The Colorado River Basin’s hidden, below-ground reservoir — which spans parts of Colorado and six other states — has lost about 13 trillion gallons of water, and it’s shrinking faster than it has in the past, according to researchers at Arizona State University.
Groundwater, stored in the cracks in rock and spaces between soil and sand below our feet, is the oft-overlooked stepchild of the basin, which provides water to 40 million people around the West. Policymakers, water managers and others spend much of their time talking about the basin’s shrinking above-ground supplies — even more so since two reservoirs, Lake Mead and Lake Powell, fell to historic lows around 2022.
This Fresh Water News story is a collaboration between The Colorado Sun and Water Education Colorado. It also appears at wateredco.org.
Meanwhile underground, more than a Lake Mead-worth of water has disappeared, according to a study published May 27 in the American Geophysical Union’s peer-reviewed research journal, Advancing Earth and Space Sciences.
“We know groundwater depletion is happening. We’ve known it for a long time,” said James Famiglietti, global futures professor at Arizona State University and lead author on the recent study. “The part that surprised me is the changes that happened over the last 10 years.”
The Arizona State University researchers used satellite data, on-the-ground information and models to look at the basin’s below-ground storage between 2002 and 2024.
The groundwater loss during that period equals about 72% of the basin’s total storage capacity in federal reservoirs.
Not only is the total decline large, but the groundwater depletion also happened about three times faster over the last 10 years compared with the decade before, Famiglietti said.
The water loss is more prominent in downstream states, like Arizona, than upstream states, like Colorado, according to the study. The dynamic is part of the increasing tensions between basin states about how to cut back on water use in future dry years.
People are using it faster and at a larger scale than it can be replenished, Famiglietti said. That has to do with scant groundwater use regulation in some regions and a century of industrial-scale agriculture, he said.
The basin’s ongoing struggles with drought, rising temperatures and tightening surface water supplies make it harder to restore groundwater supplies when they are used, he said.
And some groundwater aquifers are incredibly old, nonrenewable resources, he said.
“We essentially started tapping them a hundred years ago, and we’re burning through all that,” Famiglietti said.
What about Colorado River groundwater in Colorado?
Colorado and the three other states in the Upper Basin — New Mexico, Utah and Wyoming — lost less groundwater than downstream states, the research found.
The Upper Basin’s groundwater shrank by about 11.8 million acre-feet between 2002 and 2024. One acre-foot equals 325,851 gallons, or roughly the amount of water used by two to four households in a year.
That would be like emptying all 17 reservoirs that serve Denver Water and its 1.5 million Denver-area customers — and then doing it again 16 more times.
Although western Colorado is in the Colorado River Basin, Colorado is less reliant on the basin’s groundwater supply than other states, Jason Ullmann, who directs Colorado’s Division of Water Resources, said.
At higher elevations, the groundwater supply is limited from the get-go. The Western Slope’s steep, mountainous terrain mostly creates small underground aquifers near streams, he said.
For Ullmann, the study’s findings weren’t terribly surprising: It’s been clear for a while that groundwater supplies are being used faster than they are being replenished, and much of that loss is happening in other states, like Arizona, he said.
He noted that 18% of Arizona is subject to groundwater management, according to the study. Colorado’s groundwater policies cover 100% of the state, Ullmann said.
Colorado has been managing its groundwater since the 1960s to comply with interstate water-sharing agreements and to use groundwater more sustainably, he said.
Downstream groundwater loss
In the Lower Basin — Arizona, California and Nevada — groundwater has declined by 29.2 million acre-feet.
The Lower Basin’s groundwater loss rates are highest in parts of Arizona, like the Prescott area, Hualapai Valley area, and parts of southeast Arizona near the U.S.-Mexico border. These regions use groundwater for 80% to 100% of their supply, mainly for agriculture, according to the study.
Groundwater use and depletion is a hot-button issue in Arizona.
The city of Phoenix is very slowly sinking into the ground because of groundwater depletion, according to a study published in early May.
Arizona is also the first to face water cuts when surface water supplies fall too low under Colorado River water law. When water from the Colorado River isn’t enough, many Arizona communities would increasingly rely on their below-ground water storage, Famiglietti said.
For Famiglietti, the big takeaway was that the Colorado River Basin uses more groundwater than above-ground supplies, but the groundwater is way less protected.
“That’s huge. People need to understand that,” he said.
Famiglietti’s main hope for the research study? The governor’s office here in Arizona uses it as a scientific justification for the need to expand groundwater protection.
“(We’re) just trying to raise awareness that you can only do this so many times before you run out, and therefore we need to protect it,” Famiglietti said.
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