Snow water equivalent

From HydroWiki

The snow water equivalent (SWE) of a snowpack is the volume of water per unit area that would result from the total melting of that snowpack. Thus, analogous to typical measurements of liquid precipitation (rain), SWE is given in units of length. Snow water equivalent is sometimes denoted by the symbolic term ‘h_m’ (melt height.)

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Importance:

The snow water equivalent of a snowpack is one of the most important measurements of a snowpack from a hydrologic standpoint because it is a direct measure of the liquid water content of a snowpack. When some or all of this water is released during the seasonal melt, it becomes an important component of local surface water and groundwater budgets. In some areas of the world (e.g. some regions of the western United States), snowmelt provides the majority of annual groundwater recharge, so monitoring of snow water equivalent of snowpacks is an important part of water resources management. Links to available SWE data from monitoring sites in the United States can be found below.

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Measurement:

The most obvious but impractical way to measure SWE is to melt a section of snowpack cut from a known area of ground surface (A) and measure the volume of the meltwater (V_m). Calculation would be as follows:

A more practical and popular way to measure SWE involves the use of a snow tube. The snow tube is pushed fully into the snowpack to measure the height of the snow (h_s), then it is carefully lifted with the snow inside and weighed on a calibrated scale which gives the SWE directly. The density of the snowpack (ρ_s) can also be calculated using this method using the following relationship:

where ρ_w is the density of water (1 g/cm³). More detailed instructions for snow sampling and snow tube measurement of SWE can be found in the links below.

Snowpack densities typically range from ~0.07 g/cm³ to 0.40 g/cm³, so the snow water equivalent of a snowpack is typically 1/14 to 2/5 the height of the snow. When snow density measurements are unavailable, a rough estimate of 1/10 is sometimes used to estimate the relative density of new snow.

For continuous measurements of SWE, a snow pillow is sometimes used. A snow pillow is simply a device deployed on the ground which is designed to weigh the overlying snow. However, because the internal strength of snow can relieve a significant amount of the mass from the base, snow pillows must be rigorously maintained (often consisting of cutting free the block of snow above the snow pillow.) SWE can also be measured with a warmed or anti-freeze equipped precipitation gage, although these gages can be difficult to set up and only measure precipitation input, not the SWE of a changing snowpack.