The Upper Colorado River Basin lost groundwater at a notably faster rate than the Lower Basin during a recent period of extreme drought, despite expectations that its snow-fed water system would make it more resilient, according to a new study.
Researchers said the Virginia Tech-led study demonstrates the need to manage supplies above and below ground together, as prolonged drought places increasing pressure on the river system.
The Colorado River is a vital resource for the West, stretching 1,400 miles and supplying water to tens of millions of people across seven states: Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming. It also irrigates millions of acres of farmland and supports hydropower generation, recreation, fish and wildlife habitat.
But the river has been strained by a multidecadal drought, with its two largest reservoirs, Lake Mead and Lake Powell, falling to record lows this summer.
...
Upper Basin Suffers Steeper Groundwater Decline
The researchers used satellite and hydrological data to track changes in groundwater in the basin between water year 2004 and 2023. Water experts and hydrologists use October as the start of the water year as this is when precipitation transitions from rain to mountain snow and snowpack begins to build up, according to the Water Education Foundation.
Their findings, published in Water Resources Research, suggested that during the Colorado River Basin’s recent “extreme dry period from water year 2020-2022” groundwater storage experienced a sharper decline in the Upper Basin than the Lower Basin.
“We think the most interesting results or interpretations are that [Lower Basin groundwater] storage decline seems less coupled to surface water than in the Upper Basin,” Virginia Tech assistant professor of hydrology and remote sensing Susanna Werth told Newsweek.
The researchers said groundwater losses in the Upper Basin closely mirrored water shortages on the surface—pointing to a strong connection between both systems. In the drier Lower Basin, on the other hand, groundwater storage depleted before surface levels dropped, suggesting human-driven water demand could be the “dominant stressor.”
Additionally, the Upper Basin lost approximately 4.6 trillion gallons of groundwater over this period—almost twice as much as in the Lower Basin, which saw an estimated loss of 2.5 trillion gallons, according to Virginia Tech.
"We generally think of the Upper Basin as more resilient because of its snowmelt-driven recharge,” lead author Esther Oyedele, a geosciences graduate student at Virginia Tech, said in a release from the university. "I did not expect to see so much groundwater loss there during the recent drought. What surprised me even more was that groundwater did not track surface-water shortages in the same way in the Upper and Lower Basins.”
Werth said in the release: "Continued groundwater depletion, especially in the Upper Basin, could further reduce groundwater contributions to rivers and intensify the feedback between groundwater loss and surface-water shortages.”
Werth added that the study shows that water levels both above and below ground must be managed in order to protect the Colorado River Basin.
Contact Newsweek editor on this story: Edward Pearcey.