Post-fire hydrological response in an ENSO dominated environment

Elevated rates of erosion following wild-fires are often reported in the literature. The extent of the increase is usually related to the first rainfall events and sometimes to the prevailing rainfall-runoff conditions in the years following the fire. We set out to examine the rainfall-runoff response after several wild-fires over a 40-year period in the Nattai catchment, Sydney Basin Australia, in order to quantify the hydrological response. The fires differed in the total area burnt (14 - 530 km²) and hence the degree of disturbance to the Nattai catchment (701 km²) but wee chosen on the basis of having burnt the same area during each fire, thereby allowing us to compare between fires. Hourly instantaneous discharge from a gauging station in the lower part of the Nattai catchment was used to determine river flow and infer runoff. Daily rainfall totals from eight gauges throughout the catchment were used to determine rainfall events, and radar imagery was used to calculate rainfall intensity over fifteen minute time intervals for storms after the most recent fire in 2001-2. The discharge record showed a good relationship with daily rainfall and failed to indicate any change in the timing of runoff such as double-peaks from burnt and un-burnt parts of the catchment, during rainfall events within the first twelve months after all fires. A change in the rainfall-runoff response however, was noticed for the first rainfall events after three of the fires whereby the Nattai River only showed a small rise, despite average rainfall across the catchment of between 20-60 mm day-1. Analysis of rainfall intensity data after the 2001-2 fire revealed that the storm events were non-convective, resulting in broad scale rainfall with intensities of <10 min hr-1 over several days. In contrast, rainfall events with smaller daily totals but which showed a significant rise in the Nattai River were found to be the result of isolated convective storm cells with 15 minute rainfall intensities exceeding 10 mm. Hence, it appears that the extent and amount of erosion after wild-fires is determined by storm characteristics which govern rainfall intensity, rather than simply time after fire. This has significant implications for future wildfire management given predictions of more extreme climatic events associated with climate change.