Higher temperatures will be a key driver of future decreases in Colorado River flow, according to a new report from a team that includes University of Arizona scientists.

Previous scientific studies indicate by about 2050, the Colorado River will shrink because of climate change. However, their estimates of how much range from 6 percent less water to 45 percent less water.

To help water managers and policy makers understand why the wide range of estimates, a multidisciplinary team of scientists reviewed 16 scientific studies that project future Colorado River flows.

The team's paper, "Understanding Uncertainties in Future Colorado River Streamflow," was published online June 25 in the Bulletin of the American Meteorological Society.

"These previous studies all suggest less water in the future, and we confirm this," said co-author Jonathan T. Overpeck, co-director of the UA Institute of the Environment.

Lead author Julie Vano, who recently earned a University of Washington doctorate in civil and environmental engineering, said, "The different estimates have led to a lot of frustration. This paper puts all the studies in a single framework and identifies how they are connected."

Overpeck said, "The paper does the best job thus far in framing the challenge of maintaining water supply in the seven-state-plus-Mexico region served by the Colorado."

Knowing how much water will be in the river is crucial for water managers trying to plan for the coming decades: The Colorado River provides water for more than 30 million people, including those in the fast-growing cities of Las Vegas, Phoenix and Los Angeles.

 

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The new paper establishes what is known about the river's future. Warmer temperatures will lead to more evaporation and thus less flow. Whether precipitation will change is less certain, but climate change is likely to decrease the amount of rain and snow in the Colorado basin.

"Even if the Colorado headwaters get no change in precipitation, the temperature increases – a sure bet – will drive lower flows in the river," said Overpeck, a UA professor of geosciences and of atmospheric sciences.

"It's surprising to me how much temperature increases alone will decrease flow in the Colorado," he said.  

The team’s study does not provide new estimates of future annual flows of the Colorado River. Instead, it provides context for comparing the various scientific studies and explains why the different types of previous studies produced different predictions.