Groundwater pumping in the last century has contributed as much as 50 percent to stream flow declines in some U.S. rivers, according to new research led by a University of Arizona hydrologist.
The new study, published in Science Advances, has important implications for managing U.S. water resources. Laws regulating the use of groundwater and surface waters differ from state to state. Some Western states, Arizona among them, manage groundwater and surface water separately.
“We’re trying to figure out how that groundwater depletion has actually reshaped our hydrologic landscape,” said first author Laura Condon, a UA assistant professor of hydrology and atmospheric sciences. “What does that mean for us, and what are the lasting impacts?”
Condon said this is the first study to look at the impact of past groundwater pumping across the entire United States. Other researchers have examined how groundwater pumping affected surface waters, but at smaller scales.
Using a computer model, Condon and her co-author, Reed Maxwell of the Colorado School of Mines in Golden, determined what U.S. surface waters would have been like without significant consumptive uses and compared that with surface water changes since large-scale groundwater pumping began in the 1950s.
The scientists focused particularly on the Colorado and Mississippi River basins and looked exclusively at the effects of past groundwater pumping because those losses have already occurred.
The U.S. Geological Survey has calculated the loss of groundwater over the 20th century as 800 cubic kilometers, or 649 million acre-feet. That amount of water would cover the states of Montana, Idaho, Wyoming, Nevada, Utah, Colorado, Arizona, and New Mexico, plus most of California, with water one foot deep.
“We showed that because we’ve taken all of this water out of the subsurface, that has had really big impacts on how our land surface hydrology behaves,” she said. “We can show in our simulation that by taking out this groundwater, we have dried up lots of small streams across the U.S. because those streams would have been fed by groundwater discharge.”
Groundwater helps provide water to existing vegetation, including crops, Condon said.
Receding water tables and dwindling streams can make irrigating crops more difficult and costly. Some native vegetation including cottonwood trees will eventually die if the water table drops below their roots.
Groundwater is often the slowest component of the terrestrial hydrologic system to recover from losses, Condon said.
The scientists found that streams, lakes, and rivers in western Nebraska, western Kansas, eastern Colorado, and other parts of the High Plains have been particularly hard hit by groundwater pumping. That finding agrees with other smaller-scale studies in the region.
“With this study, we not only have been able to reconstruct the impact of historical pumping on stream depletion, but we can also use it in a predictive sense, to help sustainably manage groundwater pumping moving forward,” Maxwell said.
The team’s computer model maps the groundwater and surface waters onto a grid of squares 0.6-mile on a side that covers most of the contiguous U.S. The model, which does not include the coastal regions, includes all the groundwater down to 328 feet below the land surface.
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