MODELLING THE IMPACTS OF WILDFIRE ON SURFACE RUNOFF IN THE UPPER UBERABINHA RIVER WATERSHED USING HEC-HMS

Abstract

Fire significantly affects hydrological processes in the watershed because it changes land cover and it creates a double layer of hydrophobic soil covered with ash, increasing the surface runoff and the production of debris flow in the basin. Assessing the impacts of fire on overland flow requires the use of modeling softwares capable of simulating post-fire discharge. Because a total of 760 wildfires were detected in the Upper Uberabinha River subbasin in the last nine years, it is of dire importance to understand the consequential impacts of fire on hydrological processes in this basin. In this study, the HEC-HMS model was used to evaluate post-fire discharge in the Upper Uberabinha River watershed. Model was previously calibrated and validated using two representative storms observed in the wet season. After calibration, the 5-, 10-, 25-, 50-, 100-, and 200-year storms were simulated in scenarios with increasing burn severity. The calibrated model performed well in the prediction of discharge values at a daily basis (0% difference in peak timing; 0% difference in peak flow; 31.8% BIAS). Peak flow and discharge volume increased and peak timing shifted to the left as severity of burn increased. The highest increment in peak discharge was 74.7% for the 10-year storm, whereas overall discharge volume raised in up to 31.9% for the 50-year storm, both after simulation in the most fire-impacted scenario. The results reveal that fire highly affects hydrological characteristics, e.g. peak timing and flow and discharge volume, in the Upper Uberabinha River watershed. The authors suggest further investigations concerning the impacts of wildfire on other processes, such as the production of debris flow in the basin.