Spatial patterns of rainfall and shallow landslide susceptibility

[1] We quantify the effect of spatial patterns in climatological rainfall on shallow landslide susceptibility by forcing a physically based model of slope stability (SHALSTAB) with the rainfall pattern produced by a high-resolution atmospheric model (MM5) over the western Olympic Mountains of Washington State. Our results suggest that for two small basins in the Olympics, 10 km–scale variations in rainfall have a nontrivial effect on landslide susceptibility. Assuming uniform rainfall equal to the average rainfall over the basins results in a moderate underestimate of landslide susceptibility. Using climatological data from a lowland station to characterize the rainfall over the basins results in a substantial underestimate of susceptibility. The effect of spatial variability in rainfall on variations in stability is comparable to the effect of moderate-to-large variability in soil parameters (such as ±30% variations in soil thickness). At a practical level, these results imply that accounting for persistent patterns of rainfall may aid in discerning regions within the same watershed where similar land use practices will lead to differing landslide risk.