HYDROLOGICAL MODELLING OF KARNALI RIVER BASIN, NEPAL

Abstract

In this study, the Soil and Water Assessment Tool (SWAT) is used for hydrological modelling of the Karnali River basin (45,954 km2) Nepal. The main objectives of the study are: to test the applicability of SWAT model in the Karnali river basin, to carry out the water balance study of the basin and to determine the snowmelt contribution in the river flow. Further, sediment yield from the basin is also estimated. Snowmelt Runoff Model (SRM) is also used to compare the snowmelt runoff simulated from the SWAT model. Calibration, validation and uncertainty analysis of the SWAT model, both daily and monthly basis, is carried out with SWAT-CUP Sequential Uncertainty Fitting (SUFI-2) program. To assess the goodness of calibration, two measures are used: ) (I) p-factor which is the percentage of data captured by the 95% prediction uncertainty, and (II) d-factor, which is the ratio of the average thickness of the 95PPU band and the standard deviation of the observed discharge. In addition, Nash-Sutclifle Efficiency (NSE), Percentage bias (PBJAS), and Coefficient of determination (R2) are also used for the model performance evaluation. The value of p-factor and the r-factor for daily calibration (1993-2005) is 0.88 and 0.76 respectively and it is 0.95 and 0.83 respectively for monthly calibration. During model validation (1980-1989), p-factor is 0.89 and r-factor is 0.71 for daily time step and 0.93 and 0.71 respectively for monthly time step. Similarly, other statistical result also shows that performance of the SWAT model in the Karnali river basin is quite good. The study revealed that average annual runoff volume available at the basin outlet is about 47.16 billion cubic meter out of which about 12% of runoff volume is contributed by the snowmelt runoff. About 25% of annual precipitation (1481.30 mm) seems to be lost as an evapotranspiration. Long term average specific sediment yield from the basin is estimated to be 6,321 ton/km2/yr. Based on the results obtained in this study, it is concluded that both the hydrologic models, SWAT and the SRM can be efficiently utilized in the mountainous river basins of Nepal for planning and management of water resources.