Dust on Snow
As a snow hydrologist, the primary question guiding my research is, “How much water is held as snow in mountains?” and relatedly, “When, and how fast, will it melt?” These are particularly relevant in the Western US, where 75% or more of our water can come from seasonal snowmelt. I focus primarily on understanding what provides the energy to initiate and drive seasonal snow melt in the mountains, and how that might be changing over time. Although many people assume it is warming air temperatures in the spring that melts snow, in most snow-covered environments it is actually the amount of absorbed sunlight.
The term albedo describes how much sunlight is reflected by the snow surface. For example, freshly fallen snow, which is one of the brightest natural surfaces on earth, has an albedo of 0.9 indicating that it reflects 90% of incoming sunlight. As snow gets older, its albedo decreases, meaning that it reflects less, and absorbs more. For clean snow the decline in albedo is gradual, snow grains round and grow larger over time and absorb more sunlight.
More immediate declines in snow albedo follow deposition of dark aerosols, like dust or soot, tree debris, or even growth of snow algae. These, known as light absorbing particles (LAPs), are very effective at accelerating snowmelt because they make snow darker in the visible wavelengths, where solar radiation peaks. We understand this process intuitively, for example, on a sunny day we know that we will be cooler if we wear a white shirt rather than a black shirt, because the white shirt reflects all colors of visible light whereas the black shirt absorbs all colors of visible light.
In the Western US, the primary aerosol that darkens the snow and accelerates melt is dust, the deposition rates of which have increased dramatically since modern settlement of the West. Disturbance associated with land use change has made surface soils more susceptible to erosion, and some of the largest dust producers in North America, like the southern Colorado Plateau, are upwind of our mountain water towers.
Dust is particularly effective at accelerating snowmelt because the majority of dust comes from episodic events that happen in the spring, when arid regions dry out/wind speeds increase, and when days are getting longer/snowmelt is initiating. Once deposited in the snow dust stays in the layer that it was deposited (it isn’t carried away with melt water) and as melt progresses all the dust layers combine at the surface, compounding snow darkening and albedo decline.
We have been studying the impacts of dust on snow over the last ten years in the Colorado Rockies, the headwaters for the Colorado River, using well instrumented snow study sites and regular snow and dust observations. This research has established that dust deposition happens consistently every season, although the number of dust events, and amount of dust deposited, varies from year to year. In most years, dust accelerates snowmelt by a month, but in extreme dust years’ snowmelt can be accelerated by up to two months, shifting both runoff timing and magnitude. This impact is so powerful that surface darkening by dust that controls the rate of snowmelt in each season, as opposed to other factors, like the number of days above freezing.
Dust not only makes snow runoff earlier and faster, but can also lead to plants greening up earlier- shifting evapotranspiration rates- a process that can lower water yields. Because dust on snow is not currently accounted for in operational runoff models it also leads to errors in snowmelt forecasting. Now that we better understand dust on snow processes, we are working with regional water managers to account for albedo decline and accelerated melt following dust deposition.
So, how are things looking this season? I was recently out in the field, digging snow pits and collecting snow samples, with the help of two graduate students in my research group at the University of Utah. We first visited Senator Beck Basin Study Area in the San Juan Mountains, CO, the first high altitude point of contact for dust events originating out the southern Colorado Plateau, and the following day we visited Grand Mesa Study Plot, which is on the northern rim of Grand Mesa in west central CO.
Depending on where you are, there have been 3-4 dust events so far this season, but the dust is not yet strongly impacting melt because it has been a big winter- and it keeps snowing! In fact, we had to sample through a snowstorm and 6 inches of new snowfall while we were visiting Grand Mesa Study Plot. There was a visible dust layer near the surface at both study plots, but the freshly fallen snow brightens the surface, and delays the darkening effect of the dust, which has the greatest impact when it is right at, or just below, the snow surface. With a few more storms in the forecast, it may still be a few weeks before dust is accelerating melt.
I also study the impacts of dust and other aerosols on snow in other regions of the world, including the mountains in my backyard. When I moved to Salt Lake City, I began studying the impacts of dust on snow in the Wasatch Mountains, focusing on the Wasatch Front, which provides >80% of water to the Salt Lake City metropolitan area. Here, the dust comes primarily from arid and disturbed regions in the Great Basin, but using observations at my study plot near Alta, UT, which has been set up to sample dust both in the air and in the snow, we have also shown that it may be coming in part from the dry lake bed of the Great Salt Lake. Like many saline lakes globally, the Great Salt Lake has been in steady decline, due to upstream water withdrawals, and with no protections- minimum lake level or water rights- the lake will continue declining.
We are just now beginning to understand dust emission rates and patterns from the exposed dry lake bed, and how that might be impacting snow water resources, and the Greatest Snow on Earth, in the adjacent Wasatch Mountains.