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What is Snow Water Equivalent?

Snow Water Equivalent, or SWE, is a commonly used measurement used by hydrologists and water managers to gage the amount of liquid water contained within the snowpack. In other words, it is the amount of water that will be released from the snowpack when it melts.

Snow Water Equivalent is one of the primary measurements taken at SNOwpack TELemetry (SNOTEL) and snowcourses across the western United States. Most snowpack reports issued by the Natural Resource Conservation Service (NRCS) use SWE to summarize snowpack conditions for spring and summer water supply forecasting. 

Why not use Snow Depth?

Because we are primarily interested in water supply during the spring and summer, snow water equivalent is used as the standard for summarizing individual site and basin snowpack conditions. Snow Depth can vary greatly from hour to hour, due to settlement and compaction, but the amount of water contained within the snow remains consistent. 

How is Snow Water Equivalent Measured?

Common SNOTEL site layout. Snow Pillow, Precipitation Gauge, Snow Depth Sensor, Air Temperature

Snow Pillows - SNOTEL Measurements

There are over 830 SNOTEL stations across the western United States operated by the NRCS to measure snowpack, precipitation and a number of other parameters for seasonal water supply forecasting. These sites transmit data hourly for use by hydrologists, water managers, recreationists, and the public.

All SNOTEL sites have snow pillows, a large bladder filled with antifreeze and water, to measure the snow water equivalent of the snowpack. In most locations, SNOTEL sites replaced a snowcourse with a long measurement history.   

SNOTEL pillow and Depth Sensor. The Pillow measures Snow Water Equivalent (SWE) by weighing the snowpack on top of it. The Snow Depth sensor measures the depth of snow on top of the pillow using an acoustic signal.

Snow pillows have plumbing which runs into the SNOTEL shelter allowing for displacement of the fluid as snow accumulates in the winter. A specially calibrated sensor inside the shelter coverts the pressure into inches of water.  

As such, snow pillows are actually measuring the weight of the water in the snowpack through the winter. 

A typical SNOTEL site also has a Snow Depth sensor which allows users to also know the depth of snow that has accumulated on the pillow. 

A map of SNOTEL sites in the west can be found here. 

 

 

Common snowcourse transect

Federal Snow Sampling Tubes - Snowcourse Measurements

Federal Snow Samplers (formerly the Mt. Rose Sampler) have been used since 1909 to make measurements of Snow Water Equivalent at snow courses across the Western United States. 

There are over 1100 active Snowcourses in the western United States, and many have a long period of record (>50 years).

Taking a core sample of the snowpack

Snow surveyors from across the west visit these established measurement locations monthly and take measurements of snow water equivalent, snow depth and snow density. 

Snow courses are typically measured from January 1st - June 1st. 

 

 

How is a Snowcourse measured?

Snowcourses are typically 5 to 10 points that surveyors measure and then average. All points are measured and an average is created for all the points measured. 

The Snow Sampler is driven through the snowpack to the ground surface. This takes a core sample of the snowpack and the depth of the snow is recorded. 

A core sample of the snowpack is visible in the snow tube, and allows the surveyors to ensure that they have a good sample. A good sample core should be close to the depth of the snow they measured. Some compaction will occur from driving the snow tube through the snowpack

Weighing core sample to measure Snow Water Equivalent

The diameter cutter bit on the the snow tube is specifically sized so that 1 oz = 1 inch of water. A calibrated scale is used to measure the snow water equivalent of the core sample. 

Density for the snowpack is calculated by dividing the snow water equivalent by the depth of the snow.

14.0" SWE / 42" Snow Depth = 33% Density

Density can be used to measure variability across the snowcourse and is also used to quality control the samples taken.  

Once the snowcourse has been sampled the data is sent in to the responsible Data Collection Office and is entered into the database and is available to the public.

The value from the snowcourse can then be compared to the historical data to come up with a percentage of normal for that location on a particular date. 

A map of snowcourses in the West can be found here.

 

 

 

 

Additional Resources:

Additional information on the SNOTEL system network can be found here. 

A history of the Snow Survey and Water Supply Forecasting Program can be found here.

 

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