In ResSim water quality simulation, reservoirs are modeled as a one-dimensional (1D) stack of vertical layers, while river reaches are represented by 1D longitudinally segmented cells. Mass conservation is the fundamental governing principle of a water quality model. The mass balance equation for a constituent within a control volume in both reservoirs and river systems accounts for changes due to transport (advection and dispersion), as well as physical, chemical, and biological (or biochemical) processes occurring within the system. The mixing coefficient included in the ResSim water quality model represents either a longitudinal dispersion or vertical diffusion coefficient, quantifying the degree of mixing between adjacent water quality cells.
Mixing coefficients are specified in the Mixing Coefficient Set Editor and can be either user-defined parameters or computed using empirical formulations provided by the model. Users create a mixing coefficient set linked to a geometry dataset (Chapter 4). Within the mixing coefficient set, users specify the vertical diffusion method for each reservoir in the linked geometry dataset and assign the dispersion coefficients for each reach in the dataset. Additionally, users define another important source of mixing in reservoirs, known as entrainment. This refers to the mixing that occurs as a stream flows into a reservoir and is also parameterized in the Mixing Coefficient Set Editor.
The prerequisite for creating and completing a Mixing Coefficient Set is the completion of the Geometry Dataset (Chapter 4). It's important to note that the Mixing Coefficient Set is not used in any water quality datasets directly.