Dust may be snowpack’s nightmare
July 2, 2007
Ever noticed dust on snow during the spring time?
Tom Painter, a scientist who is now calling Kimball Junction home, has found that dust blow into the high mountains can speed runoff by about a month.
Painter, who is now an assistant professor with the University of Utah, has been conducting the experiment for three years in the San Juan Mountains of Colorado. However, he believes the results have implications for other mountain ranges around the world, particularly at a time of increased climatic uncertainty.
But the study has the greatest immediate implications for the 35 million people who depend in whole or part upon waters from the Colorado River and its tributaries. Included in those cities are Denver, Salt Lake City (through trades) and Los Angeles.
Painter grew up in Fort Collins, Colo., but attended the University of Utah and also took a break from college, skiing at Snowbird for a year. Although gifted in mathematics, he found his passion after enrolling at the University of California at Santa Barbara. While there, and skiing weekends at Mammoth, he more clearly understood he wanted to conduct research involving snow.
The seed for the current experiment was planted during a climbing trip near Aspen in late June of 1998. As he and his father Ralph J. Painter, set out to climb 14,000-foot peak South Maroon, the younger Painter scrapped off the surface of a section of snow along the trail. The surface was quite dirty, but the scraped surface was white.
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They climbed the mountain, ate their lunches, and in late afternoon returned by the thoughtless experiment. The darker snow had melted several more centimeters than the clean white snow.
That was not particularly surprising. We all know that dark absorbs solar radiation, while white surfaces reflect it, something called albedo. Snow has the greatest albedo of any material on the planet.
But where did the dust come from? That’s a question Tom Painter couldn’t answer.
Nor did he try – until after he got his Ph.D. in 2002 and then was aligned with the National Snow and Ice Data Center at the University of Colorado-Boulder.
To conduct the experiment, he needed a relatively pristine place – one not carved up by skiers, and certainly not by snowmobiles. It had to be away from obvious sources of air pollution, like smokestacks or roads, yet accessible for weekly visits.
That site was found midway between the towns of Ouray, Silverton, and Telluride. There, in a base adjacent to Red Mountain Pass, measurements are taken at elevations ranging from 11,050- to 13,500-feet.
In the American Southwest, dust storms most frequently occur from February through May. As well, the affect of dust in causing snow to melt is more pronounced as the days get longer and the sunshine falls more directly on the snowbanks.
In a paper published in a journal called Geophysical Research Letters, Painter says he has shown that dust speeds the melting of snow anywhere from 18 to 35 days, depending upon the year and whether the locations was in trees at a lower elevation or above treeline.
Does it matter when the snow melts?
Yes, says Painter and other scientists. Furthermore, they say that the dust that lands on the San Juan Mountains, at least much of it, is not natural, but rather the result of disturbance of deserts in northern Arizona and New Mexico
The dusts of spring
How do they know this? First, satellite images allow them to see plumes rising from the desert and then moving toward the Rocky Mountains. Second, scientists can analyze the chemistry of the dust specimens, looking for the chemical isotopes that can then be traced to the isotopes found in the bedrock of regions.
An expert in identifying the fingerprints of dust is Jason Neff, a biogeochemist with the Geological Science Department at the University of Colorado-Boulder. The isotopes clearly tell the story of the dust, he says.
"With that information, we can say very definitely it’s not local," he says. "It’s not from wood-burning, it’s not road dust, and it’s not stuff flying off ridges from near the sample sites. It’s traveling into the state and being deposited on the snowpack."
It’s not possible, given current tools, to precisely pinpoint the source of the dust – say, for example, a location 20 miles from Flagstaff. But Neff and other scientists believe that relatively little of this dust is natural. Some areas, such as the playas, or dry river beds and lakes, are natural sources of dust. But most of the dust is the result of soil disturbances.
Neff participated in a study in Canyonlands National Park that found a progressive loss of surface soil in areas that have been grazed, as compared to areas not by domestic livestock.
"One-hundred years of grazing in the American Southwest, at least in that particular place in Utah, has led to a pretty large loss of soils," he says.
Neff is also conducting a study in the San Juans. There he is studying core sediments taken from above-timberline lakes – including the dust that has been deposited. The goal is to determine whether there has been a big human effect in the last 200 years, during the time of Euro-American settlement.
"We actually had a lot more grazing 100 years ago than we do now, although we now have more cars, home construction and recreational use" in the desert Southwest, he said.
Neff hopes to publish his study results later this year.
Desert dust isn’t natural
From Moab, Jayne Belnap has also been studying changes in the deserts of the Colorado Plateau. She’s a soil scientist with the U.S. Geological Survey, and among her projects she has used advanced equipment to simulate the effect of wind storms in the desert.
The goal was to see what sticks and what flies.
Belnap says some deserts are naturally dusty, but they are few. In the American Southwest, the story largely is of disturbances – from livestock grazing, from recreational vehicles and from home construction.
"Anything that has a compressional force. It doesn’t matter whether it’s a hoof, a bike, or an ATV or a tank – anything will disturb that surface," she says.
This is, she repeats, not normal. Desert soils will mostly stay put if they haven’t been disturbed.
But the larger story here, she says, is of the altered hydrology caused by the vagrant dust in the Southwest. Even if the reservoirs can hold the flood waters of a rapid spring runoff, she says, the rhythm is changed.
Among the water quality problems will be increased sediment loading.
Brad Udall, director of the Bolder-based Western Watershed Assessment, says he believes Painter’s study has not yet caused major eyebrow lifting among climate scientists. But water managers are paying attention.
"It’s certainly a wild card," he says. "It’s something about which we need to know a lot more."
The study has potentially large implications for the Colorado River Basin. About 75 percent of the water in that basin arrives in the form of snow.
Much of the water, some 16 times the total annual flow of the river, is held back in reservoirs in Wyoming, Colorado and New Mexico, but with half in the two big reservoirs: Powell and Mead. But the most efficient reservoirs are the alpine snowbanks that, even into July, continue to melt in the high country. Lower-elevation reservoirs lose much water to evaporation. Lake Powell loses six feet per year to evaporation.
Already, the Colorado River Basin is fully used, with virtually none getting into the Sea of Cortez, and most of what gets to Mexico being brackish, of poor quality.
With global warming almost certain to turn up the heat in the Southwest, climatologists are now in general agreement that less water can be expected, if for no other reason than increased evaporation.
If the deserts, their soils already disturbed, become more dry, they could yield even more dust on the snowpacks of the San Juans and, to lesser extents, across the Colorado Rockies.
Jeff Dozier, a professor of snow hydrology at the University of Santa Barbara, says the study is important far beyond Colorado. Similarly conditions to those of the San Juans exist in other areas of the nation and world. Altogether, about a billion people in the world depend on melt from snow or glaciers for water supply.
"In considering future climate changes, we often focus on warmer temperatures," says Dozier. "In the high alpine zone, however, snow is more sensitive to the amount of solar radiation absorbed than it is to the air temperature."
"For this reason, a change in the amount of dust from the desert (perhaps caused by drier conditions there) is probably more important to the San Juan snow than a change in temperature."