, when vegetation starts to lose access to shallow groundwater as storage is depleted with warming.

Adam Atchley, Los Alamos National Laboratory"Initially, plants might not be experiencing stress because they still have existing shallow groundwater available, but as we continue to have warmer conditions, they can compensate less and less, and changes are more dramatic each year," Condon said. Shallow groundwater appears to be buffering the response to warming, but when it is depleted, it cannot do that anymore, she said.

The study’s simulations were set up to keep precipitation patterns the same and only increase atmospheric temperatures according to projections ranging from 1.5° C to 4° C. Even with 1.5° C of warming, 119 million cubic meters of storage were lost from groundwater. At 4° C, groundwater losses were projected at 324 million cubic meters.

“We are facing a crisis in global groundwater storage," Condon said. “Huge groundwater reservoirs are drying up at an alarming rate, and that's a problem because they nourish major growing regions around the world.”