Impulse radar snow surveys - influence of snow density

Abstract

Snow cover water equivalent (SWE) is of major importance for planning of e.g. hydropower production in areas where a large proportion of the annual precipitation falls as snow. Radar technique can be used to determine SWE from the two-way travel time (twt) of a radar-wave propagation through a snowpack. SWE is usually related to twt through an empirical relationship, SWE = -b + a twt, where the values of a and b are determined by linear regression from simultaneous measurements of SWE (with snow-courses) and twt (with radar technology). In this paper a theoretical relationship between twt and SWE is developed showing the need for introducing the density when relating twt to SWE. Use of different empirical relationships for the real dielectric constant showed that the a-value for dry snow with a density of 350 kg m-3 (a typical value at the end of the accumulation season in the Nordic countries) is 0.040 m ns-1 (twt given in nanoseconds). When the snow density deviates considerably from this value a corrected a-value has to be used. A density of 300 and 400 kg m-3 gives a = 0.036 and 0.045 m ns-1 respectively. The b-value should theoretically be zero for measurements at the snow surface, non-zero values are probably due to the use of the direct wave between transmitter and receiver antennas as reference. The theoretically derived equations were confirmed by laboratory and field measurements as well as by measurements taken from literature.