Aspect Controls on the Spatial Re-Distribution of Snow Water Equivalence in a Subalpine Catchment

Abstract. Quantifying subalpine snowpack parameters as they vary through time with respect to aspect and position on slope are important for estimating the seasonal storage of snow water resources. Snow depth and density are dynamic parameters that change throughout the progression of the accumulation and melt periods, with direct implications on the distribution of Snow Water Equivalence (SWE) across a landscape. Additionally, changes in density can infer physical processes occurring within the snowpack such as compaction, liquid water pooling, and lateral flow. This study measures snow depth and density throughout a 0.25 km² watershed in northern Colorado USA using L-Band (1.0 GHz) Ground Penetrating Radar (GPR) and coincident depth probing. GPR snow densities were calibrated using bulk densities from snow pits and a SNOTEL station. A physical snowpack model, SNOWPACK, with input from local Remote Automated Weather Station and SNOTEL station produced models of snow depth, snow density, and liquid water content (LWC). The model simulations indicate mid-winter melt events produced LWC on the south aspect that are less present in the north aspect and flat areas. These midwinter melt events resulted in the lateral flow of LWC downslope, and the redistribution of SWE as observed in GPR surveys. Further observations show a steady increase of soil moisture throughout the winter in the flat terrain and ice layer formation on the south aspect snow pits during mid-winter surveys. Other key observations include pooling of liquid water at the base of the north aspect during the later spring season melt phase evidenced by pit observations and GPR transects. We further develop a conceptual model for the aspect controls on the distribution and movement of SWE during the winter and spring seasons. In summary, mid-winter melt events are observed on south aspects, causing a redistribution of SWE downslope while spring melt brings liquid water pooling at the base of north aspects. These differences in snowmelt dynamics based primarily on aspect, providing important processes to consider for spatially and temporally extensive SWE measurements moving forward.