Improving snow water equivalent estimates with ground penetrating radar: laboratory test of snow wetness influence on electrical conductivity of snow

Snow water equivalent (SWE) of annual snowpacks is of major importance for Scandinavian hydropower industry, since reliable predictions of snowmelt are needed for an efficient energy production. SWE over large areas can be estimated using ground penetrating radar operated from helicopters or snowmobiles from the two-way travel time of radar wave propagation through a snowpack. Radar estimates of SWE can either be based directly on empirical relationships with the two-way travel time, or calculated from snow density and snowpack depth, which can be measured manually at selected locations or estimated from the radar wave two-way travel time and propagation velocity in snow. However, it is known that presence of liquid water in a snowpack creates uncertainties, which for a typical snowpack with 5% (by volume) liquid water can lead to an overestimation of snow water equivalent by about 20%. It would therefore be beneficial if radar data could also be used to determine snow wetness.

Radar wave amplitude is reduced when the wave passes through a snowpack. This attenuation depends on electrical properties of snow: permittivity and electrical conductivity, which in turn depend on snow wetness. The relationship between wave attenuation and these electrical properties can be derived theoretically from Maxwell’s equations. The connection between snow permittivity and snow wetness and density follows an empirical formula known to be highly accurate. However, to be able to determine snow wetness from wave attenuation the relationship between electrical conductivity and snow wetness also has to be known. The present work attempts to establish this relationship experimentally.

A laboratory test was set up to study the relationship between snow wetness and electrical conductivity of snow. Three sets of radar amplitude measurements, two with “old” and one with new-fallen snow, were made on initially dry one-meter thick snow samples contained in a plywood box with cross-section area about 0.5 m2. Snow wetness was controlled by stepwise adding water to the snow in between radar measurements. Permittivity of snow was obtained in two different ways: estimated using Looyenga’s empirical formula for mixtures, and calculated from the radar wave one-way travel time and path length (both methods produced similar results). Electrical conductivity of snow was calculated from snow permittivity, snowpack depth, and radar wave attenuation.

A tentative relationship between electrical conductivity and snow wetness was found, but further tests including studies of the effect of variations of salt content in snow are needed to assess the generality of the result.

Author: Granlund, Nils

Source: Lulea University of Technology

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