Numerical simulation of the residence time distribution dynamics of a restored agricultural wetland under varying environmental conditions.

This work demonstrates the development, calibration, and use of the three-dimensional Environmental Fluid Dynamics Code (EFDC) hydrodynamic and mass transport model to simulate mixing patterns and study the environmental controls on the residence time distributions of a 1.3 ha agricultural wetland in central Iowa. Incorporating time-varying flow boundary conditions and atmospheric forcing, the model was calibrated against observed state variables, including water temperatures, basin hydraulic characteristics, and dye concentrations monitored at the outlet for six tracer studies conducted under varying flow and atmospheric conditions when submersed aquatic vegetation was mostly absent from the basin. EFDC reasonably reproduced observed basin internal hydraulics, temperatures, and mass transport dynamics, with mean absolute relative errors ranging from 0.02 to 16.3%. Sensitivity analyses suggest that wind shear exerts the greatest control on the modeled, and by extension observed, RTD for this system, primarily affecting measures of short-circuiting and, to a lesser degree, basin-wide mixing, particularly in the absence of atmospheric thermal forcing. Thermal forcing was found to significantly influence short-circuiting and mixing during warmer periods, with this effect being highly influenced by wind. Transient flows nominally influenced most RTD characteristics, save for mean and median residence times.