The Colorado River Basin’s groundwater supplies are dwindling, thanks to a combination of both natural events and human pumping activities, a new study has found.
The critical Western system has lost about 42 million acre-feet of water storage since 2003 — with 65 percent of those declines, or 28 million acre-feet, attributable to groundwater depletion, according to the study, published on Tuesday in Geophysical Research Letters.
Over the past century alone, the authors noted, the Colorado River’s flow has plunged by about 20 percent, with climate models predicting additional reductions of up to 30 percent by mid-century, due to escalating temperatures and diminishing Rocky Mountain snowpack.
“The decline of the river poses a severe threat to both agricultural and municipal water supplies, which are heavily reliant on the river,” the researchers warned.
The Colorado River system, which serves about 40 million people in the U.S. and Mexico, is divided on this side of the border into a Lower Basin and an Upper Basin, which respectively include California, Arizona and Nevada, and Wyoming, Colorado, Utah and New Mexico.
The actual amount of water available in the Colorado River Basin varies every year, based on snowpack, other environmental conditions and usage. But historic treaties allotted annual allocations of 7.5 million acre-feet to each domestic basin, as well as 1.5 million acre-feet to Mexico.
For context, Western U.S. households tend to consume about half an acre-foot of water every year.
Harnessing satellite observations of the region’s water storage, the scientists found that groundwater depletion accounted for about 53 percent of the total water supply loss in the Upper Basin and 71 percent in the Lower Basin — far greater than the amounts lost by the system’s two largest reservoirs, Lake Powell and Lake Mead.
With much less surface water available, the demand for groundwater is expected to climb — and in the Lower Basin, this resource already accounts for 40 perent of the water supply, the researchers explained.
Yet although groundwater is seen as “a crucial buffer” in arid environments, it is also “rapidly disappearing due to excessive extraction one one hand and insufficient recharge and management on the other,” the authors added.
About 80 percent of the basin’s water is devoted to irrigation, which bolsters a $1.4 billion agricultural industry in Arizona alone, according to the study.
“This situation places immense pressure on the region’s groundwater resources,” the scientists stressed.
Looking forward, the scientists emphasized a need to identify factors that contribute to the system’s groundwater loss — with the goal of creating “sustainable water management strategies that can help secure water resources for the region’s future.”
These considerations may prove particularly important in the current moment, as the region’s states negotiate long-term operational and conservation guidelines for the 1,450-mile artery. The current interim rules, set in 2007, will expire at the end of 2026.
“As climate change intensifies and demands on the Colorado River continue to grow, the inclusion of groundwater in interstate [Colorado River Basin] water discussions has become a national imperative,” the researchers concluded.
