HYDROLOGICAL MODELLING OF MANAHARI RIVER BASIN OF NEPAL USING SWAT

Abstract

Manahari River basin is a minor class river basin of Nepal originating from mid hill of Central Nepal. It drains south west to East Rapti River and then Narayani River (called Gandak in India) which is the important left bank tributary of Ganges River. In this study, Hydrological modelling of the Manahari river basin (Area 429.68 km2) has been envisaged using semi distributed SWAT model for assessment of water availability. The key objective of this study is to develop a suitable model to represent hydrology of Manahari river basin for estimation of water balance components and to detect trend in hydro metrological parameters in the basin scale. ArcSWAT model simulation was carried out for discharge on monthly time scale for 15 years (1992 to 2006). The SWAT model was calibrated and validated for the years 1995-1998 and 1999-2001 respectively, considering observed stream flow data using SWAT CUP and sequential uncertainty fitting (SUFI2) technique. Calibration of the SWAT model was carried out using (i) p-factor which is the percentage of data lying within the 95% prediction uncertainty, and (ii) r-factor, which is the ratio of the average thickness of the 95% prediction uncertainty band and the standard deviation of the observed value of discharge. Furthermore, The Coefficient of determination (R2), Nash-Sutcliffe Efficiency (NSE), Percentage bias (PBIAS), and ratio of root mean square error to the standard deviation of measured data (RSR) were also used to evaluate the performance of model. For monthly flow, the value of p-factor and the r-factor for calibration were found to be 0.82 and 0.93 and for validation were found to be 0.77 and 0.64 respectively. The results obtained from the model calibration and validation showed reliable estimate of monthly stream flow (R2: 0.79, NSE: 0.78, PBIAS: -6.6 and RSR: 0.47) for calibration period. However, for the validation period, model performance was high as compared to the calibration period with parameter for flow (R2: 0.85, NSE: 0.83, PBIAS: 12.5 and RSR: 0.41). The SWAT statistical analysis/results indicates that performance of SWAT model is very good for the basin. Furthermore, the water balance study of the basin revealed that 33.62% of the average annual rainfall contributed to evapotranspiration. The annual volume of water available at the basin outlet is 572.89 MCM. The average annual sediment yield is 8.73 Mt/ha, which is moderate for basin average & lies under moderate erosion class. A trend analysis of annual and seasonal rainfall was also performed and no significant trend was found at the confidence level of 95%. This study would be useful for assessment of possibility of storage type project in the basin in terms of water availability and sediment yield and, in turn, in integrated water resources management and sustainable development of the Manahari river basin and Chitwan valley.