Global warming is drying up the Colorado River, UCLA says

When it comes to how much water is actually running down the Colorado River to quench seven Western states, a new study makes clear that “low” is the new “average.”

Higher temperatures caused by climate change stole 10 trillion gallons of water out of the Colorado River Basin from 2000 to 2021, robbing vital resources from 40 million people across the southwestern U.S., according to a stark new study from UCLA hydrologists. The decline will put added pressure on ongoing negotiations about how to share the river’s limited resources.

Not only that, but it is part of a long-term drying trend in the basin, which is even worse in snowpack areas like Colorado’s mountains, according to the study by Benjamin Bass and other UCLA hydrology experts published in the AGU journal “Water Resources Research.”

“The fact that warming removed as much water from the basin as the size of Lake Mead itself during the recent megadrought is a wake-up call to the climate change impacts we are living today,” Bass told AGU, also known as the American Geophysical Union.

Previous research shows the current drought in the river basin is the worst in the 1,200 years of history scientists can track through tree ring studies and other methods.

The modeling by the UCLA researchers shows 2.7 degrees Fahrenheit in warming since 1880, at the beginning of the fossil fuel industrial age. The model also accounts for how much changes in vegetation from higher temperatures and growing carbon dioxide might affect snowpack runoff into the basin’s rivers.

When changes in plant absorption are factored in, UCLA concludes rising temperatures took 10.3% out of expected runoff.

Areas of the basin that are “usually snow-covered in winter are now losing water about twice as fast as typically snowless regions,” AGU said in summarizing the study. “The rapid water loss in snowpack regions is a sign that the Rocky Mountain West is transitioning to a more arid climate rather than simply undergoing periodic droughts.”

The results will add pressure to ongoing negotiations among the seven Colorado River Basin states over how much to cut water use in order to match the new realities of supply.

The study makes it clearer than ever that when river compacts were negotiated as far back as the 1920s, agencies overestimated how much water would be available in an average year. The 1922 Colorado River Compact, for example, split the river’s supply equally between the upper and lower halves of the basin, allotting 7.5 million acre-feet to each. Later agreements tacked on more water for Mexico.

It should also add urgency to the recently launched meetings of Colorado’s own new drought task force.

“The original compact’s 15 million acre-feet annual delivery estimate, which is based on a stationary climate, is likely not sustainable under present-day conditions,” Bass said, in email responses to Colorado Sun questions. The compact gives the Upper and Lower Basins 7.5 million acre feet a year from the Colorado, but heat and long term drought may have lowered the available water to only 10 to 12 million acre feet a year.

The Colorado River Basin feeds major cities from Denver, via intermountain transfers; Las Vegas, Phoenix and Los Angeles, as well as lucrative agricultural interests and 30 Native American tribes. The headwaters rise near Rocky Mountain National Park, and Colorado supplies most of the snowpack that serves the entire length of the river.

The river basin has also been in a prolonged drought since 2000, with a more intense, short-term drought in 2020 and 2021. In 2021, conditions were so bad that the Bureau of Reclamation declared a water shortage for the first time in the basin’s history.

This year, federal, state and tribal officials launched a new round of negotiations focused on how to share the river’s water in times of shortage over the long term. These negotiations, along with more near-term planning for 2023 through 2026, assess water storage and releases from Lake Mead and Lake Powell, the basin’s two largest water storage reservoirs.

Over the past decade, researchers have been studying the river’s shrinking supply with renewed vigor, and past studies have said the river’s flow could shrink by anywhere from 2% to 12% for each additional 1.8 degrees Fahrenheit of warming.

“This study comes to a conclusion that is right about in the middle of that range. That’s, in some ways, kind of a convenient finding for planning purposes,” said Peter Goble, a climatologist with the Colorado Climate Center at Colorado State University who was not involved in the UCLA study.

While temperature trends fluctuate naturally, researchers also link warming temperatures since 1880 to human-caused actions, like releasing higher levels of the greenhouse gas carbon dioxide into the atmosphere.

And these increased temperatures have interrupted the Colorado River’s entire water cycle. Not only can hotter conditions impact precipitation patterns, but they also change soil moisture and how plants use water to grow.

The role of plants was a particularly novel part of the study, Goble said. The UCLA researchers found that plants lessen the outsize impacts of warming temperatures on the river basin’s water supply.

Plants can use water more efficiently when the amount of carbon dioxide in the atmosphere is at higher concentrations. So while warming alone decreases the river’s supply by 8.1% per degree Celsius of warming, plants soften the blow: When their role is considered, the river’s supply drops by 6.8% per degree Celsius of warming.

The end result, however, is still a decrease in runoff every decade, according to the study.

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Climate scientists say the basin’s changing climate is leading to aridification, a long-term, increasing dryness rather than a seasonal variation. And this drying pattern is more pronounced in snowpack regions, where runoff is dropping at double the rate of the decline in nonsnowpack regions.

“Despite only making up (about) 30% of the basin’s drainage area, 86% of runoff decreases in the Colorado Basin is driven by water loss in snowpack regions,” the researchers say in the study.

Because of warming, more precipitation is falling as rain, rather than snow, in snowpack regions like the Western Slope of Colorado.

There’s another big factor: how much heat snow absorbs. The whiteness of snow helps reflect solar radiation, but when the layer of snow isn’t as thick — or when dust blows onto it — its surface can absorb more heat. In April, a widespread dust event left fields of thick snowpack with a layer of dirty, brown dust and helped shorten the snowmelt by about a month, according to experts.

Earlier snowmelt means there is less snow in mid- and late-summer to feed the basin’s rivers, streams and croplands.

“A high fraction of the runoff in Colorado, especially Western Colorado, comes from our annual snowpack accumulations. That’s the primary source of our water,” Goble said. “Anything that disproportionately impacts snowpack-fed watersheds disproportionately affects us here.”

In Colorado, the water supply typically begins to suffer most after multiple, consecutive years of low snowpack, Goble said.

“Because our snowpack was so much above average in 2023, we were able to see the best recovery in our state’s water supply since at least 2019,” he said.

But there are still concerns, he said. The summer monsoon season typically starts in mid-July and lasts through late August or early September. This year, it hasn’t delivered very much water to southern and western Colorado. For example, the Durango-La Plata County Airport received about 0.08 inches of rain in July, while other areas of the state saw record rainfall.

After a wet spring with lots of plant growth, the dry period could cause plants to lose moisture and increase wildfire risk, Goble said. And although Colorado’s reservoirs have recovered, the Colorado River Basin’s largest reservoirs, lakes Mead and Powell, would need consecutive years like 2023 to recover.

“The likelihood of those occurring back-to-back is low,” Goble said. “We’re still definitely concerned about the water system as a whole, especially long term, given how we know warmer temps affect the entire river system.”

Shannon Mullane writes about Western water issues for The Colorado Sun and her work is funded by a grant from the Catena Foundation. She focuses on the Colorado River Basin, tribal affairs related to water, and West Slope water issues.Born in East Tennessee, Shannon has been in... More by Shannon Mullane

Michael Booth is a Colorado Sun reporter covering health, health policy and the environment.Email: [email protected] Twitter: @MBoothDenver More by Michael Booth