Quantifying model output uncertainty due to spatial variability of rainfall

Traditionally in the application of hydrologic/water quality (H/WQ) models, rainfall is assumed to be spatially homogeneous and is considered not to c ontribute to output uncertainty. The objective of this study was to assess the uncertainty induced in model outputs solely due to rainfall spatial var iability. The study was conducted using the AGNPS model and the rainfall pa ttern captured by a network of 17 rain gauges. For each rainfall event, the model was run using the rainfall captured by each rain gauge, one at a tim e, under the assumption of rainfall spatial homogeneity. A large uncertaint y in the modeled outputs resulted from the rainfall spatial variability. Th e uncertainty in the modeled outputs exceeded the input rainfall uncertaint y. Results of this study indicate that spatial variability of rainfall shou ld be captured and used in H/WQ models in order to accurately assess the re lease and transport of pollutants. A large uncertainty in the model outputs can be expected if this rainfall property is not taken into account.