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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鈥檚 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), , 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鈥攌ey information for decision makers.

鈥淲e know, for example, that warmer temperatures will lead to more evaporation and less flow,鈥� said co-author Bradley Udall, who contributed to the study as director of the CIRES , a -funded program at the . Udall is now director of 抖阴旅行射-Boulder鈥檚 at Colorado Law. 鈥淎lthough projections of future precipitation aren鈥檛 as clear, it鈥檚 likely that we鈥檙e going to see a reduction in overall flow in the Colorado,鈥� Udall said.

The is published this week in the .

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.

鈥淭he different estimates have led to a lot of frustration,鈥� said lead author , who recently earned a UW doctorate in civil and environmental engineering. 鈥淭his paper puts all the studies in a single framework and identifies how they are connected.鈥�

In evaluating recent scientific papers that estimate future flow of the Colorado River, she and her co-authors identified several reasons for different flow estimates. Among them are differences in:

• Climate models and future-emissions scenarios used. Models run with higher future greenhouse gas emissions, for example, typically produce warmer and often drier climates, and smaller Colorado River flows.
• The models鈥� spatial resolution, which is important for capturing topography and its effects on snow distribution in the Colorado River鈥檚 mountainous headwaters. Models with coarser resolution tend to overestimate the sensitivity of runoff to climate change.
• Representation of land surface hydrology, which determines how precipitation and temperature changes will affect the land鈥檚 ability to absorb, evaporate or transport water.
• Methods used to downscale from the roughly 200-kilometer resolution used by global climate models to the 10- to 20-kilometer resolution used by regional hydrology models.

The new paper also highlights several important realities faced by Western water managers reliant on Colorado River flows. The early 20th century, for example, is the basis for water allocation in the basin, but was a period of unusually high flow, so Colorado River water is already overallocated. Moreover, tree ring records suggest that the Colorado has experienced severe droughts in the past and will do so again, even without any human-caused climate change. If exacerbated by steadily decreasing flows due to climate change, such a 鈥渕egadrought鈥� could result in decades of extremely low streamflow, much lower than observed in the last century.听 听 听 听 听听

Paper authors include leaders in Western water issues, ranging from specialists in atmospheric sciences to hydrology to paleoclimate: at CIRES鈥檚 Western Water Assessment and the University of Colorado Boulder; , Tapash Das and Hugo Hidalgo at the University of California, San Diego; , Holly Hartmann and Kiyomi Morino at the University of Arizona in Tucson; Levi Brekke at the U.S. Bureau of Reclamation; at the U.S. Geological Survey in Denver; and Martin Hoerling at the in Boulder; and Kevin Werner at the National Weather Service in Salt Lake City.

CIRES is the Cooperative Institute for Research in Environmental Sciences, a joint institute of 抖阴旅行射-Boulder and NOAA.

The research was funded by NOAA through its and its .听

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For more information, contact lead author Julie Vano at 206-794-7946 or jvano@uw.edu; Dennis Lettenmaier at 206-543-2532 or dennisl@uw.edu; or Bradley Udall at 303-492-1288 or bradley.udall@colorado.edu.

Hannah Hickey
University of Washington
202-543-2580
hickeyh@uw.edu

Katy Human
CIRES
303-735-0196
kathleen.human@colorado.edu

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