William H. McAnally

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Water resources modelers face the challenge of dealing with numerous uncertainties due to the lack of knowledge of the natural systems, numerical approaches used in modeling (equations, parameters, structures, solutions), and field data collected to set up and evaluate models. Propagation of parameter uncertainty into model results is a relevant topic in environmental hydrology. Uncertainty analyses improve assessment of hydrological modeling. There is a need in modern hydrology of developing and testing uncertainty analysis methods that support hydrological model evaluation. In this research the propagation of model parameter uncertainty into streamflow model results is evaluated. The Hydrological Simulation Program – FORTRAN (HSPF) supported by the US Environmental Protection Agency was evaluated using hydroenvironmental data from the Luxapallila Creek watershed located in Mississippi and Alabama, USA. The uncertainty bounds of model outputs were computed using the Monte Carlo simulation and Harr’s point estimation methods. Analysis of parameter uncertainty propagation on streamflow simulations from 12 HSPF parameters was accomplished using 5,000 Monte Carlo random samples and 24 Harr selected points for each selected parameter. The comparison showed that Harr’s method could be an appropriate initial indicator of parameter uncertainty propagation on streamflow simulations, particularly for hydrology models with several parameters.

A streamflow-driven gas-liquid transfer rate model was developed. This model was developed in terms of total surface renewal rate and gas diffusion coefficient. The total surface renewal rate was considered to be the sum of the surface renewal rates caused by the turbulence from the air-water interface and that from the water-bed interface. A general mixing length formula and a vertical fluctuation velocity formula was developed to formulate the surface renewal rate. Predictions of this model show good agreements with the measurements in previous studies and the predictions of Churchill's, O'Connor-Dobbins', and Owens-Gibbs' formulae. It was indicated in some previous studies that the existing streamflow-driven gas-liquid transfer rate formulae have limited application ranges. The model developed in this study can be applied for all the normal ranges of flow velocity and water depth in natural rivers, which generalizes and simplifies the applications.