Linking hydrodynamic and water quality models with different scales

Procedures have been developed to allow linkages between finite-difference hydrodynamic and water quality models having different temporal and spatial scales by defining appropriate values for the mean flow and dispersion coefficients. These procedures account for numerical dispersion, based on model formulation and solution parameters, as well as temporal and spatial averaging between different model scales, which introduce additional dispersion effects. Temporal averaging of velocity generates an equivalent dispersivity, which is estimated from the root-mean-square velocity fluctuation, evaluated over the averaging period. Dispersivity due to lateral and longitudinal averaging depends on the distributions of velocity and dispersion over the areas being averaged. The calculations are based on preservation of predicted mass distributions when different modeling scales ate used, and results from a specific case study are used to confirm the validity of these procedures.