Author(s):
Karl Birkeland - USDA Forest Service Nat'l Avalanche Center
Kalle Kronholm - Montana State University
Kathy Hansen - Montana State University
Abstract:
Knowledge of the spatial variability of snowpack properties at the scale of individual slopes is critically important for assessing snow avalanche danger. Such variability evolves through time, and understanding that evolution will improve avalanche forecasting, enhance public safety and save lives. We have spent the last two years collecting field data in southwest Montana to better understand temporal changes in the spatial variability in snow, and we have developed a cellular automaton model to explore the effects of some of the possible changes. However, we require a theory for the temporal evolution of spatial variability. In essence, the snowpack goes through periods of stability and instability in response to current snowpack and weather conditions. We theorize that this change from an unstable to a stable snowpack is due at least in part to important changes in spatial variability that can be quantified with a variogram. The stabilization process is accompanied by an increase in the local (within a meter or two) variability due to areas of the weak layer that strain soften while adjacent areas age harden. This characteristic spatial divergence is shared with other nonlinear dynamical systems, like soils. The increase in local variability, independent of the average conditions on the slope, results in conditions that are less conducive to crack propagation and avalanche release than a more uniform slope. Quantitatively demonstrating this process with field data has proven to be difficult, but understanding the temporal evolution of snowpack spatial variability is vital for improving avalanche forecasting efforts.
