Ground-breaking study quantifies impacts of human land-use patterns in southwestern deserts
By Summit Voice
SUMMIT COUNTY — Desert dust falling on snow in the high country may be cutting Colorado River flows downstream by as much as 5 percent each year — about twice the amount of water that Las Vegas uses annually.
Reducing dust deposition could help boost the Colorado River’s yield, but that would mean changing land-use patterns and human disturbances in the southwestern desert regions, according to Brad Udall, director of the Western Water Assessment.
“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 Jeffrey Deems, who does research for the Western Water Assessment and the National Snow and Ice Data Center.
Earlier snowmelt then lets the growing season of snow-covered vegetation start earlier, resulting in more water lost through evaporation and transpiration, Deems said 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 downwind on the mountains that feed the Colorado’s headwaters.
“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, Calif. and an affiliate scientist of the National Snow and ice Data Center. “But it can get to the point where it looks like cinnamon toast.”
To evaluate how the dust impacts snowmelt, the team used a reliable hydrology model for the Colorado Basin. Given how important the river’s water is, there are good data sets available for flows. Using the model, the researchers compared current flow rates at Lees Ferry in Arizona to rates during “lower dust” conditions prior to the disturbance of desert soils.
Snowmelt in the current dusty conditions occurred nearly three weeks earlier than in pre-settlement conditions, the results showed, and an average of 5 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 dust study was published in the September 20 issue of the Proceedings of the National Academy of Sciences. Coauthors include Jayne Belnap of the US Geological Survey in Utah, Alan Hamlet of the University of Washington and Christopher Landry of the Center for Snow and Avalanche Studies in Colorado. Funding from the project came from the National Science Foundation, the National Aeronautics and Space Administration and the Western Water Assessment.