A new study from the University of Arizona may have important implications for managing water resources. Groundwater pumping over the last century, the researchers say, has contributed as much as 50 percent to stream flow declines in some U.S. rivers.

How has this happened? First, water supplies in the ground—which occur in layers called aquifers—have been tapped for crop irrigation, bottled water production, municipal drinking water, and hydraulic fracking for petroleum products, on a very large scale, for many years.

Using a computer model, UA Assistant Professor and lead author Laura Condon and her co-author, Reed Maxwell of the Colorado School of Mines, figured out 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 primarily focused on the Colorado and Mississippi River basins and looked exclusively at the effects of past groundwater pumping, since those losses have already occurred.

But these scientists aren’t alone in realizing the vast scale of water usage over the past century.

The U.S. Geological Survey has determined that the loss of groundwater over the 20th century was 800 cubic kilometers, or 649 million acre-feet. That would cover Montana, Idaho, Wyoming, Utah, Nevada, Colorado, Arizona, and New Mexico, plus most of California, with a foot of water.

“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,” Condon 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.”

The regions most sensitive to a lowering of the water table are areas east of the Rocky Mountains where the water table used to be shallow, at depts of 6 to 33 feet. In those regions, the fates of groundwater and surface water are very closely linked, and the reduction of available water at a shallower depth is more disruptive to vegetation, streams, and rivers.

Groundwater also provides water for the crops we rely on for food. With receding water tables and dwindling streams, crop irrigation is becoming increasingly difficult and costly.

This is a problem because parts of the Midwest where the amount of precipitation used to equal the amount of evaporation of water are becoming more arid due to climate change. Groundwater pumping is being used in the Midwest on a larger scale than before, and that has reduced surface waters in the region. Streams, lakes, and rivers in western Nebraska, western Kansas, eastern Colorado, and other parts of the High Plains have been particularly harmed by large-scale pumping.

“In the West, we worry about water availability a lot and have many systems in place for handling and managing water shortage,” Condon said. “As you move East, where things are more humid, we don’t have as many systems in place.”

Do you live in an area where there’s a lot of groundwater pumping? Have you seen the impacts of it in your local ecosystem and environment? Sound off in the comments!

Photo: A pivot irrigation system in a large crop field. Pivot systems rely on groundwater pumping in order to function. Credit: Shutterstock