Application of SWAT Model for Estimating Runoff of Nethravathi River Basin using Sequential Uncertainty Fitting Technique

Nethravathi river basin, located in Dakshina Kannada district of Karnataka experiences from water scarcity during summer, severe runoff and soil loss during rainy season. In the present study, an attempt was made to predict runoff from Nethravathi river basin using SWAT model for 36 Years (1970-2005). SWAT-CUP (SWAT-Calibration and Uncertainty Programs) was used to ascertain the model sensitivity, calibration and validation by Sequential Uncertainty Fitting (SUFI-2) technique. Both monthly and daily discharge data calibration was performed for the period from 1995 to 1999, and then validated for the period 2000–2005 using Central Water Commission (CWC) discharge data recorded at Bantwal station. Modeling results indicated that monthly time step yield better results than that for the daily time step during both calibration and validation. For monthly calibration, the R2 and NS values were 0.96 and 0.94 and for validation it was 0.91 each. On the other hand, for daily calibration, the R2 and NS values were 0.88 and 0.84 and for validation, it was 0.8 and 0.79 respectively. A simulation that exactly corresponds to observed data would be described by a P-factor of 1. The value of simulated results indicated that p-factor and r-factors during monthly and daily calibrations were satisfactory. The estimated average annual runoff is equivalent to 30% of average annual rainfall of the entire river basin. The runoff varied spatially from 774 mm to 1527 mm. The average annual runoff resulted from different land use, land cover patterns inferred that minimum runoff (1068 mm) was observed in the evergreen forest land and the maximum was in orchard and agricultural crop area (1394.1 mm). From the results of estimated runoff during the above normal, normal and drought years, it can be suggested that appropriate soil and water conservation structures are needed for the sustainable management of the study area.