Slope Effects on SWAT Modeling in a Mountainous Basin

Abstract

The soil and water assessment tool (SWAT) is a distributed basin model that includes the option of defining spatial discretization in terms of terrain slope. Influence of terrain slope in runoff results from mountain basins is a determining factor in its simulation results; however, its use as a criterion for basin discretization and for the parameter calibration has not yet been analyzed. In this study, this influence is analyzed for calibrations using two different cases. Ten discretization cases were carried out to evaluate the relative importance of slope discretization compared with other discretization criteria. Data from 1999–2005 were used for model calibration, and those from 2006–2009, for model validation. Parameter identification and specification were performed with the combined latin hypercube and one-factor-at-a-time (LH-OAT) and the shuffled complex evolution-uncertainty analysis methods (SCE-UA), respectively. All cases resulted in very good statistical values, with the Nash-Sutcliffe efficiency coefficient of 0.82–0.85, a bias of 2–10%, and the observations standard deviation ratio of 0.4–0.3. More realistic calibrated parameters were found when terrain slope variation was not included in the spatial discretization criteria. The inclusion of slope did not significantly improve simulations results when a good set of parameters was used, but it did enhance the calibration when a reduced number of subbasins was used.