Item: Long-term observation of the water content of an alpine snowpack
Title: Long-term observation of the water content of an alpine snowpack
Proceedings: Proceedings of the 1994 International Snow Science Workshop, Snowbird, Utah, USA
Authors: Charles Fierz and Paul M. B. Fohn, Swiss Federal Institute for Snow and Avalanche Research, CH-7260 Weissfluhjoch I Davos, Switzerland, e-mail: email@example.com
Abstract: Relatively little is known about both the infiltration of water in subfreezing snow covers and its influence on the snow strength as well as on release mechanisms of wet snow avalanches. This is partly due to the fact that reliable measurements on wet snow are difficult to perform and therefore our understanding relies mostly on qualitative observations. To get more insight into the properties of wet snow, a quantitative knowledge of the water content in the snow cover has to be known over both a given area (up to now punctually) and longer periods of time. Indeed, the strength of wet snow is said to decrease significantly with a water content above 7% by volume, i. e. near the transition from the pendular to the funicular regime. Over the last few years, systematic measurements of the water content in both time and space were performed on the SFISAR study plot (2540 m a.s.I.) and its surroundings. Over 50 water content profiles representing 518 single measurements were taken and compared with the results obtained by the hand test; the correlation shows a significant discrepancy with the International Classification for Seasonal Snow on the Ground. However, water contents above 6% by volume were rarely measured and the daily variation did not exceed 1 to 2 % by volume. On the other hand, the long term observation of the water content shows the importance of the time elapsed since the snow cover first became isothermal. The quite different run-off patterns (5m2 Iysimeter) at the beginning ofthe melting period end ofApril in 1993 ( - 5 mm/day) and 1994 (- 50 mm/day) can be related to both the different structure and the history of the snow cover, influencing the ability of the snow cover to retain and transmit water. However, around 3 weeks after run-offs started to be continuous these differences were fading out. Finally, a few measurements were taken in the starting zone of a wet snow avalanche minutes after it had released. There the gliding plane was within the snow cover and water evidently ran along this failure plane although we could not record water contents above 7.2% by volume, corresponding to a pore volume saturation of 9,9% at a density of 323 kg/m3.
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Keywords: snow cover, snow strength, water content
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