Watershed Delineation and Runoff Estimation using GIS and HEC-HMS Model in the Gwayi Catchment, Zimbabwe

Abstract

Climate change poses significant challenges to hydrological systems by causing erratic rainfall patterns and increased flooding. Understanding hydrological cycle patterns and changes are essential for effective watershed management. Hydrological modelling is widely applied globally to estimate hydrological behaviour. However, its application has been limited in the data-scarce Gwayi catchment in Zimbabwe. The aim of this study was to utilize the Hydrologic Engineering Centre Hydrologic Modelling System (HEC-HMS) model to delineate watersheds and predict the surface runoff at the sub-basin level in the Gwayi catchment. The HEC-HMS model was calibrated and validated over a period of five years in five sub-basins from 2017-2021 based on the completeness of the data. The model efficiency was evaluated using the Nash Sutcliffe Efficiency (NSE). During the calibration period, the NSE values ranged from 0.19 to 0.44, and during the validation period, they ranged from they ranged from -2.7 to 0.22. The trial-and-error method was used in calibration. The model predicted a total runoff volume of 102.19mm for the specified period. The analysis highlights the significant influence of land use, soil, and slope parameters on surface runoff generation because they were the most sensitive during calibration. Findings indicated that surface storage data is an important input in HEC-HMS to evaluate the model’s potential to accurately estimate discharge. This is because reservoirs play a critical role in regulating river flow and water levels and therefore have a substantial effect on the timing and magnitude of flows. The study findings contribute to the body of knowledge in hydrological modelling in the Gwayi and similar environment. It is recommended that further research be conducted at smaller scales, less than 5000km² to reduce model complexity and enhance precision.