Temporal changes in the slope–scale spatial variability of the shear strength of buried surface hoar layers

Determining whether the snowpack is becoming more spatially variable or uniform is important for accurate avalanche forecasts. Greater variability increases uncertainty in extrapolation and prediction. Our results offer a look at the evolution of the spatial variability of shear strength of two buried surface hoar layers in southwestern Montana, USA, over time. We studied the layers from shortly after burial until they were no longer the weakest layer in the snowpack. We selected study sites with planar slopes, uniform ground cover, and wind-sheltered locations. This simplified the comparison of the plots by minimizing initial spatial differences so we could focus on temporal change. Within each site, we sampled four 14 m × 14 m plots with more than 70 shear frame tests in a layout optimized for spatial analysis. At both sites, the layers gained strength at a rate that slowed as the layers aged. Although there was little change in the relative variability, absolute variability increased through time. Temporal change was more pronounced when the layers were younger and were gaining strength more rapidly. Additional tests at one plot suggested a correlation length, or the distance at which test results are related, for shear strength of just a few meters. At the other plot, the surface hoar layer collapsed during the initial sample. An initial dramatic decrease in shear strength occurred after this collapse followed by strengthening during that day and into the following day. Though we measured increasing absolute variability through time, uncovering changes in our other measures of spatial variability proved elusive. Developing methods and techniques for adequately characterizing variability, and temporal changes in that variability, will continue to be challenging.