Year-to-year changes in preferential flow development in a seasonal snowpack and their dependence on snowpack conditions

Water percolation into a snowpack can be classified as either uniform or preferential percolation through water channels. To expand the one-dimensional water movement model for simulation of multidimensional water movement in a snowpack, it is necessary to understand the conditions that generate the preferential percolation of water through the water channels in a natural snowpack. The Snow and Ice Research Center, National Research Institute for Earth Science and Disaster Resilience, has operated a multi-compartment-lysimeter (M.C.L.) (3 × 3 m in area; consisting of nine small lysimeters each of 1 × 1 m area) to measure the distribution of water discharged from the bottom of the natural snowpack since 2006. The measured data indicate strongly heterogeneous distribution of water discharge (SHD). Specifically, significant differences were observed between the discharges at each small lysimeter. Such SHD should be attributable to the preferential flow of water concentrated in water channels due to considerable horizontal water movement in the snowpack. We compared nine years of M.C.L. data with corresponding meteorological data and found that SHD often occurred under the following conditions: 1) during winters, when the maximum snow height was greater than the average maximum snow height over 30 years (1.4 m); 2) during the latter part of winter, specifically, the period after the maximum snow height was recorded; and 3) in the case of a large amount of water input from melting snow and rain. Moreover, we analysed the SHD conditions using snow pit data and found that SHD mostly occurred when there were strong differences in the capillary force between snow layers, such as that between fine and coarse-layered structures, as in the shallow part in the snowpack.