Spatial and Temporal Variability of Temperate Snowpack Properties in northern Maine and southeast Alaska with Multi-offset Ground-penetrating Radar

Glaciers and surrounding snowpacks in mountainous terrain serve as freshwater reservoirs for nearby communities and ecosystems. Their presence and meltwater runoff support hydroelectricity, irrigation, drinking water, nutrient flux, and navigation. However, the amount of water held within these reservoirs, or the snow water equivalent (SWE), is difficult to quantify, especially in remote, often mountainous regions at mid to high latitudes. Current methods for estimating SWE in mountainous terrain are limited to point sources (e.g., SNOTEL sites), remotely sensed data (e.g. satellite, aircraft, unmanned aerial vehicles (UAVs)), or noninvasive ground-based geophysical techniques (e.g., ground-penetrating radar (GPR)). Notably, Maine and Alaska, USA, exhibit some of the greatest SWE uncertainties and subsequent meltwater runoff predictions. In the Chapter 2 pilot study, I investigate the spatial variability of snow depth, density and SWE using coupled in-situ snow pit and ~70 km of ultra-high frequency (UHF) 900 MHz common-offset (CO) and 1000 MHz multi-offset (MO) GPR surveys during a temperate northern Maine winter. CO GPR consists of a single transmitter-receiver pair, and relies on assumptions of radio wave velocity, density and liquid water content (LWC). However, the MO GPR system consists of multiple transmitter-receiver pairs, enabling the characterization of radio wave velocity from the raypath geometries and two-way travel times (TWTT) of the electromagnetic energy. I compare depth, density and SWE estimations from each radar system to the in-situ snow pit and find the CO GPR generally underestimates these properties, while MO was more realistic estimation. I also evaluated the influence on terrain variables (e.g., elevation, aspect, northness, and biomass) on GPR-derived snowpack properties to find a weak correlation, likely due to the conditions of the surveyed snowpack. In Chapter 3, I quantify the sub-seasonal spatiotemporal variability of snowpack properties from two repeat ~32-km-long 500 MHz MO GPR transects across the temperate Juneau Icefield system. The transects span from the low elevation marine-proximal ablation area on the lower T’aakú Kwáan Sít’i to the high elevation continental accumulation zone at the Matthes-Llewellyn-Tulsequah ice divide. With boundary conditions from firn cores, paired with advanced velocity analyses and dielectric mixing methods, I quantify the stacking and interval velocity, permittivity, dry and wet density, and LWC across the transect. Repeat surveys show variable ablation processes across the elevation profile, with meltwater infiltration into the firn below the equilibrium line altitude (ELA) and surface ablation with refreezing and densification processes above the ELA through to the firn. These results emphasize the important role LWC plays in estimating snowpack properties with MO GPR. These successful research campaigns evaluated the advantages and limitations of CO and MO GPR systems, constrained our understanding of the discrepancies between in-situ snowpack data and CO and MO GPR-derived snowpack properties across two different temperate snowpacks, and laid the foundation for future surveys across Maine and southeast Alaska.