Thinking About Thaws

I can hardly believe that the end of March is upon us already. That being said, I feel ready for spring weather.  March in Colorado often feels like a temperature toss-up, with some of us ready for spring to look like this:

Pansies. Image Credit: Amy Rice

Although in the mountains, spring more often looks like this:

Snow near Glenwood, March 2011. Image Credit: ECO

As ready as I am to thaw out my own chilly limbs, however, an early spring thaw isn't always a good thing in nature. Timing of thaws and snowmelt have impacts that last throughout the entire summer, long after the last of the snow has vanished. Some animals, like the wolverine, produce their young at the end of winter and need a lingering snowpack to insulate their dens and keep their young warm until air temperatures rise. Snow depth and date of snowmelt have broad impacts too, though, spanning the entire watershed.

Snow offers a form of water storage. It also keeps the ground underneath from drying out early in the spring. Once the snow melts completely in the spring, water begins evaporating from the ground as temperatures rise, and soil begins drying.  A deep snowpack saturating the soil in early spring creates an important moisture base to provide plants with water until the next heavy summer rain event. If you look at the graph below, showing rain events and soil moisture at three depths for the 2013 growing season, you can see that rain events--even big ones--rarely soak into the ground enough to have a large impact on soil at the 20 inch depth. This means that for the deeper soil layer, the amount of snow that soaks into the ground in the spring may determine moisture availability for the entire season. How saturated the soil becomes in spring also determines the base moisture amount for the 2 and 8 inch depths, although a heavy rain mid-season can help replenish soil moisture moisture at the shallower depths.

Soil Moisture at 3 depths and temperature for the 2013 growing season. Image Credit: AGCI.

Because snowmelt plays such an important role in soil saturation, the snow water equivalent (how much water you get if you melt the snow) on April 1st is sometimes used as an indicator for how much water will be available later in the year. If the snow melts rapidly or there is not much snow, then it may run off early or evaporate rather than soaking deep into the ground. This can have impacts for the entire water table, and thus for water availability in our streams and rivers.

Below is a graph showing the snow water equivalent on April 1st for an Independence Pass SNOTEL site in Colorado. The graph was created by AGCI for the City of Aspen using data from the National Resources Conservation Service. There has not been a strong trend in snow water equivalent since 1981.

Snow Water Equivalent on April 1st on Independence Pass in Colorado. Created using data available through NRCS.

Snowmelt date and snow depth also influence the risk levels for the fire season. The earlier the soil dries out, the sooner the fire season begins. Drier soil means less water available to plants. It also means warmer ground temperatures because when soil is dry, is is less buffered against temperature change, so it heats more quickly if the air temperature is high. This past winter we were up above 100% snowpack. With the combination of cold temperatures and deeper snowpack, the soil has thawed later than it did last year when the winter was dry.

The two graphs below show data from our Sky Mountain soil moisture station for this year and last year. You can see that soil moisture at the 2 inch depth thawed over a week later in 2014 than it did in 2013 (March 9th vs. March 1st, respectively). The thaw is visible as a sudden spike in soil moisture. The 8 and 20 inch depths that later in the season. Last year, the 20 inch depth began thawing around April 1st. As of April 1st this year, the 20 inch thaw has yet to happen. These graphs also reveal that the soil is starting out the spring with a higher moisture content this year than last year.

Soil Moisture data from Sky Mountain, Colorado. Image Credit: AGCI

In addition to the amount of snow we get each year, timing of runoff (the time of year when river flows are the highest) impacts human activities. Low snowpack, warmer temperatures, or a combination of both can lead to earlier than usual date of peak runoff. Farmers, hydro-electric plants, even municipal users also have ebbs and flows in the timing of when they use the most water, so timing of peak flow can be important to farmers, city planners, and others.

Changes in snowmelt timing also impact phenology: the timing of events in the natural world. Ecological impacts of earlier snowmelt are still being investigated by scientists. Warming average air temperatures across the globe (and here in the Roaring Fork Valley) may lead to snowpack melting sooner in the season. Once the insulating layer of snow is gone, and thaw begins, some plants to begin grow, even if the date is earlier than it has been in previous years. Of flowering plants that have been studied in the mountains, some begin growing and blooming earlier in the season as a response to early snowmelt. Others sprout from the ground earlier, but still bloom around the same time. Some flowers don't even begin growing early at all. 

The timing of plant bloom, seeding, and growth is important to the survival of animals that eat them and, indirectly, the carnivores that eat those herbivores. As a result, a shift in timing the timing can have ecological impacts that move from plants all the way up the food chain.

Snow may be an inanimate object, but its role in providing insulation and soil moisture have widespread consequences for the plants and animals living in this valley.