There may be a wetter future ahead for the Colorado River, new CU Boulder research says

New climate research offers a dose of optimism for the overstressed Colorado River: Wetter conditions may be on the horizon.

Past studies found that 2000 to 2021 marked the driest period in the Colorado River Basin in the last 1,200 years. But that is likely to change, according to recent research from the University of Colorado and the University of Hawaii. Precipitation in the basin naturally rises and falls, and it may be due for an upswing — good news for a river system that provides for 40 million people whose reservoirs sit at historic lows.

At the same time, the research also indicates that there’s a small chance that conditions could get worse. Officials, the researchers said, need to prepare for both outcomes as they finalize new rules to manage the river for years to come.

“Sometimes science has the first word in a process that is political. It rarely has the last word, so I don’t have any false expectations,” said Martin Hoerling, a climate scientist on CU’s Boulder campus  and the lead author on the study. “But I think what’s important to recognize here is that the future doesn’t have to be seen as a downhill-only prospect.”

Precipitation is the main driver of what happens in the Colorado River Basin, according to the research published in the Journal of Climate in April.

Most of the water, about 85%, that feeds the Colorado River begins as snow in the basin’s headwaters in mountains above 10,000 feet in Colorado and Wyoming. The Colorado River and its tributaries then flows through cities, farms, industries and environments across the western U.S. before ending in Mexico.

Over the past two decades, precipitation in the basin has been less than average — part of a long pattern of peaks and valleys in the river’s history. Drawing from that history and new climate models, the researchers found that there’s a 70% chance the river system will see an increase in precipitation compared to the last two decades.

That’s good news for the river’s flows, which are closely linked to the amount of water falling into the basin, Hoerling said.

From 2000 to now, about 12.5 million acre-feet of water per year flowed down the Colorado River, well below the 14.5 million acre-feet that was once its norm.

But the new research indicates that those flows could rise to 13.5 million or 14 million acre-feet on average between 2026 and 2050. That’s significantly higher than the slight possibility that precipitation will fall, and the river’s flows could drop to 10 million acre-feet on average.

One acre-foot roughly equals the annual water use of two to three households.

“So much of the conversation has been that there’s almost a foregone conclusion that we’re only going to see less water in the Colorado (River),” said Nanette Hosenfeld, senior hydrometeorologist at the Colorado Basin River Forecast Center. “I think this [study] just does a good job of raising awareness that that’s not necessarily true.”

A slim chance of devastating conditions 

Rising temperatures will, however, undercut some of the gains from increased precipitation.

The region has already warmed by about 1.8 degrees, and climate experts expect temperatures to keep rising. Warming depletes water in the basin, dries out soils, stresses plants and allows more water vapor to be stored in the atmosphere.

There is also a small chance that precipitation could decrease. About 4% of the samples indicate that the flow in the river could be as low as 10 million acre-feet in the next quarter century.

“That’s the bad news. That would be devastating,” Hoerling said. “The good news is, the chances are better than even that we’re going to have a bounce back from where we were the last 20 years.”

An average of 10 million acre-feet per year means the river’s flow could be as low as 6 million acre-feet in some years.

The basin’s water users need to be prepared for both wetter wets and drier dries, said Eric Kuhn, one of the study’s authors and former general manager of the Colorado River District.

“The range of what we need to be prepared for, and what might be reasonable, is now enlarged,” he said.

Planning for all outcomes

The Bureau of Reclamation is analyzing proposed guidelines for storing and releasing water from the basin’s reservoirs after 2026, when the current rules expire.

Basin states released competing proposals in March after tense negotiations stalled earlier this year. Tribal nations, environmental groups and academics also shared their own ideas. Reclamation is expected to release options for managing the reservoirs in December.

Under the Upper Basin proposal, if reservoir levels fall below certain points, then the Lower Basin would have to start conserving water with maximum cuts of 3.9 million acre-feet.

Under the Lower Basin’s proposal, all seven states would take a painful hit in low-storage years. One way to split the shortages is for the Upper Basin to cut its use by 1.2 million acre-feet and for the Lower Basin to cut by 2.7 million acre-feet.

In 2022, the Upper Basin used 4 million acre-feet of water, and the Lower Basin used 10.1 million acre-feet, according to the Colorado Water Conservation Board, a state water policy agency.

“I don’t know how the law of the river survives in its current form [with] a 10 million acre-foot river,” Kuhn said. “It appears to me that the negotiations, or the lack of negotiations between the basins, that the next level of cuts is really difficult.”

While the study outlines a more optimistic future, the possibility of worsening conditions are important to keep in mind going forward, according to Peter Goble, a climatologist for the Colorado Climate Center at Colorado State University.

It would be a gambler’s fallacy to assume that, since the conditions have been dry, the basin is “due” for a wet period, especially considering rising temperatures, Goble said. That means continued conservation efforts and remembering the lessons of the past two decades is important.

“Losing our focus on conservation is a mistake when we look at the whole range of possible outcomes and consider the fact that even in light of this study, there’s a chance that things continue to get worse,” Goble said. “I hope that doesn’t happen.”

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