Development of a hillslope model for predicting erosion and water quality impacts of wildfire in SE Australia

Forest fire is known to lead to increases in sediment and nutrient yield from burnt catchments. A program of hillslope-scale field measurement was initiated following a severe forest wildfire in Victoria in 2003 to quantify the changes in site properties and erosion processes due to the fire. Experimental methods included rainfall simulation, overland flow studies, water repellence testing, concentrated flow erosion measurement, vegetation cover assessment, and ring infiltrometer measurements. [GRAPHICS] The results from a series of 100 mm h(-1) rainfall simulation experiments over a two year period are shown in Figure 1 and Figure 2, and indicate that; there is a large natural seasonal oscillation in water repellence and runoff generation ( from rainfall simulation), however burnt and unburnt sites generate similar rates of runoff in summer ie. fire does not result in large increases in repellence and runoff in summer strong water repellence persists into the wet season at the burnt sites, contributing to greater winter runoff generation ( from rainfall simulation) the soil infiltration capacity ( the maximum rate of water intake) remained very high 400-1300 mm h(-1)) throughout the study for both burnt and unburnt sites the sediment concentration of rainfall-excess overland flow increased approximately 20 fold following the fire and declined exponentially to pre-fire levels two years after the fire [GRAPHICS] The results were used to develop a conceptual model of the hillslope processes driving increases in sediment exports following the fire, including; a function to represent runoff generation from areas with spatially variable infiltration capacity; a function to represent the seasonal oscillation in water repellence and runoff generation, and post-fire modification to this seasonal pattern, and; a function to represent the change in interill sediment generation rates as a function of time since the fire. The unexpected conclusion from this study is that the source of elevated post-fire stream sediment loads in these catchments is not broadscale hillslope erosion. Rather, the very high-spatially variable infiltration capacities result in infiltration excess overland flow and associated erosion from only a small hillslope fringe adjacent to the stream edge.