Aeolian dust deposition to Rocky Mountain snowpacks

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This article is part of the Fall 2017 issue of the Earth Science Matters Newsletter.

Mountain snowpacks are a critical resource in arid and semi-arid regions of the world, with more than one-sixth of the world’s population relying on snowmelt for their water supply. Snowpacks act as a large natural reservoir, storing water during winter and releasing it during the summer and fall, when human and ecological demands are greatest. Current and projected climate warming poses a substantial risk to snowpack resources, causing shifts in precipitation from snow to rain, and shifting the timing of snowmelt towards earlier in the year.  The western United States (U.S.) is an area where snowpack resources are especially important, providing up to 85% of annual runoff; however, springtime warming and declining snowfall may pose a substantial risk to summer/fall water supplies.  In addition to climate warming, the Rocky Mountains are downwind from major sources of aeolian (wind-blown) dust in the southwestern U.S. and northern Mexico. When deposited on snow, this dust can further accelerate snowmelt by darkening the snowpack and making it absorb more energy from the sun.

snowpack with buried dust layer

Photo of snowpack showing dust layer deposited on February 14-15, 2006. The dust layer is visible as a tan-colored band in the snow, approximately 15 cm below the snow surface. (Featured in Figure 1 from Clow et al., 2016).

(Credit: David Clow, USGS. Public domain.)

Recent studies have suggested that increasing dust deposition is the main driver of recent changes in snowmelt timing in the Rocky Mountains; however, lack of regional, long-term monitoring data on dust deposition to snow has made this concept difficult to test. USGS researchers recently developed a novel way to infer long-term trends in dust deposition using snowpack chemistry as a surrogate for dust deposition. When aeolian dust interacts with melting snow, it imparts a unique chemical signature as certain soluble minerals (calcite) dissolve in the snowmelt.  The USGS study takes advantage of a long-term snowpack chemistry monitoring network, which the USGS began in 1993 to address concerns about effects of air pollution on ecosystems in the Rocky Mountains.  The study also uses data from three other long-term chemistry- and snowpack-monitoring networks to piece together trends in snow-, rain-, and fine-dust chemistry, and link them to changes in climate and snowmelt timing.

Results from the study show that dust deposition to snow increases from north to south in the Rockies, and it increased by 81% in the southern Rockies between 1993 and 2014. Dust deposition to snow was a significant, but not the only, factor linked to earlier snowmelt in the Rockies. Decreases in springtime snowfall were important as well.  These two factors accelerated snowmelt timing by 7—18 days between 1993 and 2014. This shift is important because it can lead to lower soil moisture during the summer, which stresses trees and other vegetation, making them more vulnerable to insects, disease, and wildfire.

Previous studies have shown that aeolian dust emissions may have doubled globally during the 20th century, possibly due to drought and land-use change. Climate projections for increased aridity in the southwestern U.S., northern Africa, and other mid-latitude regions of the northern Hemisphere suggest that aeolian dust emissions may continue to increase, compounding the risk that climate warming poses to snowpack water resources in arid/semi-arid regions of the world.  Information from the current study is being used by the National Park Service, US Forest Service, and local government agencies and water providers to guide decisions about water management and air pollution control strategies that may affect ecosystems and future snowpack water resources in the western U.S.

The paper, “Increasing aeolian dust deposition to snowpacks in the Rocky Mountains inferred from snowpack, wet deposition, and aerosol chemistry”, was published in Atmospheric Environment.  It is available at: http://www.sciencedirect.com/science/article/pii/S1352231016305179.

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