PHOENIX – Scientists have documented how climate change is sapping the Colorado River, and new research shows the river is so sensitive to warming that it could lose about one-fourth of its flow by 2050 as temperatures continue to climb.

Scientists with the U.S. Geological Survey found that the loss of snowpack due to higher temperatures plays a major role in driving the trend of the river’s dwindling flow. They estimated that warmer temperatures were behind about half of the 16% decline in the river’s flow during the stretch of drought years from 2000-2017, a drop that has forced Western states to adopt plans to boost the Colorado’s water-starved reservoirs.

Without changes in precipitation, the researchers said, for each additional 1.8 degrees of warming, the Colorado River’s average flow is likely to drop by about 9%.

The USGS scientists considered two scenarios of climate change. In one, warmer temperatures by 2050 would reduce the amount of water flowing in the river by 14-26%. In the other scenario, warming would take away 19-31% of the river’s flow.

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“Either of the scenarios leads to a substantial decrease in flow,” said Chris Milly, a senior research scientist with USGS. “And the scenario with higher greenhouse-gas concentrations decreases the flow more than the scenario with lower greenhouse gas concentrations.”

The findings, which were published Thursday in the journal Science, refine previous estimates and indicate the impacts of warming will likely be on the high end of what other scientists calculated in previous research.

The research has major implications for how water is managed along the Colorado River, which provides water for about 40 million people and more than 5 million acres of farmland from Wyoming to Southern California.

Snow provides a 'protective shield'

Looking at trends over the past century, the researchers examined recorded measurements from 1913-2017 and found the average temperature across the Upper Colorado River Basin increased by 2.5 and the river’s flow decreased by about 20%.

They estimated that more than half of this lost flow was attributable to higher temperatures. That equates to a loss of roughly 1.5 million acre-feet of water per year, which is more than half of the annual water allotment for the entire state of Arizona.

In other previous studies, estimates of potential future declines in the river’s flow — leaving aside any variations in precipitation — ranged from 2% to 15% for each 1 degree C of warming.

Milly and fellow USGS scientist Krista Dunne zeroed in on their estimate by pinpointing the reflectivity of snow, known as albedo, as a key element in the river’s sensitivity to warming.

They used measurements of albedo across the Upper Colorado River Basin recorded over decades by instruments called MODIS (short for Moderate Resolution Imaging Spectroradiometer), which orbit the Earth aboard two NASA satellites.

Milly and Dunne focused on the role of snow cover as a “protective shield” for water in the river basin.

Milly likened the flowing river to the leftovers of the “meal” of snow and rain that falls across the basin, after evaporation has “eaten” its share.

“The more that's consumed by evaporation, the less that's left for the river and the people downstream,” Milly said.

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And the amount consumed by evaporation is driven by how much energy the basin absorbs in the form of sunlight. The snow cover in the Rocky Mountains reflects back to the sky and space a significant fraction of the sunlight.

As the world gets hotter with the burning of fossil fuels, more of the precipitation falls as rain instead of snow. And the snow melts away earlier in the year. As the snow cover in the mountains is progressively lost, the river basin absorbs more energy.

“This drives more evaporation. And that makes for a smaller plate of leftovers for the river and its users,” Milly said.

Research suggests 'the loss is very high'

The researchers also looked at whether possible future increases in precipitation could counteract the trend. They concluded that changes in precipitation might moderate the effects brought on by warmer temperatures, but likely won’t be enough to fully counter the “temperature-induced drying.”

“Increasing risk of severe water shortages is expected,” they said in the study.

Brad Udall, a water and climate scientist at Colorado State University, said the research confirms the findings of a 2018 study that he co-authored with scientists at the University of California, Los Angeles, in which they also found that about half of the loss in river flow since 2000 has been due to higher temperatures.

“It shows very high temperature sensitivity for the Colorado River — for every 1C rise, you lose almost 10% of flow,” Udall said in an email.

The researchers’ latest estimate of temperature-driven flow losses is at the upper end of what Udall and fellow climate scientist Jonathan Overpeck suggested in a 2017 study, when they estimated declines of 3% to 10% for every degree of warming.

“This new paper suggests the loss is very high,” Udall said. “This has important implications for water users and managers alike.”

In their 2017 study, Udall and Overpeck used climate models to estimate a business-as-usual scenario of greenhouse gas emissions. They projected that without changes in precipitation, warming will likely cause the Colorado River’s flow to decrease by 35% or more this century.

For decades, the Colorado River has been so heavily used that it seldom reaches the sea. Its delta in Mexico has shriveled, leaving only small wetlands in a dusty stretch of desert.

Difficult talks ahead on long-term plans

The river’s largest reservoirs have dropped dramatically since 2000. Two decades of mostly dry years and overuse have taken a toll, and rising temperatures have added to the strains.

Lake Powell now sits 50% full, and Lake Mead is 43% full.

Arizona, Nevada and Mexico have begun taking less water from the river this year under a set of agreements aimed at reducing the risk of the reservoirs falling to critically low levels.

The two states agreed to leave a portion of their water allotments in Lake Mead under a deal with California called the Lower Basin Drought Contingency Plan, which the states’ representatives signed at Hoover Dam in May.

California agreed to contribute water at a lower trigger point if reservoir levels continue to fall. And Mexico agreed under a separate accord to take steps to help prop up Lake Mead, the nation’s largest reservoir near Las Vegas.

The drought contingency plans — one for the three Lower Basin states and the other for the Upper Basin states of Colorado, Wyoming, Utah and New Mexico — are designed to help boost the reservoirs’ levels between 2020 and 2026.

Federal officials plan to start a review by the end of this year to examine how the existing rules have worked and how the guidelines for potential shortages could be improved after 2026.

The latest research underscores the growing challenges for water managers and policymakers as they consider how to adjust the rules or change the system to adapt to a river with less water.

Milly and Dunne said in their study that declines in mountain snowpack unleashed by climate change will have far-reaching effects on the availability of water in other regions beyond the Colorado River Basin.

“Many water-stressed regions around the world depend on runoff from seasonally snow-covered mountains,” they wrote, “and more than one-sixth of the global population relies on seasonal snow and glaciers for water supply.”

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