Nondestructive three-dimensional observations of flow finger and lateral flow development in dry snow using magnetic resonance imaging

Flow fingers develop due to the infiltration of snow meltwater or rain water into a dry snowpack. The width, spacing, and areal coverage of the flow finger path and occurrence of capillary barriers have been demonstrated using dye tracer experiments that were conducted in the field and in cold laboratories. Because dye tracer experiments require the destructive analysis of the snow, the initial shape, and the temporal development of fingering and lateral flows are unclear. To examine this, we carried out nondestructive observations of flow finger development, and the formation of capillary barriers using two different dry snow samples in a cold laboratory using magnetic resonance imaging (MRI) with a three-dimensional rapid imaging method. X-ray micro-computed tomography (mu CT) was also used to determine the relationship between the water flow phenomena and the snow microstructure. The experimental results showed that flow fingers developed in dry snow samples. The number of flow finger paths increased, and a lateral flow developed through the continuing water supply. The small discontinuity of the pore size and snow density in the vertical direction acted as a capillary barrier and induced lateral flow. These results showed that both the persistence of the flow finger path and the increase in the number of flow paths are important for understanding the infiltration of water into dry snow and for developing a snowpack model that includes water-moving processes. Nondestructive observation using MRI is a promising technique to follow the development of preferential flow path and lateral flow. Detailed information regarding the heterogeneity of the pore distribution and snow microstructures in a layered snowpack is required to understand the water flow phenomena in a natural snowpack.