Terrain and Canopy Controls on Liquid Water Content in a Continental Snowpack: Implications for InSAR Remote Sensing Techniques

Radar systems have been widely used to measure snow water equivalent (SWE) and Interferometric Synthetic Aperture Radar is a promising approach for accurately measuring SWE at high spatiotemporal resolution from spaceborne platforms. At lower frequencies (eg., L-band, C-band), radar propagates through the snowpack at a velocity determined by the dielectric permittivity of the snowpack. Dielectric permittivity estimates are a significant source of uncertainty in radar methodologies. In dry snow, dielectric permittivity can be estimated directly from snow density, which can be calculated from models or obtained from automated in-situ snow observation stations. However, wet snow dielectric permittivity is a function of both liquid water content (LWC) and snow density. The LWC, or the interstitial water in a snowpack, can cause spatiotemporal variability in dielectric permittivity. We used ground-penetrating radar (GPR), probed snow depths, and snowpit-derived densities to estimate SWE and LWC for seven survey dates at Cameron Pass, Colorado (~3120 m) from April to June 2019. For the survey dates when LWC was observed, we found that radar SWE-retrievals overestimated the mean observed SWE, with a maximum of 40% overestimation late in the melt season. This uncertainty is problematic for radar systems measuring SWE when LWC is present, but improvements to modelling the spatial variability of LWC can be made. We calculated leaf area index (LAI) from Sentinel-2 Level 2A imagery to characterize canopy cover and characterized the terrain using the aspect and slope products derived from the National Elevation Dataset. We found a consistent negative correlation (mean r² = 0.31 for four survey dates) between LAI and LWC. In addition, we found median LWC values within 0.4 vol. % for four survey dates at two sites with contrasting slope and aspect characteristics, one flat and the other sloping (12-20°) with southerly aspect. This highlights the important role of hillslope processes in draining melt water from the snowpack and the need for further research to better constrain these processes.