Does dusty snow mean less water in the Rockies?

Have you been skiing in the Western U.S. and been surprised by brown snow? We recently talked to a research team that studies these coats of dirt. The dust storms that cause dusty snow appear to be getting bigger, thanks to climate warming drying out this region. Worse, dust on snow may increase flooding and slowly smother water supplies in the southwestern U.S. How bad is this situation and who will be affected by it? Can anything be done?

The what and where of dust

Dust particles on snowpack

On snow, dust particles absorb sunlight and quickly transfer that energy into warmth, causing the snowpack to melt faster. (Credit: Chris Landry, Center for Snow and Avalanche Studies)

The more dust, the darker the surface and the faster snow will melt, because dark surfaces absorb more solar energy. “Sunlight is the major snow melter,” said Jeff Deems, a research scientist at NSIDC. “It’s not air temperature.” Fresh snow reflects anywhere between 90 to 95 percent of solar energy. “Very dusty snow, on the other hand, may reflect as little as 30 percent, meaning 70 percent is absorbed,” Deems said. If you more than double absorbency, you’re dramatically increasing the rate at which snow melts.

Dust storms boil out of the arid regions of the West. Jason Neff, a professor of Geology and Environmental Studies at the University of Colorado Boulder, said, “We know humans have increased dust by disturbing desert soils: grazing animals, driving vehicles, building roads and towns.” With the settlement of the West, post 1860s, dust has increased. “It wasn’t completely dust free before,” Deems said, “but by the mid to late 1800s the dust levels jumped up seven-fold.”

Is the extreme the new norm?

In recent years, increased temperature and prolonged drought have been drying the West. 2009, 2010, and now 2013 saw some very large dust storms. “There is indication from our colleagues out in the desert that the extreme scenarios could be a more frequent occurrence,” Deems said.

The extreme dust masses witnessed in 2009 and 2010 absorbed two to four times the solar radiation, and shifted peak snowmelt four to seven weeks earlier, compared to before the West was settled. Timing is everything. Scientists estimate that earlier soil exposure can decrease annual runoff by about 5 percent. Exposed land evaporates more water. Earlier snowmelt also triggers an earlier growing season, allowing for vegetation to take up and evaporate water. “Nothing can be done about the loss of water to evapotranspiration,” said Thomas Painter, a research scientist at the NASA Jet Propulsion Laboratory. “It’s permanent.”

Depleted water levels of Hoover Dam

The water intake towers of the Hoover Dam expose their bones under depleting water levels in Lake Mead. (Credit: Wikimedia Commons)

The Colorado River brings water to more than 40 million people in cities like Denver, Las Vegas, Phoenix, and Los Angeles, punctuated by dams that store its water in reservoirs. Lakes Powell and Mead combined can store about four years worth of Colorado River flow, but for the Upper Colorado Basin, snowpack is the most important reservoir. “We can store way more water as snow in the mountains than we can in our reservoirs—vastly more,” Deems said. “So the longer the snow sticks around, the longer we have water. If flow comes early, then we’re stuck in late summer with only what could be stored in surface water reservoirs.”

Faster snowmelt also suggests a surge in river flow, and when rivers come up faster, water management gets complicated. “It’s just like if you turn on your sink, and if you only have a 101 gallons of water potential and your faucet blasts a hundred gallons out before you even get a cup underneath, then the person behind you in line may only fill their cup half full,” Painter said.

Climate models project that by 2050 the Colorado River will lose 5 to 20 percent of its total runoff. “It’s not good,” said Painter. “To put a dent in the flow like that—people are going to feel it.” The river irrigates 5.5 million acres of agriculture. But something can still be done. If the amount of dust blown onto Colorado River snowpacks can be reduced, a more stable snow pack lingers, giving way to a trickling river system rather than a roaring, untamable beast. Painter added, “By reducing dust loading, we may stem some climate change tide in terms of water loss.”

References

Deems, J. S., T. H. Painter, J. J. Barsugli, J. Belnap, and B. Udall. Combined impacts of current and future dust deposition and regional warming on Colorado River Basin snow dynamics and hydrology. 2013. Hydrology and Earth System Sciences Discussion 10, 6237-6275, doi:10.5194/hessd-10-6237-2013.

Painter, Thomas H., J. S. Deems, J. Belnap, A. F. Hamlet, C. C. Landry, and B. Udall. Response of Colorado River runoff to dust radiative forcing in snow. 2010. Proceedings of the National Academy of Sciences, doi:10.1073/pnas.0913139107.

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