Evaluation of the SWAT model performance for simulating river discharge in the Himalayan and tropical basins of Asia

This study evaluated the Soil and Water Assessment Tool (SWAT) model performance for 11 basins located in two contrasting climatic regions of Asia: the Himalayan and the Southeast Asian tropics. A large variation existed among the case study basins in relation to basin size (330-78,529 km(2)), topography (377-4,310 metres above sea level) and annual rainfall (1,273-2,500 mm). Performance of the model was evaluated using R-2 and wR(2) for a low discharge event; Nash-Sutcliffe efficiency (NSE), R-2 and RMSE-observation standard deviation ratio (RSR) for high discharge events; and NSE, R-2, PBIAS, RSR, NSErel and wR(2) for the overall hydrographs. SWAT was found to be suitable for both climatic regions but yielded better performance in the Himalayan basins (NSE 0.72-0.81 at calibration) compared to the tropical basins (NSE 0.36-0.72 at calibration). Although most of the models underperformed in either low or high discharge events, a few of those remaining showed a balance between the extremes, proving that it is possible to achieve a balanced hydrograph with the SWAT model. The consistency of model performance across numerous Himalayan and tropical basins in the area confirmed the versatility and reliability of SWAT as a hydrological model and suitable tool for water resources planning and management.