How stable are inter-annual patterns in snow accumulation on glaciers? Results from ground-penetrating radar surveys on Wolverine and Gulkana glaciers, Alaska between 2013 and 2017

Snow accumulation can vary abruptly over short distances in complex terrain due to interactions between topography and meteorological processes, potentially hindering the viability of glacier mass balance methods that solely rely on limited point observations. We collected 10s of kilometers of 500 MHz common-offset ground-penetrating radar data (GPR) at two glaciers in Alaska, Wolverine (maritime climate) and Gulkana (continental climate), for five years (2013 - 2017). We used a terrain parameter based multi-variable regression to extrapolate snow water equivalent (SWE) across the unmeasured regions of the glacier. We found that this approach explains, on average, 75% of the variability in SWE derived from the GPR observations, with elevation providing the greatest explanatory power, followed by terms describing wind redistribution (Sb, Sx, curvature). Over the five-year interval, mean glacier wide SWE is more than twice as large at Wolverine (2.4 vs. 1.1 m. w.eq.) than Gulkana, in line with their respective climate settings. Both glaciers exhibit similar variability over this interval ( 0.3 m w.eq.), which is comparable to the variability observed at Gulkana over the 50 year USGS Benchmark Glacier program record, but only 1/3 of the long-term variability at Wolverine. We derived coefficients of variation to quantify inter-annual variability in spatial patterns, and find that 90% of the glacier area experiences less than 25% variability over this interval, with the greatest variability found in the lowest reaches of the ablation zone, where the surface roughness is high and many of the features are transient. This finding suggests that spatial patterns in SWE across the majority of these glaciers is stable from year to year, and that the absolute values scale with inter-annual variability in mean SWE. This has important implications for glacier mass balance work, particularly site selection for long-term observations.