2013 brought record levels of dust to Colorado’s mountains
By Bob Berwyn
FRISCO — Dust storms that darken the Rocky Mountain snowpack and speed snowmelt are becoming more extreme, according to new research. Particularly heavy dust-on-snow events can speed the melt-out of the snowpack by a full six weeks, all other factors being equal, said Jeffrey Deems, a researcher with the Western Water Assessment and the National Snow and Ice Data Center.
“In 2006 we were impressed at how much dust there was. Then 2009 turned up, and 2010, and 2013 was the dustiest year we’ve recorded in the San Juans,” Deems said, explaining that the latest study, put together by researchers with NOAA’s Cooperative Institute for Research in Environmental Sciences was aimed at updating previous work with data from those heavy dust years.
Last spring, on April 8, a single 16-hour dust storm dropped more dust on the San Juans than the annual totals in any previous winter since scientists started taking detailed measurements, said Chris Landry, director of the Center for Snow and Avalanche Studies in Silverton, which tracks the dust-on-snow events via a network of observation sites.
The April 8 storm deposited about 419 pounds of dust per acre, or about 47 rams per square meter, Landry said, adding that the melt-out equation also has to include year-to-year weather variations. For example, 2011 saw substantial dust deposition on the snowpack, but a cold and snowy spring delayed the snowmelt to a record late date. A year later, the opposite: Heavy dust on a low snowpack and dry weather starting in March resulted in one of the earliest melt-outs on record.
The CIRES research tried to look at the combined effects of regional warming and dust on the Colorado River, which is fed primarily by snowmelt. The findings, published in Hydrology and Earth System Sciences, shows that the incremental additional dust in the “extreme” years not only speeds snowmelt, but further reduces total water yield from the snowpack.
Add the regional warming expected in the future, and the situation seems likely to grow more dire for the 40 million people who depend on the Colorado River for water. Some models project that Colorado River flows could drop as much as 20 percent by 2100.
“But we may be able to do something about dust,” said Deems. “If the future normal is this extreme dust scenario and we can push that scenario back to lower dust levels with land restoration or management, we could keep the snow in the mountains longer, and maybe even get some of that water back,” he said.
Since the mid-1800s, human land use activities have disturbed Southwestern desert soils and broken up the soil crust that curbs wind erosion, leading to increased dust.
That earlier work was based on the moderately dusty years of 2005–2008, with about five times as much dust than in the 1800s. But during 2009, 2010 and 2013, unprecedented amounts of desert dust fell on Colorado snowpacks, about five times more than observed from 2005–2008.
Other researchers have reported that climate change is likely to increase the frequency and intensity of drought in the Southwest, which could increase dust problems further by harming the grasses and shrubs that reduce surface wind speeds.
The shift in the timing of the runoff was the most robust result of the latest research effort, Deems said, explaining that it has different implications for different parts of the Colorado River Basin. Downstream states in the Lower Basin are most concerned about the total amount of water flowing down the river, but users in the Upper Basin — ranchers and agriculural producers, for example — are more dependent on the timing.
Shifting peak water levels to earlier in the spring leaves less water for later in the year.
“In the Upper Colorado River Basin, the snowpack is our most important reservoir,” said co-author Thomas Painter of NASA’s Jet Propulsion Laboratory. “With continued dusty years and greater warming, water managers will have to make their decisions very early in the season. No longer will they have the nice long snowmelt season, shortened as it already has been, to see how snowmelt runoff is going.”
The research team also found a subtle shift on the total water loss in the Colorado River, from a loss of 5 percent estimated during the moderate dust years of 2005–2008, to a total loss of about 6 percent lost during extremely dusty years. This relatively small change is due primarily to the fact that as snowmelt creeps earlier and earlier in the year, the Sun’s angle in the sky is shallower and provides less energy for evaporation than it does later in the spring.
“Our results suggest that if we can adopt dust-reducing land management strategies and rehabilitate major dust sources, we can keep our snow on the mountains longer, and perhaps offset some of the emerging climate impacts,” said co-author Brad Udall, director of the Getches-Wilkinson Center for Natural Resources, Energy and the Environment at CU-Boulder. “Dust reduction could be a very powerful strategy to help us adapt to the growing impacts of climate change on our precious water supplies in the American Southwest.”