Hydrological Modeling of West Rapti River Basin of Nepal Using SWAT Model

Abstract

In this study, hydrological modeling of the West Rapti River, Nepal was carried out for estimation of runoff and sediment yield using the Soil and Water Assessment Tool (SWAT) model. The water balance components of the SWAT model, viz. precipitation, surface runoff, lateral flow, groundwater recharge, actual evapotranspiration and potential evapotranspiration, were studied. The SWAT model setup was carried out for simulation of discharge and sediment yield on a monthly basis for the years 2000–2013 (14 years). Calibration and validation of the model were carried out using SWAT-CUP with Sequential Uncertainty Fitting (SUFI-2) technique. The model was calibrated and validated for the years 2003–2006 and 2007–2009, respectively, using measured streamflow and rating curve generated sediment data. The model performed well for both calibration and validation periods. The model showed reliable estimates of monthly runoff (R₂ = 0.96, NSE = 0.95, PBIAS = 4.7 and RSR = 0.22) and sediment yield (R₂ = 0.71, NSE = 0.68, PBIAS = 15.10 and RSR = 0.57) for the calibration period. During validation period, model results were lesser than the calibration period flow runoff (R₂ = 0.78, NSE = 0.78, PBIAS = 5.3 and RSR = 0.47) and sediment yield (R₂ = 0.69, NSE = 0.69, PBIAS = −9.70 and RSR = 0.56). The water balance study revealed that evapotranspiration is more predominant and accounting 48.60% of the average annual precipitation falling over the basin. The annual volume of water available at the basin outlet is 4.5 billion cubic meters (BCMs). The average annual sediment yield of the basin is 17.67 t/ha/year, and the study area lied under high erosion class. Further, the validated SWAT model was also employed for evaluation of the best management practices (BMPs) in the study area.