J. E. VanderKwaak

E. A. Sudicky

Abstract A fully-integrated numerical model is presented which considers the flow of water and transport of multiple solutes on the two-dimensional land surface and in the three-dimensional, dual-continua subsurface, under variably-saturated conditions. Linkage between the various continua is through first-order, physically based flux relationships or through pressure head and concentration continuity assumptions. Full coupling of flow and transport is achieved by assembling and solving one system of discrete algebraic equations such that water and solute fluxes between continua are determined simultaneously. To achieve a high degree of computational efficiency, robust and efficient discretization and solution techniques are utilized. The numerical model was tested by simulating a controlled field experiment described by Abdul & Gillham (1989) involving coupled surface–subsurface flow and tracer transport in a small subcatchment at CFB Bordon, Ontario. Observed surface discharge volumes and timings arising from the application of artificial rainfall containing a tracer were simulated with reasonable accuracy using published or measured parameter values and minimal calibration. The observed dynamic response is shown to be a nonlinear function of surface and subsurface flow processes that are affected by subsurface permeability, surface roughness, topography, and initial conditions. Excess rainfall and groundwater seepage that flows overland initially, generate surface ponding within microtopographic depressions, including those located in the initially dry stream channel. The ponded surface water forms an internal, transient constraint on the porous medium pressure head near the land surface which is not reflected in solutions making use of traditional seepage face algorithms. The dominant streamflow mechanism deduced from the simulations is infiltration excess over an increasing contributing area, with the contributing area controlled by rapid response of the capillary fringe. Water originating above the initial water table overshadows groundwater contributions in this case.