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 5 percent of its water each year, says a new study co-authored by researchers from the University of Colorado at Boulder-based Cooperative Institute for Research in Environmental Sciences, or CIRES.
The quantity of lost water is enough to supply Los Angeles for 18 months, said study co-author Brad Udall, director of the Western Water Assessment, or WWA, a joint program of CIRES and the National Oceanic and Atmospheric Administration. "By cutting down on dust we could restore some of the lost flow, which is critical as the Southwest 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 study co-author and Research Associate Jeffrey Deems of CU-Boulder's National Snow and Ice Data Center and a scientist at 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 U.S. states and Mexico.
The study is being published in the Sept. 20 issue of the Proceedings of the National Academy of Sciences. Co-authors on the PNAS study 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 Silverton, Colo. Funding for the project came from NASA, the National Science Foundation and the WWA.
Heavy dust coatings on the snowpack are a relatively recent phenomenon beginning about 150 years ago, when human activities like livestock grazing and road building began disturbing the desert soil and breaking 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 in the San Juan mountains where the river's headwaters form, Deems said.
"The compressed mountain runoff period makes water management more difficult than a slower runoff," said lead study author Tom Painter, a snow hydrologist at both NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the University of California at Los Angeles. "With the more rapid runoff under dust-accelerated melt, costly errors are more likely to be made when water is released from and captured in Colorado River reservoirs."
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 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.
The researchers believe steps can be taken to reduce the severity of dust-on-snow events in the Colorado River Basin. They point out the impact of the Taylor Grazing Act of 1934 for potential guidance on how dust loads can be reduced. The act regulated grazing on public lands to improve rangeland conditions. Lake sediment studies show it decreased the amount of dust falling in the Rocky Mountains by about one quarter.
"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.
"Restoration of desert soils could increase the duration of snow cover, simplifying water management, increasing water supplies and reducing the need for additional reservoir storage of water," said Painter. "Peak runoff under cleaner conditions would then come later in summer, when agricultural and other water demands are greater.
"It could also at least partially mitigate the expected regional impacts of climate change, which include reduced Colorado River flows, increased year-to-year variability in its flow rate and more severe and longer droughts," Painter said. "Climate models project a 7 to 20 percent reduction in Colorado River basin runoff in this century due to climate change."