Estimation of Streamflow Using SWAT Model under Climate Change in the Upper Wangchhu Watershed, Bhutan

Saran Pradhan, Naruemol Kaewjampa, and Piyapong Tongdeenok



The objectives of this study were to assess the suitability of Soil and Water Assessment Tool (SWAT) model to simulate streamflow and study the effect of climate change on streamflow in the Upper Wangchhu Watershed (UWW) in Bhutan. The model was set up using the Digital Elevation Model, land use, meteorological data and soil data. The data were collected from various agencies in Bhutan to setup the model on a monthly time step. The Sequential Uncertainty Fitting (SUFI-2) in SWAT-Calibration and Uncertainty Procedures (SWAT-CUP) was used for calibration and validation of the model. The results from model calibration (NSE = 0.76, R2 = 0.79, PBIAS = 15.80) and validation (NSE = 0.61, R2 = 0.66, PBIAS = -5.00) indicate that the model performance was satisfactory. The calibrated model showed that the average annual streamflow was 752.89 m3/s during 2003-2013, which was not significantly different (p > 0.05) from the measured average annual flow. The model also satisfactorily simulated the streamflow for a future climate change scenario RCP8.5. Compared to 2013, the streamflow in 2050 and 2070 is predicted to significantly decrease during the dry seasons (p < 0.01) and increase during the rainy seasons (p < 0.05). The climate change will affect streamflow in the UWW and the SWAT model is suitable for streamflow study in such a scenario. For future studies in the UWW with the SWAT model, it is recommended to have a better soil map, establish more hydro-meteorological stations, and also to include snowmelt, land use, and water removal processes as these can affect the SWAT output.