Carl Sandburg said that the fog comes in on little cat feet. The dust that smothered the mountain snowpack in Colorado one afternoon last April arrived like a hard-charging Doberman.
Satellite images showed the dust originated in Arizona and New Mexico, just north of Interstate 40. Picked up by strong winds, the desert dust blew high into the San Juan Mountains, in Colorado’s southwest corner. There, the dust settled on the snowpack – at least until a carpet of fresh snow had fallen.
Neither rare nor normal, such dust storms have been significantly denting the flow of the Colorado River, scientists report in a paper published recently in a journal, the Proceedings of the Academy of Natural Sciences. They say the dust causes the snowpack to absorb more solar energy during the spring, hastening runoff and robbing the river of water of 5 percent of its flow before it reaches the Grand Canyon. They believe the dust is mostly caused by human activities in desert regions.
This conclusion has enormous implications for the 27 million to 30 million people between Denver and San Diego who depend upon Colorado River water.
‘A large chunk of water’
Brad Udall, managing director of the Western Water Assessment and a co-author of the study, points out that this volume of water, 750,000 acre-feet annually, is twice the water right owned by Las Vegas, about half of what gets drawn through the Central Arizona Project, and about twice what the city of Denver uses.
“It’s a large chunk of water,” he says in a video posted at in a video posted at the Cooperative Institute for Research in Environmental Sciences website.
Udall goes on to say that this understanding presents opportunity. Pinched by rapid population growth and what looks to be rapidly rising temperatures, cities of the Southwest have already begun studying their options.
The Southern Nevada Water Authority is boring a $700 million tunnel deeper into Lake Mead, the reservoir behind Hoover Dam. The deeper tunnel will be needed to draw water for Las Vegas if drought and shortages intensify, as many models predict. Despite relatively good water years in the Rocky Mountains, the reservoir now is only 42 percent full.
Other studies have explored options for increasing water supplies. One research effort in Wyoming is testing how much more water can be delivered by seeding winter clouds. Other studies have even thinly examined the idea of importing water from other basins, such as the Mississippi River.
If causes of the desert dust can be abated, says Udall, more native water will be available to cities and farms of the Southwest.
However, the lower-basin states of Arizona, California and Nevada have so far shown little interest in the dust-on-snow findings, says Eric Kuhn, general manager of the Colorado River Water Conservation District, an agency based in Glenwood Springs, Colo. It contributed $10,000 this year to the dust-on-snow research.
“I have my doubts that it will ever get much traction,” Kuhn adds. “My guess is that many will consider dust a problem without a real solution. What are the options, reducing recreation and development in the deserts?”
This growing understanding of how activities in desert country can affect high-mountain snowpacks was triggered by a casual observation one June day in the late 1990s. Tom Painter, then a graduate student, was hiking up a mountain peak near Aspen with his father. They paused that morning near a lingering patch of dirty-looking snow. Idly, the younger Painter scraped off the top layer of dirty snow, leaving a gleaming, white surface.
Returning late in the afternoon, the Painters observed the darkened snow had shrunk significantly. The snow scraped clean that morning stood higher, like a mushroom, as it had melted far less.
Painter understood what had happened. White surfaces reflect more solar radiation than dark surfaces, a phenomenon called albedo. After all, snow on an asphalt driveway melts more rapidly than on a concrete one. What Painter didn’t know was the source of the dust and its larger effect in melting mountain snowpacks.
After getting his Ph.D., Painter teamed with Chris Landry, who had recently established the Center for Snow and Avalanche Studies in Silverton. Landry had grown up in Aspen and Whitefish, Mont., where he learned to ski. He had recently received a master’s degree in snow and avalanche studies at Montana State University.
Settling in Silverton, an old mining town located at an elevation of 9,300 feet, Landry found an above-timberline valley called Senator Beck Basin suitable for ongoing alpine research. Although close to a paved road, it’s inaccessible to four-wheelers, unused by snowmobilers, and was never substantially tainted by mining.
But the snow is by no means pristine. Pits dug to the ground in May or June at elevations of 11,000 to 12,200 feet reveal a snowpack that in places looks like an angel-food cake layered with chocolate frosting. Each layer represents a different storm. As the snow melts, the dust remains. By late May during the last two years, the snowpack in the San Juan Mountains has looked like a beige carpet.
Where does the lost water go?
Frequency of dust storms varies. In eight years of record-keeping, there have been as few as three and as many as 12. They have come anywhere form October to June, although most often in spring. Depending upon origin, including Utah, California and even Mexico, the dust sprinkled on snow looks red, black or tan.
Humans cause much of this dust. Studying sediments from an above-timberline lake accumulated during the last 1,200 years, scientist Jason Neff found an enormous change beginning in the 1800s. Dust deposition increased 600 percent. Later, after the 1930s, dust levels fell back to 500 percent.
This increase in dust coincides with American settlement of the desert southwest. Later, in the 1930s, Congress adopted restrictions on grazing of public lands – possibly explaining the small decrease since then.
Working in the desert country of southeast Utah, U.S. Geological Survey researcher Jayne Belnap documented how dust gets picked up and blown. Setting up portable wind tunnels to simulate storms, she found undisturbed land yielded little dust. Disturbed lands
were another matter.
Still, a nagging question from water-user groups remained. “They would say, ‘We understand the influence on timing and rates (of runoff), but how does this affect yields?’” says Landry. “I had to say, ‘We don’t know yet. We don’t have a good answer for you yet.”
For that answer, Painter and associates set out to model the findings from Senator Beck Basin more broadly across the upper Colorado River Basin. Nearly all the water in the river comes from the upper basin, which also includes portions of Wyoming, Utah and New Mexico.
Doing much of the modeling work was Jeff Deems. Passionate about snow, he had spent several years skiing hard and banging nails, first in Colorado ski towns and then in Bozeman, Mont. By then, he was skiing the backcountry as much as lifts. Driven to understand avalanche formation, he enrolled at Montana State’s snow studies program, later earning a Ph.D. at Colorado State University.
To help answer the question about effect on water yield, Painter and Deems adapted a hydrology model developed to describe runoff in the Colorado River Basin. To understand how snowpacks without significant dust, they compared data with Switzerland, which gets only slightly sprinkled by the Sahara.
Where does the lost water go? The researchers explain that snow itself loses only a little water to the atmosphere, a process called sublimation, before melting. But with the snow absent three weeks earlier, more water is lost to evaporation and in the case of plants, transpiration. Even in the cooler temperatures of high mountains, this adds up.
In short, what always seemed normal was not at all normal. If the research by Deems, Painter and others is correct, the Colorado River all along has been carrying less water due to dust.
How much water is at stake?
Could losses increase as global warming heats the Southwest? As deserts heat, vegetation will struggle, resulting in higher wind speeds at ground level. Combined with continued land disturbance, this would allow more dust to be blown onto mountain snowpacks. This could cause even more loss than 5 percent, says Udall.
“I think this study gives us some hints that we could lose even more water if we’re not careful about how we manage those lands,” says Udall.
Laurna Kaatz, climate scientist, describes the study as interesting, but says the effect of accumulating greenhouse gases on Denver’s water supply remains unclear.
“I don’t think we’re at a point we can say definitely that we know what will happen in the future and what we need to be prepared for in the future,” says Kaatz. “At Denver Water, we are doing our best to be prepared for the uncertainties that unfold.”
Kuhn admits he was surprised by the model that delivered the 5 percent estimate, which he believes could change with additional study. “The number is only as good as the model, but whether it’s 4 percent or 6 percent, that’s a lot of water.”
Driving frequently for the last 30 years between Glenwood Springs and Flagstaff, Ariz., where his parents live, Kuhn believes that livestock grazing no longer disturbs the soil significantly. Instead, he blames recreation, development and roads for breaking the microbial layers on desert soils, allowing dust to be picked up by winds.
The researchers are continuing their work, this time with a $1.6 million grant from NASA. They also seek to expand the dust-measuring network across the Colorado River Basin to other mountain research stations. Entirely new monitoring stations are needed in places such as Wyoming’s Wind River Range, a major source of Colorado River water, says Deems.