Article: Sexstone, G. A., Clow, D. W., Fassnacht, S. R., Liston, G. E., Hiemstra, C. A., Knowles, J. F., & Penn, C. A. (2018). Snow sublimation in mountain environments and its sensitivity to forest disturbance and climate warming. Water Resources Research, 54. https://doi.org/10.1002/2017WR021172
Do you want to build a snowpack?
Much of the time that we hear about the importance of winter snowpack in the mountains is in reference to when ski season starts. But even if you are like me and your idea of an extreme winter sport is having to go outside when it’s below 50 degrees Fahrenheit, snowpack is still really important. Seasonal mountain snow represents an important source of freshwater in many places. When the snow melts, it becomes water that can go towards supporting vegetation, adding to groundwater, and flowing downslope in rivers and streams. (Check out this before and after image showing fields greening up after mountain snow melts). In some places like California, a substantial portion of the water across the entire state originates as snow in the Sierra Nevada. As a result, there is a lot of interest in understanding how much snow falls in the mountains and what happens to it.
In mountains with seasonal snowpack, after the snow falls, it will slowly ablate. Ablation is the term used to describe the disappearance of the snow and can happen by a number of different processes. Most simply, and most commonly, the snow melts. However, another important process is sublimation in which snow goes straight to water vapor and escapes to the atmosphere as gas. There are a few different types of sublimation. It can occur directly from the surface of the snow (surface sublimation), when show gets caught in the trees (canopy sublimation), or when snow is blown by the wind (blowing snow sublimation).
But there’s snow much we still don’t understand
Despite the fact that sublimation can be an important process in determining how much of the snowpack will eventually be available as water for us to use, there is still a lot that we do not know about the role that sublimation plays in the disappearance of snowpack. In order to better understand the role of sublimation now and in the future, a team of scientists, led by Graham Sexstone, conducted a study where they measured and modelled snow sublimation. Specifically, they sought to answer the following questions:
- How much does sublimation contribute to snowpack ablation?
- How good are we at simulating snowpack sublimation?
- How much does sublimation change with different disturbances (disease, beetle infestation, etc.) and climate change?
In order to answer these questions, Sexstone and colleagues used both computer models and measurements taken at a field site. Their field site was an area of the Rocky Mountains in north, central Colorado. Within the field site, there were a number of different environments. There were alpine areas above the treeline, forested areas, and nonforested areas that were below the treeline. They measured snowpack levels, sublimation rates, and weather data at observation points across the field site for five years. They then took this data that was measured in the field and compared it to predictions for the same area made by a computer model. Finally, to assess the potential impact of disturbances and climate change, they ran the computer model again with these different scenarios to see how it impacted sublimation.
So where does the snow go?
In their data from the field site, the researchers found that there was a lot of variability in sublimation rates and type of sublimation process across the different environments and landscapes within the field site. On average, they found that 28% of the winter precipitation was sublimated, although this fraction varied with overall amounts of snowfall. In years with lower amounts of winter snow, the total amount of snow sublimated was lower, but represented an overall larger portion of winter precipitation. Even with this range of different outcomes they observed in the field, the research team still found that the results from their computer model were pretty good at predicting these observations from the field.
In the different future scenarios that were tested in the computer model, there were various impacts on sublimation. For both the climate change scenario and the bark beetle infestation (which causes tree death), there was only a slight decrease in the amounts of snow that was sublimated in each case. However, this decrease was not a result of lower rates of sublimation. In the case of the bark beetle infestation, they saw a shift in the type of sublimation processes that were occurring, as a result of there being fewer trees. In the climate change case, the researchers attributed the decrease in overall sublimation to a decrease in the amount of snowfall. So while both of these cases did not show a drastic change to sublimation, they did highlight some of the complexities in the environment that these scientists and others are still working to figure out.
Since the Olympics are over, now you can spend your time following snowpack levels!
Overall, this study showed that sublimation is an important process in terms of understanding what happens to snowpack. A significant portion of the winter snow in the mountains ends up being lost as water vapor and this process can vary a lot from place to place. As the impacts of climate change become more severe and water resources become more strained, it will become more and more critical that we can predict what will happen to the snowpack we rely on. Snowpack levels are often one of the first things that scientists, engineers, and policymakers look to when they are trying to predict water supplies since it’s that melting snow that flows downhill to support ecosystems, cities, and agriculture. Low snowpack levels can often be one of the first warning signs of drought, both because it means less water will become available from snowmelt as well as an indication that precipitation levels are below average. If you are interested in following along with the current snowpack levels, you can find up-to-date snowpack reports from the USDA’s National Water and Climate Center or, if you are specifically interested in California and like your science in daily tweet form, you can follow one of my favorite new accounts, @snowbot_SN. Currently, much of the western United States is experiencing below average snowpack, as you can see below. Some are still hoping for a “Miracle March” with lots of snow to make up the difference, but you’ll have to stay tuned to find out!