Accelerated melting due to dust exposes surface vegetation earlier in the year, and the growing plants suck water out of the soil. As a result, the team calculated, there is 5% less runoff available to flow into rivers.

To quantify the effect on runoff, Painter and his colleagues plugged historical data into a computer model that projected what annual runoff would have been from 1916 to 2003 under the cleaner snow conditions that existed prior to 1880.

Researchers had already shown that dust emissions in the southwest have increased fivefold since the mid-19th century, when settlers and their livestock poured across the frontier, breaking up the fragile crusts atop desert soils. That extra dust absorbs more sunlight, melting the snowpack sooner and shortening the duration of snow cover each year by three to four weeks, Painter said.

The lost water amounts to more than 250 billion gallons — enough to supply the Los Angeles region for 18 months, said study leader Thomas H. Painter, a snow hydrologist at the Jet Propulsion Laboratory in Pasadena. “That’s a lot of water,” said Painter, whose study was published online by the Proceedings of the National Academy of Sciences.

The dark dust thrown up by human activity in the deserts of the southwestern United States hastens the melting of Rocky Mountain snow and ultimately reduces the amount of water flowing into the upper Colorado River by around 5%, scientists reported Monday.

The model did not factor in the likelihood that a longer-lasting snowpack also cools the atmosphere, probably resulting in less evaporation and more runoff, Painter said. This means the 5% figure is a minimum estimate of the amount of Colorado River water that is lost, he said.

It also suggests that mitigating dust in the region could also counteract some of the effects of climate change. It would “allow snowpack to hang around longer, cool the atmosphere, and contribute to regional cooling,” Painter said.

The study of how color affects the rate at which snow melts goes back to a sunny day in the 1720s, when none other than Benjamin Franklin arrayed pieces of colored cloth on snow and observed how quickly the patches beneath them liquefied.

“In a few hours, the black, being warmed most by the sun, was sunk so low as to be below the stroke of the sun’s rays,” he remarked in a letter to a friend. “The other colors [melted] less as they were lighter; and the quite white remained on the surface of the snow.”

More recently, scientists have used field instruments and satellites to measure dust’s impact on snowmelt on small research plots or watershed areas of an acre or two, said University of Colorado snow hydrologist Mark Williams, who was not part of Painter’s team. The new study marks the first time that scientists have measured this effect on a vast, watershed-wide scale.

While Williams said he would “quibble” with some of the parameters of Painter’s computer model — including estimates of how much water the early-growing plants would draw out of the system — he said that improving the model “would affect the magnitude, but not the conclusion,” of the study.

“These are intriguing results,” he said.

Both Painter and Williams noted that the work has striking — and potentially controversial — ramifications for water and land use policy in the Southwest. The Colorado River and the vast system of dams and reservoirs along it supply water to 27 million people in seven U.S. states, including California, and Mexico. Restoring some of its flows by cutting dust emissions could help relieve the longstanding, intractable water shortages that have shaped the history of the West.

Dust has already decreased since 1934, when the Taylor Grazing Act limited the amount of grazing allowed on public lands. Scientists have previously analyzed lake deposits and found that dust production fell about a quarter as a result, Painter said.

But finding the collective will these days to cut dust emissions further would not be easy. Potential measures include banning the use of ATVs and further restrictions on grazing. “I can’t see too many politicians with enough backbone to make it work,” Williams said.

-- Eryn Brown

Photo: The Gunnison River is a tributary of the Colorado River. Desert dust settling on the Rocky Mountains speeds spring snow melt and plant growth, depleting runoff into the Colorado River, scientists say. Credit: Shaun Stanley/AP.