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As the climate warms, the dry southern regions of the Western United States will have less groundwater recharge while the northern regions will have more, researchers report.

“Our study asked what will be the effect of climate change on groundwater recharge in the Western US in the near future, 2021-2050, and the far future, 2070-2100,” says first author Rewati Niraula, who worked on the research as part of his doctoral work in the University of Arizona hydrology and atmospheric sciences department.

“…for those places that are already having problems, climate change is going to tighten the screws…”

The new study covers the entire US West, from the High Plains states to the Pacific coast, and provides the first detailed look at how groundwater recharge may change as the climate changes, says senior author Thomas Meixner, professor of hydrology and atmospheric sciences at the University of Arizona.

“For the southern region of the Western US there will be a reduction in groundwater recharge, and in the northern region of the Western US we will have an increase,” says Niraula, now a senior research associate at the Texas Institute of Applied Environmental Research at Tarleton State University.

Groundwater is an important source of freshwater, particularly in the West, and is often used to make up for the lack of surface water during droughts, the authors note. In many areas of the West, groundwater pumping currently exceeds the amount of groundwater recharge.

“The portions of the West that are already stretched in terms of water resources—Arizona, New Mexico, the High Plains of Texas, the southern Central Valley—for those places that are already having problems, climate change is going to tighten the screws,” Meixner says.

Niraula says, “2021 is pretty close, so we need to start acting now. At the individual level and water-manager level there are many things we can do.”

The researchers tested how future precipitation and temperature projections would interact with aspects of the land surface such as vegetation and soil type to affect groundwater recharge during two time intervals: 2021-2050 and 2071-2100.

Various changes

Although generally the dry areas are going to get drier and the wet areas will get wetter under climate change, the new research indicates that future changes in groundwater recharge are more complex.

“The future is saying there’s going to be less recharge. That doesn’t mean you drain the aquifers dry…”

“Changes in recharge don’t necessarily map onto changes in precipitation even at a very local scale,” Meixner says. “The geology and the ecology of the landscape have an effect.”

Because the various climatic regimes in the West will affect recharge differently, the team divided the West into five hydro-climatic regions: south (Texas, Oklahoma, and Kansas), southwest (Utah, Colorado, Arizona, and New Mexico), west (California and Nevada), northwest (Washington, Oregon, and Idaho), and the northern Rockies and Plains states (Montana, Wyoming, North Dakota, South Dakota, and Nebraska).

To estimate groundwater recharge for the baseline period of 1971-2000, the researchers used a model known as Variable Infiltration Capacity, or VIC. Information from VIC is available for the entire coterminous US on a grid of about 7.5 miles (12 km) on a side.

In addition to temperature and precipitation, VIC’s groundwater recharge estimates take into consideration a particular location’s land surface, vegetation and soil type. Those factors influence whether water on the landscape evaporates, runs off, or soaks into the ground and recharges the aquifer.

For projections of future temperature and precipitation during the near future (2021-2050) and the far future (2071-2100), the researchers used 11 different global climate models. The scientists then plugged the future climate estimates from each of the 11 models into the VIC model to generate projected groundwater recharge scenarios.

For each region and time period, the researchers compared the projected groundwater recharge with the recharge during 1971-2000.

For the near future, the majority of models projected that recharge will increase in the northern Rockies and Plains region. The models agreed that groundwater recharge would decrease for the west and southwest regions. For the south and northwest regions, the projections were more uncertain and decreased and increased, respectively.

The difference among the recharge projections from the 11 global change models reflects the difference in future regional precipitation that the models project, the authors write.

Deposits in the ‘bank’

This new research provides a broad picture of how climate change may alter groundwater recharge in the future, Meixner says.

“Groundwater represents a bank. We can store water from decade to decade, and arguably millennium to millennium—but when we take a withdrawal from that bank, we have to hope there are deposits making up for our withdrawal,” Meixner says. “If there aren’t deposits making up for the withdrawals, we have less water in the future to face water resource challenges with.”

Managing groundwater now and in the future is the role of management and policy, Meixner says.

“The future is saying there’s going to be less recharge. That doesn’t mean you drain the aquifers dry,” Meixner says. “Whether we drain the aquifers dry is a management decision.”

Additional coauthors of the study are from the University of Illinois; the University of Michigan; NASA Goddard Space Flight Center; the University of California; the National Center for Atmospheric Research; and the University of Arizona.

The researchers report their findings in the journal Geophysical Research Letters. The US Geological Survey’s John Wesley Powell Center and the National Science Foundation funded the research.

Source: University of Arizona