Abstract: Traditionally in the application of hydrologic/water quality (H/WQ) models, rainfall is assumed to be spatially homogeneous and is considered not to contribute to output uncertainty. The objective of this study was to assess the uncertainty induced by model outputs-runoff and sediment yield- solely due to rainfall spatial variability. The study was conducted using the SWAT model and the rainfall captured by a network of 24 rain gauges in Lushi Basin. For each rainfall event, the model was run using the rainfall captured by each rain gauge, one at a time, under the assumption of rainfall spatial homogeneity in the study area, A large uncertainty in the modeled outputs was resulted from the rainfall spatial variability. The uncertainty in the modeled outputs exceeded the input rainfall uncertainty. The uncertainty of simulated sediment yield outranges that of the runoff. The uncertainty of the simulated runoff increases with the spatial variability of rainfall. The more homogeneous of the rainfall, the more certainty of the modeled runoff. The uncertainty of the modeled sediment yield increased with the increase of spatial variability of rainfall, and the uncertainty of the modeled sediment yield is inversely related with the simulated sediment yield. Results of this study indicate that spatial variability of rainfall should be captured and used in H/WQ models in order to accurately assess the model outputs.In the application of H/WQ models, the assumption of the spatial homogeneity of the rainfall may not be valid. Spatial variability of rainfall introduces uncertainty into model outputs when uniformity of rainfall is assumed. Spatial variability of rainfall should be captured and used in H/WQ models in order to accurately assess the release and transport of pollutants. Since rainfall is a driving force behind many kinds of pollutant release and subsequent transport mechanisms, ignoring this property of rainfall in the application of H/WQ models will put a limit on the accuracy of the model results.

Key words: rainfall, runoff and sediment yield, SWAT, uncertainty, variability