Uncertainty in the model parameters due to spatial variability of rainfall

Most hydrologic/water quality (H/WQ) models that use rainfall as input assume spatial homogeneity of rainfall. Under this assumption this study assesses the variability induced in calibrated model parameters solely due to rainfall spatial variability. The AGNPS model and a network of 17 raingauges were used. Model parameters were estimated using rainfall observed at each gauge location, one at a time, as though that rainfall covered the entire catchment. A large uncertainty in the estimated parameters resulted from the spatial variability of rainfall. The uncertain- ty in the estimated parameters using the rainfall observed by a single gauge exceeded the rainfall measurement error. A large uncertainty in estimated model parameters can be expected if detailed variations in the input rainfall are not taken into account.