Spatially variable wildfire impacts on sediment mobilization: event-scale evidence from eastern Australian catchments

Abstract. Wildfires may profoundly affect forested catchments by altering hydrological processes, soil properties, and sediment dynamics. While post-fire increases in water quality constituents are well documented, the underlying divers for such changes remain poorly understood. This study examines how fire and short-term hydrologic conditions interact to shape sediment dynamics using a novel Bayesian Hierarchical modelling framework. The analysis is at event-scale based on multi-year, high-frequency turbidity and streamflow data collected from 14 forested catchments in eastern Australia, which were burned to varying severities during the 2019/2020 Black Summer fires and affected by subsequent floods. Further, we explored how these effects vary across catchments with the extent of burning and other catchment characteristics.

Model results show that severe burning had a clear steepening effect on the slopes of event concentration-discharge (C-Q) in 5 out of the 10 severely burnt catchments studied, indicating enhanced sediment mobilization following extreme burning. The influence of short-term hydrologic conditions on C-Q slopes was comparatively minor, suggesting that fire effects dominated post-disturbance sediment responses. The magnitude of fire effects (as post-fire changes in the C-Q slopes) did not always scale directly with the proportion of catchment burnt, which also seems to be driven by the location of extreme burning and forest types. These findings provide a large-scale, multi-catchment understanding of post-fire sediment mobilization mechanisms, which can inform future improvement of modelling and management of sediment in fire-affected forested landscapes.