Snowpack and snow water equivalent (SWE)

Snow water equivalent

Snow water equivalent (SWE) is the depth of water you would have if you melted the snowpack in place, measured in inches. Snow density varies widely: a foot of fresh powder may hold an inch of water or less, while a foot of dense spring snow can hold several inches. SWE strips that variation out, which is why water managers use it instead of snow depth.

How it is measured: SNOTEL

The Natural Resources Conservation Service (NRCS) runs the SNOTEL (snow telemetry) network: automated stations at high elevations across the western United States — more than 850 of them — plus manual snow courses. At a SNOTEL site, a snow pillow weighs the snow above it with a pressure sensor, and the weight is converted into water equivalent. Stations also record snow depth, precipitation and air temperature and transmit their data, so readings are available every day of the year. NRCS's National Water and Climate Center publishes the data and uses it for water supply forecasts.

Percent of median

A SWE reading of 10 inches means little by itself. NRCS compares each day's SWE with the station's median for that date over a 30-year reference period (currently 1991–2020). 100% means a typical snowpack for the date; 70% means 30% below normal for the date. Two cautions:

• Early and late in the season, medians are small, so percentages swing wildly. A few inches above a near-zero median can read as several hundred percent.
• Percent of median for the date is not the same as percent of the seasonal peak. A basin can be at 120% of median in March and still finish below normal if April is warm and dry.

Why snowpack matters

Snowpack is Colorado's largest reservoir. Snow that builds from roughly October through early spring melts from April through June and becomes the spring runoff that fills reservoirs, sets how long rivers stay out of call and determines compact deliveries. Forecasters at NRCS and NOAA's Colorado Basin River Forecast Center use SWE, soil moisture and weather outlooks to forecast runoff volumes, and water users plan the season around those forecasts.

A big snowpack is not a guarantee: dry soils can absorb melt before it reaches streams, dust on snow and warm spells can make it melt early, and a dry spring can reduce runoff. A thin snowpack is a strong warning, especially for junior water rights and for basins that depend on a single season's runoff.

Reading one station

A single SNOTEL site describes one spot on one slope, at one elevation. Basin and division figures average several stations, which smooths out the quirks of any one site. When you look at a station, check its elevation and aspect, and compare it with neighbors before drawing conclusions about a whole watershed.

The water year

Hydrologists count years from October 1 to September 30. The water year that begins on October 1 is named for the calendar year in which it ends, so it lines up with a full cycle of snow accumulation, melt and late-summer low flow.

Other snow data

SNOTEL measures snow at points. NOAA's SNODAS model estimates snow across the whole landscape on a grid, combining observations with a model; it is useful between stations but is an estimate, not a measurement.

How BasinMind reports snowpack

BasinMind reads current SWE and the NRCS 1991–2020 median for the SNOTEL stations it lists in each water division, and reports the division's percent of median when stations are reporting. The live box above shows it for Division 5. Outside the snow season many stations read zero or have no median for the date, so BasinMind says so instead of showing a percentage. NRCS's interactive map is the full record.