MODELS AND MECHANISMS OF POST-FIRE EROSIONAL RESPONSE IN RANGELANDS: A CASE STUDY FROM THE BOISE FOOTHILLS

Larger, more severe fires and prolonged fire seasons are accompanied by an increased risk of fire-related hazards such as post-fire floods and debris flows. Our ongoing work to determine post-fire erosion hazards in the Boise Foothills of southwest Idaho is part of a larger project to inform citizens of the fire and fire-induced hazards that threaten residents living within the ‘fire-plain’ of the Wildland Urban Interface (WUI). Like many cities in the western US (e.g. Denver, Reno, Salt Lake City), the Boise WUI is located at the base of a mountain front. These vegetative communities are typified by shrubs (sagebrush steppe) and grasslands at lower elevations, with open forests at higher elevations and on north-facing slopes. Despite increasing WUI development concurrent with growing fire and erosion hazards in these ecosystems, most erosion and fuel models are not catered to these systems.

Using models developed by Cannon et al. (2010), we have identified several basins predicted to pose post-fire debris flow hazards in the Boise WUI. Our work, presented here, seeks to identify discrepancies in post-fire debris flow models that may arise as the result of applying models used in forested environments to areas for which they were not calibrated; namely the Boise Foothills and, more generally, rangeland ecosystems. We hypothesize that model predictions for the Boise Foothills overestimate the volume of sediment deposited subsequent to fire and that the mode of deposition will more often be sheetflooding rather than debris flows. Prior studies in Idaho have shown that lower severity fires burning on open grassland and sagebrush-steppe dominated slopes produce more frequent erosion events of lower magnitude (smaller debris flows and sheetfloods) compared to post-fire erosion following high-severity fire in forested landscapes, where sediment storage capacity is higher and debris flows predominate (Pierce et al., 2004; Weppner et al., 2013: Riley et al., 2015). Translating this understanding of rangeland post-fire erosion response to current predictive models is needed to better inform WUI residents of the hazards that threaten them at the intersection of rangeland and forest ecosystems.