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 depe nds on snow cover. Further development of mechanistic snow behavior mo dels would be assisted by improved measurements of the disappearance o f water from snowpacks. We developed a system to measure the total wat er content (snow-water equivalent, SWE) of a snow cover based on atten uation of gamma-rays. A mixed Eu-152, 154 source (about 70 MBq) was pu shed through raceways which were placed on the soil surface prior to s nowfall. Attenuation of the emitted radiation by solid and liquid wate r in snow was measured with a Ge detector held above the snow and a mu ltichannel analyzer. Use of four energy peaks and solution of the six resulting equations reduced dependence of the measurement on source-de tector geometry. In laboratory tests, measurements of a fixed water de pth (30 mm) were constant to +/-1.5 mm following displacement of the d etector by 50 mm laterally and 100 mm vertically, a much larger reposi tioning error than occurs in the field. Field tests showed that the sy stem detected melting conditions with greater sensitivity than was att ained with collecting of snow cores. Errors in estimated SWE due to re positioning of the detector were about +/- 3 mm. Estimated energy bala nce terms were in reasonable agreement with observed melting during a field experiment. The new device will allow non-destructive SWE measur ements to assess the influences of a number of agricultural management practices on winter hydrology.