The effect of veld fires on the hydrological response of streamflow

Abstract

Veld fires are natural occurrences with the potential to impact thousands of hectares of vegetation, and in doing so, changes soil characteristics, for both urban and rural areas. It is therefore reasonable to assume that the hydrological response of a catchment could be affected by fire. The main aim of this research was to investigate the hydrological changes caused by fire on a catchment scale using a case study. On 9 March 2015, a wildfire which started in Jonkershoek nature reserve destroyed indigenous fynbos vegetation and afforested areas. Within the nature reserve, there are multiple rainfall and runoff stations, which provided a means of measuring any possible hydrological changes due to these fire events. Four catchments were used for this research, one main catchment (fynbos area) and three sub-catchments (afforested areas). Fifty-six percent of the main catchment burned, while two sub-catchments were completely burned and the other primarily unaffected by the fire. The main catchment’s hydrological response was analysed by comparing the hydrographs of comparable pre- and post-fire runoff events. Eighteen comparable events were used for the analysis. The mean runoff volume increased by approximately 7% after the fire and mean peak flow by 50%. The change was even more noticeable when comparing the two sub-catchments affected by the veld fire and the unburned sub-catchment with each other. All the sub-catchments were similar in size and were located close enough to each other to be represented by one rainfall station. Before the fire, the average daily streamflows between the unburned (control) and burned catchments were similar; however after the fire the average daily streamflow of the two burned catchments in comparison to the control catchment increased by 45% and 50%, respectively. The mean runoff volume from the two affected/burned catchments, after the fire, for individual events increased by approx. 72% and 52% in comparison to the control catchment. The mean peak flows increased by approximately 173% and 110% in comparison to the control catchment.