[env-trinity] Severe effect of dust on snowpack melting rate

[Mark Dowdle - TCRCD]

*California Water News*

*/A daily compilation for DWR personnel of significant news articles and 
comment/*

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*September 21, 2010***

*Supply --*

*Dust cuts Colorado River flow, scientists say*

*L.A. Times*


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*Dust cuts Colorado River flow, scientists say*

*L.A. Times-9/21/10*

*By Eryn Brown*

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

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 La 
Cañada Flintridge.

"That's a lot of water," said Painter, whose study was published online 
by the Proceedings of the National Academy of Sciences.

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.

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 before 1880.

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.

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 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 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 effects 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.

Though 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.

Painter and Williams said that the work had 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 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 
17% as a result, Painter said.

But finding the collective will to cut dust emissions further would not 
be easy. Potential measures include banning the use of all-terrain 
vehicles and imposing further restrictions on grazing.

"I can't see too many politicians with enough backbone to make it work," 
Williams said.#

http://www.latimes.com/news/science/la-sci-colorado-river-dust-20100921,0,6434114.story

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