ESRL Quarterly Newsletter - Fall 2010

Robbing the West of Water

New Study: Desert dust reduces Colorado River flow

Dark-colored dust that settles on snow in the Upper Colorado River Basin makes the snow melt early and robs the Colorado River of about five percent of its water each year, says a new study coauthored by ESRL and CIRES researchers. This quantity of lost water is nearly twice what the city of Las Vegas uses in a year, says study coauthor Brad Udall, director of the Western Water Assessment (WWA), a joint program of CIRES and ESRL.

“By cutting down on dust we could restore some of the lost flow, which is critical as the Southwestern climate warms,” Udall said.

Snow dusted with dark particles absorbs a greater fraction of the Sun’s rays and melts faster than white snow, said coauthor Jeffrey Deems of WWA. Earlier snowmelt then lets the growing season of snow-covered vegetation start earlier and more water is lost through evaporation and transpiration, he said. That leaves less water for the Colorado River, which supplies water to more than 27 million people in seven states and two countries.

Heavy dust coatings on the snowpack are a relatively recent phenomenon. Since the mid-1800s onwards, human activities, such as livestock grazing and road building, have disturbed the desert soil and broken up the soil crust that curbs wind erosion. Winds then whip up the desert dust—from northwest New Mexico, northeast Arizona, and southern Utah—and drop it on mountains downwind that form the river’s headwaters, Deems said.

“Dust can have an impact even when it’s too sparse to notice,” said study leader Thomas Painter, a snow hydrologist at NASA’s Jet Propulsion Laboratory in Pasadena, CA. “But it can get to the point where it looks like cinnamon toast.”

To evaluate how the dust impacts snowmelt, the team used a hydrology model that has been shown to accurately simulate snowmelt and river flows in the Colorado Basin. The researchers modeled the rate of mountain snowmelt and volume of runoff at Lees Ferry in Arizona for the “lower dust” conditions prior to the disturbance of desert soils, and for the levels of dust observed between 2003 and 2008.

Snowmelt in the current dusty conditions occurred nearly three weeks earlier than in pre-settlement conditions, the results showed, and an average of five percent less water flowed into the river above Lees Ferry. “This is the first time anyone has attempted to quantify the impacts of dust on runoff,” Udall said.

“This result suggests that if we can change our land management practices to reduce desert soil disturbance then perhaps we can extend the snowmelt season,” Deems said. “This might allow more runoff than is currently the norm.” Such a runoff boost may help offset the river’s projected runoff losses due to warming temperatures, and mitigate management tensions over the West’s most over-committed resource, Udall said.

Dust has settled on snowy mountain ranges around the world and although the impacts on annual runoff may differ, depending on the seasonal rainfall patterns in each region, the impacts of dust on snowmelt are indisputable, Painter said. “Clean your snow, it lasts longer—it is that simple.”

The study is published in the September 20 issue of the Proceedings of the National Academy of Sciences. Coauthors include Jayne Belnap of the U.S. Geological Survey in Utah, Alan Hamlet of the University of Washington and Christopher Landry of the Center for Snow and Avalanche Studies in Colorado.

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—Jane Palmer, CIRES