Researchers take new look at future Colorado River flows

The Colorado River provides water for more than 30 million people in the U.S. West, so water managers have been eager to understand how climate change will affect the river’s flow. But scientific studies have produced an unsettling range of estimates, from a modest decrease of 6 percent by 2050 to a steep drop of 45 percent by then.

A new paper by researchers at the University of Washington (UW), CIRES, NOAA and other institutions across the West investigates and explains why those estimates differ and summarizes what is known about the future of this iconic Western river—key information for decision makers.

“We know, for example, that warmer temperatures will lead to more evaporation and less flow,” said co-author Bradley Udall, who contributed to the study as director of the CIRES Western Water Assessment, a NOAA-funded program at the University of Colorado Boulder. Udall is now director of CU-Boulder’s Getches-Wilkinson Center for Natural Resources, Energy and Environment at Colorado Law. “Although projections of future precipitation aren’t as clear, it’s likely that we’re going to see a reduction in overall flow in the Colorado,” Udall said.

The study is published this week in the Bulletin of the American Meteorological Society.

While the paper does not narrow the existing range of estimates, it provides context for evaluating the current numbers. The 6 percent reduction estimate, for example, did not include some newer climate model runs, which tend to predict a drier West. And the 45 percent decrease estimate relied on models with a coarse spatial resolution that could not capture the effects of topography in the headwater regions. The new analysis, thus, supports more moderate estimates of changes in future flows.

“The different estimates have led to a lot of frustration,” said lead author Julie Vano, a UW doctoral researcher in civil and environmental engineering. “This paper puts all the studies in a single framework and identifies how they are connected.”

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