Evaluation of the swat model in simulating catchment hydrology : case study of the Modder river basin

Abstract

Hydrological models have become vital tools for understanding hydrologic processes at the catchment level. In order to use model outputs for tasks ranging from regulation to research, models should be scientifically sound, robust, and tenable. Model evaluation is therefore beneficial in the acceptance of models to support scientific research and to guide policy, regulatory, and management decision-making. The objective of this study was to evaluate the performance of the SWAT model in simulating stream flow for the Modder River Basin. The study area is situated at -29° 11’ latitude and 26° 6’ longitude at an elevation of 1335 m and drains a land area of 949 km2. The land cover is mainly grassland (pasture) with other minor land use types. The climate of the area is semi-arid with Mean Annual Precipitation (MAP) of 563 mm. Two techniques that are widely used in evaluating models, namely quantitative statistics and graphical techniques, were applied to evaluate the performance of the SWAT model. Three quantitative statistics, namely Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the mean square error to the standard deviation of measured data (RSR), in addition to the graphical techniques, were identified to be used in model evaluation. Results of calibration and validation of the model at a monthly time step gave NSE of 0.65, Pbias of 15 and RSR of 0.4, while NSE of 0.5, Pbias of 31 and RSR of 0.5 were recorded for validation. According to monthly model performance ratings, the model performed well during calibration and performed satisfactory during the validation stage.