In addition to inherent variability in falling snow, both wind redistribution and variations in radiation balance cause large spatial variability in the timing and location of input of snowmelt to the ground. However, even when these major drivers are accounted for, physically based hydrologic modeling is challenging, suggesting that there may be other processes controlling when and where snowmelt enters the subsurface. Most snowmelt models treat the snowpack as homogeneous, and assume that all meltwater moves vertically to the ground. Numerous experiments have shown that snowmelt follows a complicated path through the snowpack, moving both laterally along stratigraphic boundaries and vertically in concentrated cylindrical channels. While theoretical approaches have been developed to account for heterogeneities such as stratigraphy and vertical preferential flow paths, they are not applied in practice due to a lack of quantitative field-based measurements. We suggest that lateral movement of liquid water during melt and rain-on-snow events can be an important mechanism of water redistribution in steep, snow-dominated catchments. Flow of water within snowpacks is a complicated and poorly understood phenomenon. Permeability boundaries cause slope-parallel lateral flow over large distances, and coupled with cylindrical, vertical melt pathways, there is a resulting large spatial variability at the slope scale. An improved understanding of the lateral flow of water in snow will have impacts for a broad range of problems including: 1) rain-on-snow events which have the potential to cause flooding and landslides. The snowpack provides a large mass of water to melt, and may provide a faster route for the rainwater to enter streams. 2) A quantitative understanding of the timing and location of snowmelt within the seasonal snowpack is necessary in order to evaluate the importance of lateral flow on the seasonal hydrograph in snowmelt-dominated basins, and 3) quantifying lateral flow in snow is important for estimating the travel-time of meltwater traveling from polar ice caps and glaciers to the ocean which in turn is a basis for predicting time scales of sea level rise. Understanding the importance of lateral flow during rain-on-snow is important for forecasting floods, wet snow avalanches, and landslides, as mid-winter rain may become more common in a warming climate. This project will leverage state-of-the-art geophysical tools that already exist in-house at our institution to quantify lateral flow of water in alpine snowpacks. These include 1) measurements sensitive to microstructure which controls hydraulic conductivity, 2) rapid, non-destructive methods for measuring snow stratigraphy, depth, snow water equivalent, and liquid water content of both snow and the underlying soil, and 3) direct in-situ measurements of liquid water content. These measurements, combined with tracer experiments, will be used to characterize the snowpack conditions at the microscale that lead to lateral flow. The detailed snow and soil characterization will be used to estimate hydrologic properties for initial model conditions. Field data will be used to develop hydrologic models (using HYDRUS2D) of the experimental hillslopes to evaluate the relative importance of lateral flow in snow and soil in snowmelt-dominated catchments.
