The real culprit behind snowmelt floods isn’t temperature—it’s dirt | Science

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The real culprit behind snowmelt floods isn’t temperature—it’s dirt | Science

Desert soil blown onto mountain snow controls how fast snowmelt feeds downstream rivers regardless of how warm it is, a new study finds.

Ethan Welty/Aurora Photos

Every spring in the western United States, snow melts off mountains, feeding rivers with surges of water that can cause disastrous floods. But warm weather isn’t the main culprit, a new study finds. Instead, dusty soil that sticks to snow can darken it and accelerate its melting. The findings could establish a new way of forecasting snowmelt flooding—and suggest that the current prediction system has been getting it all wrong.

The work is “groundbreaking,” says Adrian Harpold, a hydrologist at the University of Nevada in Reno who wasn’t involved with the study. “Dust is a really big deal for snowpack.”

The desert regions of states from Colorado to California rely on snowpack as a natural water reservoir. In the spring, for example, melting snow feeds the headwaters of the Colorado River, a crucial source for irrigation, hydroelectricity, and drinking water for at least 27 million people in seven states. Current snowmelt forecasts, which depend heavily on air temperature, get it wrong by as much as 40%. That means critical decisions about water allocation often have to be made at the last minute.

Tom Painter, a snow hydrologist with NASA’s Jet Propulsion Laboratory in Pasadena, California, wanted to make the predictions better. He knew from previous research that dust from the deserts of Utah, Arizona, and New Mexico—dry red and gray soil that blows over to the Rocky Mountains—can accelerate melting and shorten snow cover there by more than a month. That’s because lots of dust falls—up to seven times more than in the mid-1800s, thanks to cattle grazing, oil and gas exploration, off-road vehicle recreation, suburban development, and farming. These disturbances perturb the delicate crust of mosses, lichens, and bacteria that would otherwise trap the dust and prevent it from blowing away. The muddy red and black snow that results absorbs more of the sun’s energy, just like a black car on a bright day. And that heat melts snow faster.

To find out how dust influences river runoff, the team examined the water flow of four rivers in the Colorado River Basin near Telluride. They then compared that activity to 10 years of air temperature data and something called radiative forcing—a measure of energy absorption—near each of the headwaters.

The more dust on the snow, the more radiative forcing, and the faster the rivers rose, the researchers reported last month in Geophysical Research Letters. Dust accounted for some 70% to 83% of the increase, whereas air temperature influenced 7% at best.

“[That] was really surprising,” says Jeff Deems, a snow hydrologist at the National Snow and Ice Data Center in Boulder, Colorado, and co-author of the study. “It almost doesn’t matter how warm the spring is, it really just matters how dark the snow is.” Painter agrees: “We were just blown away by the results.”

That’s not to say air temperature has absolutely no effect on snowmelt, Painter cautions. Air temperature now accounts for about 10% of the energy that melts snow, and its influence will be even greater as a changing climate brings warmer temperatures. But as long as dust is there, it will always beat out air temperature.

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