Abstract: Hydrological modeling is a key tool for analyzing the hydrological process of a river basin, but uncertainties in its parameters reduce the accuracy and applicability. This paper examines the influence of parameter uncertainties on runoff simulations, using a variance decomposition method of factor analysis along with a case study of the runoff calculated with the Soil and Water Assessment Tool (SWAT) model. The results show that under different floods, the influence of parameter uncertainties on the simulations is obviously different and it is larger in the case of lower flows than higher flows. From more to less sensitive to hydrological responses, the parameters can be listed as: fraction of main channel transmission losses partitioned to the deep aquifer, effective hydraulic conductivity of the channel bed, and saturated hydraulic conductivity of soil. Interaction of the first or second parameters with the third also produces a significant influence on the simulations. This study would help reduce the impact of the parameter uncertainties in hydrological modeling.

Key words: SWAT model, sensitivity analysis, uncertainty of parameters, variance decomposition method of factor analysis, flood process, Ziwu River basin