Clearing up confusion on future of Colorado River flows

Science Daily: 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. Increasing demand for that water combined with reduced flow and the looming threat of climate change have prompted concern about how to manage the basin's water in coming decades.

In the past five years, scientific studies estimated declines of future flows ranging from 6 percent to 45 percent by 2050. A paper by University of Washington researchers and co-authors at eight institutions across the West aims to explain this wide range, and provide policymakers and the public with a framework for comparison. The study is published this week in the Bulletin of the American Meteorological Society. "The different estimates have led to a lot of frustration," said lead author Julie Vano, who recently earned a UW doctorate in civil and environmental engineering. "This paper puts all the studies in a single framework and identifies how they are connected."

Besides analyzing the uncertainty, the authors establish what is known about the river's future. Warmer temperatures will lead to more evaporation and thus less flow. Changes to precipitation are less certain, since the headwaters are at the northern edge of a band of projected drying, but climate change will likely decrease the rain and snow that drains into the Colorado basin.

It also turns out that the early 20th century, which is the basis for water allocation in the basin, was a period of unusually high flow. The tree ring record suggests that the Colorado has experienced severe droughts in the past and will do so again, even without any human-caused climate change. "The Colorado River is kind of ground zero for drying in the southwestern U.S.," said co-author Dennis Lettenmaier, a UW professor of civil and environmental engineering. "We hope this paper sheds some light on how to interpret results from the new generation of climate models, and why there's an expectation that there will be a range of values, even when analyzing output from the same models."...

The bend downstream from the Glen Canyon Dam in Page, Arizona. Shot by Christian Mehlführer, Chmehl, Wikimedia Commons, under the Creative Commons Attribution 2.5 Generic license