Measuring Microscale Mechanisms of Snow Melt, Rain-on-Snow, Drainage, and Imbibition Using Rapid Magnetic Resonance Profiling

The effective hydraulic conductivity of snow is highly impacted by dynamic microstructures and time dependent phase changes at the pore scale, yet these mechanisms have never been investigated at the pore scale. This inhibits improvements on the constitutive laws for larger scale models of snow hydrology using upscaling methods. We bridge this gap by using rapid profiling of liquid water content in snow with nuclear magnetic resonance methods at a resolution of 100 micron and 50 ms. The data shows that we can clearly measure saturation profiles and velocity fluctuations during snow melt and rain on snow events in a laboratory setting. We observe the transition from matrix flow to preferential flow in snow as well as compaction in wetted porous media. We compared the data to a model media consisting of sticky-glass beads of 1 mm. These media are static and do not experience pre-wetting and phase transitions. As a result, we observe that wetting fronts and drainage processes can migrate an order of magnitude faster in snow than in static model media with similar grainsizes. This is the first time that wet snow metamorphism such as compaction of wet snow and collapses of weak layers can be observed in real time at a resolution that reveals flow and phase-change processes at the microscale. This methodology has the potential to answer many open questions in wet-snow mechanics such as the onset of wet-snow avalanches and can be applied much broader in sea-ice metamorphism and multi-phase flow in porous media.