By Emily Casey, Big Sky Watershed Corps Member, Gallatin River Task Force
True or False: 10 inches of new snowfall melts into 1 inch of water.
Let’s Investigate:
We know that snow and rain are two types of precipitation, or water that falls to the ground. Scientists call the amount of water stored in the snowpack Snow Water Equivalent or SWE for short. It can be thought of as the depth of water that would result from the snow melting instantaneously. We can use these measurements to forecast spring streamflow and predict floods.
Graph of accumulated precipitation and Snow Water Equivalent at the Lone Peak SNOTEL during the 2016 winter.
We can use the SWE equation to determine if 10 inches of snow really holds 1 inch of water:
Snow Depth (inches) X Snowpack Density (%) = Snow Water Equivalent (inches)
You might notice we are missing information for one variable in the equation: snow density. Snow density varies depending on the temperature and wind at which it snows, and how, when, and where the snow settles on the ground. For instance, new snow around Montana can range in density anywhere from 5% when its really cold to 20% at warmer temperatures.
Let’s see how snowpack densities 5% and 20% will affect our SWE:
Snow Depth (inches) X Snowpack Density (%) = Snow Water Equivalent (inches)
10 inches X 0.05 = SWE inches
0.5 inches =SWE inches
If new snow falls 10 inches at a density of 5%, the Snow Water Equivalent will be 0.5 inches.
Snow Depth (inches) X Snowpack Density (%) = Snow Water Equivalent (inches)
10 inches X 0.20= SWE inches
2 inches = SWE inches
If new snow falls 10 inches at a density of 20%, the Snow Water Equivalent will be 2 inches.
So now let’s get back to our original true or false statement: 10 inches of new snowfall melts into 1 inch of rain.
The answer? Occasionally, depending on the density of the snowpack.
Typical NRCS SNOTEL station with equipment to automatically measure temperature, precipitation, snow depth, and snow density.
Snowpack and precipitation data are collected regularly through a NRCS weather station program called SNOTEL. The closest SNOTEL site to Big Sky is at Lone Mountain. You can find other information about the Lone Mountain site here.