Snow water equivalent (SWE) is the depth of water that would result from melting a given snowpack. It is a critical measurement for water supply forecasting in snow-dominated watersheds.
Snow water equivalent (SWE) represents the amount of liquid water contained within a snowpack if it were completely melted, expressed as a depth of water (typically in millimeters or inches). SWE is calculated as the product of snow depth and snow density, where snow density is the ratio of the mass of snow to its volume. Fresh snow typically has a density of 50-100 kg/m3 (5-10% of water density), while aged, compacted snowpack can reach densities of 400-500 kg/m3 (40-50% of water density). SWE is measured using snow pillows (fluid-filled bladders that sense the weight of overlying snow), snow courses (manual snow tube measurements at established points), and remote sensing techniques including passive microwave satellite sensors and airborne lidar surveys. In western North America, the USDA Natural Resources Conservation Service operates the SNOTEL (Snow Telemetry) network of over 800 automated stations that report daily SWE measurements. Mountain snowpack serves as a natural reservoir, storing winter precipitation and releasing it as meltwater during spring and summer when agricultural and municipal demands peak. In the western United States, snowmelt contributes 50-80% of annual streamflow. Climate change is reducing SWE in many mountain regions through warmer temperatures, earlier melt onset, and more precipitation falling as rain rather than snow.