Effects of aridity in controlling the magnitude of runoff and erosion after wildfire

Abstract

AbstractThis study represents a uniquely high‐resolution observation of postwildfire runoff and erosion from dry forested uplands of SE Australia. We monitored runoff and sediment load, and temporal changes in soil surface properties from two (0.2–0.3 ha) dry forested catchments burned during the 2009 Black Saturday wildfire. Event‐based surface runoff to rainfall ratios approached 0.45 during the first year postwildfire, compared to reported values <0.01 for less arid hillslopes. Extremely high runoff ratios in these dry forests were attributed to wildfire‐induced soil water repellency and inherently low hydraulic conductivity. Mean ponded hydraulic conductivity ranged from 3 to 29 mm h−1, much lower than values commonly reported for wetter forest. Annual sediment yields peaked at 10 t ha−1 during the first year before declining dramatically to background levels, suggesting high‐magnitude erosion processes may become limited by sediment availability on hillslopes. Small differences in aridity between equatorial and polar‐facing catchments produced substantial differences in surface runoff and erosion, most likely due to higher infiltration and surface roughness on polar‐facing slopes. In summary, the results show that postwildfire erosion processes in Eucalypt forests in south‐east Australia are highly variable and that distinctive response domains within the region exist between different forest types, therefore regional generalizations are problematic. The large differences in erosion processes with relatively small changes in aridity have large implications for predicting hydrologic‐driven geomorphic changes, land degradation, and water contamination through erosion after wildfire across the landscape.