Snowpack is the snow that accumulates on the ground. When it melts as liquid water, it supplies much of the streamflow in streams and rivers, especially in a mountainous watershed like the Uncompahgre. Snowpack varies considerably over time and space according to local conditions like air temperature, relative humidity, wind, slope, and sun exposure. It is also affected by the characteristics of the snow like reflection (called “albedo”) and density.
Aerial or satellite images of snowpack are interesting to look at, but what do they tell us about the amount of water that snowpack supplies in the hydrologic cycle?
In this satellite image, snowpack is indicated as depth in inches for the central Rockies, including western Colorado. Source: NOAA
What is SWE?
A commonly used indicator of snowpack contribution to runoff and streamflow is the “snow water equivalent” (SWE). This is the amount of water held within the snowpack at a particular location. It consists of the depth of water that results when the snowpack completely melts.
The SWE is very useful in forecasting runoff and streamflow in a watershed. While it can be measured directly in the field at snow telemetry (SNOTEL) sites, it is often calculated from snow depth and bulk density measurements. In addition, aircraft-based snow surveys using LiDAR (light detection and ranging) technology provide the snow-depth data that can be used to estimate SWE variability for entire watersheds.
The graph above shows the SWE and snow depth at a SNOTEL location in the mountains near Red Mountain Pass during the 2022 and 2023 water years up until March 1.
As much as 90% of the yearly water supply in the Colorado Rockies is derived from the SWE in the snowpack. Water stored in the snowpack in the winter months is released as the runoff that eventually provides hydropower, irrigation, and drinking water. Changes in SWE can significantly affect agriculture, outdoor recreation, and the likelihood of wildfire activity. That is why it is monitored with increasing interest and accuracy today.