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What is SWE?

Snow Water Equivalent (SWE) is a common snowpack measurement used by hydrologists and water managers to gage amount of liquid water contained within snowpack. It is equal to the amount of water contained within the snowpack when it melts. It can be thought of as the depth of water that would theoretically result if you melted the entire snowpack instantaneously.  

SWE varies with the density of newly fallen snow. As a general rule 10 inches of newly fallen snow typically has about 1 inch of water content (10% density). For very light powder snow 1 inch of SWE could result in 25 inches of snow depth (4% density). Or for very heavy snow, 1 inch of SWE might total just 5 inches of snow depth (20% density).

Example: A swimming pool that is filled with 36 inches of new snow at 10% snow water density. If you could melt that snow, you would be left with a pool of water 3.6 inches deep. In this case, the SWE of your snowpack would equal 36" x 10% = 3.6 inches. 

Snowpacks increase in density through the winter. Densities in late March or early April are often 40% or more. At that time of year 36 inches of snow that is 40% density would have a SWE equal to 36" * 0.40 = 14.4 inches.