High-resolution snow-water equivalent measurement by gamma-ray spectroscopy

Frozen precipitation has important implications for water quality and soil biology. Nutrients in landspread animal manure are transported to surface waters by snowmelt, and winter survival of forages often depends on snow cover. Further development of mechanistic snow behavior models would be assisted by improved measurements of the disappearance of water from snowpacks. We developed a system to measure the total water content (snow-water equivalent, SWE) of a snow cover based on attenuation of γ-rays. A mixed Eu-152, 154 source (about 70 MBq) was pushed through raceways which were placed on the soil surface prior to snowfall. Attenuation of the emitted radiation by solid and liquid water in snow was measured with a Ge detector held above the snow and a multichannel analyzer. Use of four energy peaks and solution of the six resulting equations reduced dependence of the measurement on source-detector geometry. In laboratory tests, measurements of a fixed water depth (30 mm) were constant to ±1.5 mm following displacement of the detector by 50 mm laterally and 100 mm vertically, a much larger repositioning error than occurs in the field. Field tests showed that the system detected melting conditions with greater sensitivity than was attained with collecting of snow cores. Errors in estimated SWE due to repositioning of the detector were about ±3 mm. Estimated energy balance terms were in reasonable agreement with observed melting during a field experiment. The new device will allow non-destructive SWE measurements to assess the influences of a number of agricultural management practices on winter hydrology.