Effects of fire severity and burn patchiness on hillslope-scale surface runoff, erosion and hydrologic connectivity in a prescribed burn

Abstract

Fire severity and burn patchiness are frequently cited as important to post-fire surface runoff and erosion, yet few studies quantify their effects. A better understanding of their role is needed to predict post-fire erosion and design prescribed burns. Therefore, this study quantified the effects of fire severity and burn patchiness on surface runoff, erosion and hydrologic connectivity using 116 unbounded runoff samplers. The samplers were installed in recently prescribed burnt dry eucalypt forest in Victoria, Australia. Sediment loads over 16-months were approximately three orders of magnitude higher on burnt compared with unburnt hillslopes while differences in runoff and erosion between the low and high severity hillslopes were relatively small. Unburnt patches were often highly effective at reducing hydrologic connectivity from upslope burnt areas, with sediment loads over 16-months reduced by 1.3%, 98.1% and 99.9% downslope of 1, 5 and 10m wide unburnt patches respectively. Hydrologic connectivity was limited most effectively by wider unburnt patches (10m) and during lower magnitude storms. The results suggest overall that post-fire runoff and erosion may be substantially limited by unburnt patches while fire severity is a less important factor (within the context of prescribed burning). Consequently, post-fire erosion models should consider the spatial arrangement of unburnt patches, and unburnt patches (>10m wide) should be retained within prescribed burns to minimise erosion.