This Issue

the West

Dust on snow reduces
Colorado River runoff
by 5 percent

The cinnamon toast-like coating on snow in the Upper Colorado River Basin doesn't just ruin a scenic vista, it makes the snow melt faster and skims valuable water from the West's most important river: the Colorado.
But the news is good, in part. "By cutting down on dust we could restore some of the lost flow, which is critical as the Southwestern climate warms," said Brad Udall, director of the Western Water Assessment (WWA) — a joint program of CIRES and NOAA.

Udall is one of a team of scientists including Jeffrey Deems from CIRES WWA and the National Snow and Ice Data Center (NSIDC), who have been investigating the eyesore's impact on snowmelt and river runoff.

Snow dusted with dark particles absorbs a greater fraction of the Sun's rays and melts faster than white snow, said study leader Thomas Painter of NASA's Jet Propulsion Laboratory and an affiliate of CIRES NSIDC. "It is really like taking a relatively clean snowpack and almost doubling the sunlight hitting it." 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.

Heavy dust coatings on the snowpack are a relatively recent phenomenon: Since the mid-1800s onwards, activities such as livestock grazing and road building have disturbed the desert soil and broken

Above: Dust on snow covers a mountain range in the Rocky Mountains. Below: CSAS Director Chris Landry and graduate students Annie Bryant and McKenzie Skiles take samples to measure impurities in the snowpack.

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, said Deems.  
To evaluate how the dust impacts snowmelt, the team used a

hydrology model that has been shown to 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, the point at which the water flow is gauged and subsequently allocated.

They looked at two scenarios in their models: the "lower dust" conditions prior to the disturbance of desert soils in the 20th century and 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.

"That quantity of water lost is twice Las Vegas' current water right. It's about half of what the state of Arizona takes down through its Central Arizona Project and twice what the city of Denver uses in a year," said Udall. "It is a large chunk of water." The End