The Snow Water Equivalent (SWE) measures the amount of water in the snowpack and is a crucial indicator for water and resource managers to plan for water use. SWE is the depth of water that would cover the ground if the snow cover were in a liquid state. We use SWE as a measure of snowpack and snow season characteristics.

Why SWE Matters

Snow is an essential, life-sustaining part of the water cycle. When snow falls in the mountains, it forms a natural reservoir, storing water in the winter and then releasing it gradually through the summer melt season.

Mountain streams continue to flow throughout the dry summer months because they are usually fed by snow and ice stored high above during the previous winter. Everything living downstream of mountain snowpacks depends on water provided by melting snow.

Salmon and trout are very sensitive to stream conditions that are directly controlled by water from melting snow. Many different plants thrive on the nutrients carried by snowmelt water.

Humans also rely on snowpacks for a variety of reasons (consumption, agriculture, energy, etc.). Snowpacks provide a slow release of water to refill streams, lakes, aquifers, and reservoirs. Snow helps sustain these critical freshwater sources during dry periods.

How to Measure SWE

SWE can be measured in several ways, with snow pillows being among the most widely used instruments. Snow pillows operate by measuring the weight of the snowpack resting on their surface. This weight produces a pressure signal that is converted into an estimate of the snowpack’s water content, allowing SWE to be calculated.

Snow pillows are often associated with SNOw TELmetry stations (SNOTELs). SNOTELs can measure SWE, snow depth, temperature, solar radiation, and pressure. This data is used to understand weather conditions throughout the mountains and understand the quantity of snow and water over a watershed.

Snow sampling tubes provide a direct, manual method for measuring SWE. The tube is driven vertically through the snowpack to the ground, extracting a snow core with a known area of the tube or core. The snow core is weighed to determine its mass, which is converted to an equivalent volume of liquid water. Dividing this volume by the tube area yields the SWE for the sampled location.

A SWE tube holds a column of snow and is then measured to calculate the SWE.

The Natural Resources Conservation Service (NRCS) has been manually measuring SWE since the early 1900s. Starting in the late 1970s, nearly 300 SNOTEL sites were installed across the Western US, including Alaska.

• Other countries measure SWE, such as Canada and many European countries.

How to Report SWE

If you want to report a SWE observation or a snow depth observation, an easy way to do this is through CoCoRaHS. CoCoRaHS means Community Collaborative Rain, Hail & Snow Network. This network is a grassroots volunteer network of backyard weather observers of all ages and backgrounds working together to measure and map precipitation (rain, hail, and snow) in their local communities.

You can submit a SWE and snow depth observation, but also other weather events such as rain, hail, and ice accumulations. The CoCoRaHS webpage allows observers to see their observations mapped in "real time" and provides a wealth of information for data users.

How to View SWE

You can view SWE using the OpenSnow weather stations map overlay and selecting the SWE pulldown bar. This shows the available weather stations that measure SWE. Most of these weather stations are in the Western US because that is where the SNOTEL sites are located. Snowmelt is not as crucial in the Eastern US for summer water use compared to the West.

If you click on a specific weather station, you can view the station data and SWE data closely to view how it changes over 24 hours, 48 hours, 3 days, 5 days, and 7 days. 

Other ways to view SWE can be done through the NRCS, which maintains the SNOTEL network. The map below takes into account all of the SWE data and averages them across watersheds or basins.

The percent of average below is determined based on the SNOTEL's average over the course of its record if it spans 1991-2020. This time period of 30 years is used because it defines a climatological average. The average over that period is used as the median of that SNOTEL and can be used to understand how year-to-year SWE will compare to the average. This is important for water resources because it allows scientists to understand the hydrologic and environmental impacts of the snowpack.

Bottom Line

SWE is a snow metric used to understand how much water is in the snow. SWE is used to understand snowpack and snow season characteristics. Long-term data of SWE measured by SNOTELs is used to understand how year-to-year variability compares with a 30-year average. Finally, SWE is important because snow/water is essential for human and environmental sustainability.

Common Questions

• Does SWE relate to snow depth?
  • SWE relates to snow depth; the SWE generally increases with snow depth.
  • One SWE value will not always be associated with one snow depth value.
  • SWE changes based on the amount of water in the snow, not how deep the snow is.

• What SWE is good for skiing?
  • If you have a high SWE value, that means your snowpack is dense or deep. For a certain snow depth, the Pacific Northwest will have a higher SWE because the snow there is wetter. In Colorado or Utah, the SWE will be lower because the snow is lighter and drier.
  • For example, a snow depth of 30 inches might mean an SWE of 10 inches in Oregon, but only 5 inches in Colorado.
  • SWE relates to the snow-to-liquid ratio but is not a substitute for it. 

• Can I measure SWE?
• • Yes. To measure SWE, first measure the snow depth. Next, collect snow from a known area (for example, a small square container or a cylindrical tube) and allow it to melt completely. Weigh the melted snow (or measure its volume). Because melted snow is liquid water, its mass or volume directly represents the amount of water stored in the snow.
  • SWE is calculated as the depth of liquid water that results when the snow melts. If the melted snow is collected over a known area, the SWE is simply the volume of water divided by that area. When expressed in consistent units (e.g., millimeters or inches of water over the sampled area), this yields the SWE.
  • Don't forget you can report your SWE observation to CoCoRaHS.

Thanks for reading!

Zach Butler