Sediment availability on burned hillslopes

Erodibility describes the inherent resistance of soil to erosion. Hillslope erosion models typically consider erodibility to be constant with depth. This may not be the case after wildfire because erodibility is partly determined by the availability of noncohesive soil and ash at the surface. This study quantifies erodibility of burned soils using methods that explicitly capture variations in soil properties with depth. Flume experiments on intact cores from three sites in western United States showed that erodibility of fire-affected soil was highest at the soil surface and declined exponentially within the top 20mm of the soil profile, with root density and soil depth accounting for 62% of the variation. Variation in erodibility with depth resulted in transient sediment flux during erosion experiments on bounded field plots. Material that contributed to transient flux was conceptualized as a layer of noncohesive material of variable depth (d(nc)). This depth was related to shear strength measurements and sampled spatially to obtain the probability distribution of noncohesive material as a function of depth below the surface. After wildfire in southeast Australia, the initial d(nc) ranged from 7.5 to 9.1mm, which equated to 97-117Mgha(-1) of noncohesive material. The depth decreased exponentially with time since wildfire to 0.4mm (or <5Mgha(-1)) after 3 years of recovery. The results are organized into a framework for modeling fire effects on erodibility as a function of the production and depletion of the noncohesive layer overlying a cohesive layer.