Experimental analysis of preferential flow in dry snowpack

Preferential flow in snowpack is an important phenomenon and influences snowpack modeling, avalanche forecasting and runoff forecasting for snow-covered basins. However, the theoretical foundation is not sufficient to develop a snowpack model that includes a water movement process including preferential flow. The water ponding process, which is caused by the water-entry value of capillary pressure, is a key process for capillary pressure overshoot, water saturation overshoot and the formation of preferential flow in other porous media like homogeneous dry sand. We attempted to apply theories of preferential flow in homogeneous dry soil to initially dry snow. Infiltration experiments into homogeneous dry snow and theoretical analysis were carried out to reveal the developing process of preferential flow during infiltration into a homogeneous dry snowpack and to obtain critical conditions for that process. The capillary pressure was measured just above the interface between the dry/wet snow layer by the tensiometer. Concurrently we conducted experiments with a dye-tracer in order to observe the flow patterns. The capillary pressure overshoot and the water-entry value were observed in experiments for samples of a large grain diameter (> 0.25 mm). The capillary pressure overshoot was directly linked to the formation of preferential flow. The velocity criterion and the pressure head criterion which are used to predict preferential flow for dry sand were able to predict the preferential flow for three out of our four different samples. The velocity criterion failed, however, to predict the stable case (no occurrence of preferential flow) for the experiment with the smallest grain diameter and pore size. Good agreement experimental results and both criterions were obtained from the analysis. These results showed some similarities between the developing process of preferential flow in dry sand and in dry snowpack.

► We experimented the infiltration into homogeneous dry snow. ► The capillary pressure overshoot was observed in the coarser snow samples. ► It was directly linked to the formation of preferential flow. ► The critical condition of preferential flow in dry snowpack has been obtained. ► The developing process of preferential flow is similar to that in dry sand.