Unsaturated Water Flow in Snow:
Experiment and Simulation
Martin Schneebeli1
Unsaturated water flow in snow is an important factor for wet snow avalanche formation and
transport of nutrients stored in the snowpack. The formation of the often strongly preferential
flow patterns is not completely clear. Different mechanisms seem to be responsible for the initia-
tion of preferential flow patterns: (i) freezing of the infiltrating water due to subzero temperatures
of the snow, (ii) impermeable or partially impermeable ice layers, and (iii) capillary barriers, i.e.,
a layer with smaller grains above a layer with coarser grains. The snowpack in the Alps shows
rarely the strong subzero temperatures typical for subarctic and arctic snowpacks; however, water
flow seems to be preferential in most cases, even in an isothermal snowpack. The flow pattern
was investigated using dye tracers. Special attention was given to a minimal depression of the
freezing point of the tracers, because this can cause substantially different flow patterns. Two
main experiments have been run: (i) a rainfall in a slightly subfreezing snowpack, and (ii) a de-
signed experiment with artificial infiltration and subsequent application of three different dye
tracers. The flow pattern was detected by excavating long profiles. The snowpack was character-
ized using surface sections and conventional snow profiles, as well as water content measure-
ments. The simulation of unsaturated water flow was done with the two-dimensional finite ele-
ment code SWMS_2D (U.S. Salinity Laboratory) improved to take account of the changing hy-
draulic characteristics during infiltration. Spatial heterogeneity of the hydraulic properties was
simulated assuming Miller similarity. The experiments showed that capillary barriers seem to be
very important for the initial formation of preferential flow paths in a climate typical for the Alps
while ice layer interfaces seem to be less important. However, it could be observed that capillary
barriers often form at the location of meltfreeze crusts. Meltfreeze crusts are often very coarse
grained. It is supposed that meltfreeze crusts have often been interpreted as impermeable ice
layers instead of as a capillary barrier. The preferential flow paths visualized by the different trac-
ers have not been stable over one day, but took different routes. It is not clear if this is caused by
the not completely steady-state infiltration rate or by changing hydraulic properties due to wet
snow metamorphism. The preliminary results of the simulations showed a similar development of
flow patterns as observed in the field. The simulation of water flow in snow based on standard
snow profiles will remain a very difficult task. Much more experimental data is needed to im-
prove the relations between saturated and unsaturated hydraulic conductivity and the sorption de-
sorption curves. There is also a severe lack in understanding and experimental data of wet snow
metamorphism in partially saturated conditions. However, the experiments undertaken show that
the first occurrence of two hydraulically differing layers is extremely important to the onset of
preferential flow. This is of great importance to avalanche forecasting, because at the first inter-
face nearly saturated conditions appear in a continuous thin layer, conducive to mechanical fail-
ure.
1
Eidg. Institut fr Schnee und Lawinenforschung, Flelastrasse 11, 7260 Davos Dorf, Switzerland (E-mail:
schneebeli@slf.ch)
2
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