Effect of Model Structure on the Uncertainty of Results From Watershed Scale Water Quality Models

Two lumped parameter watershed scale hydrology and water quality models (WATGIS and QUALGIS) which describe the nitrogen loadings at the’outlet of coastal plain watersheds were examined with respect to their accuracy and uncertainty of model results. Latin Hypercube Sampling (LHS) was applied to determine the impact of uncertainty in estimating field exports and decay coefficients on the uncertainty of the simulated nitrogen loads at the outlet of a 2950 ha coastal plain watershed in eastern North Carolina. Analysis showed that QUALGIS can better predict the outflows and nitrogen loads at the outlet of the watershed as compared to WATGIS. For both models, the uncertainty in quantifying the field exports has greater impact on the uncertainty of outlet loads than does the uncertainty associated with decay coefficient. The uncertainty of predicted outputs from both model are similar.