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United States Department of Agriculture

Agricultural Research Service

Title: High-Resolution Show-Water Equivalent Measurement by Gamma-Ray Spectroscopy

Authors
item Blard, W - UNIVERSITY OF WISCONSIN
item Helmke, P - UNIVERSITY OF WISCONSIN
item Baker, John

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 26, 1996
Publication Date: N/A

Interpretive Summary: The amount of water that is present in a snowpack is an important factor in predicting spring flooding and soil moisture recharge. However, it is difficult to measure with sufficient accuracy using current methods. We developed a method that uses a low-level radioactive pellet that is moved through a track beneath the snow while a detector mounted above the snow measures the intensity of gamma radiation at several different energy levels. At each energy level the extent to which the radiation is attenuated depends on the quantity of water within the pathway of the beam. This allows simultaneous solution of equations to estimate the amount of water in the snowpack. Tests show that the method is accurate and reproducible. Further hardware improvements should result in a system that can provide accurate information for estimating quantity of meltwater, which can in turn be used for a variety of purposes.

Technical Abstract: 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 gamma rays. A mixed Eu-152,154 source (about 70MBq) 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 an intrinsic 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 plus-minus 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 was about plus-minus 3 mm. Estimated energy balance terms were in reasonable agreement with observed melting during a field experiment. The new device will allow nondestructive SWE measurements to assess the influences of a number of agricultural management practices on winter hydrology.

Last Modified: 9/20/2014
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