Coupling water flow and solute transport in a catchment scalehydrological model

A physically-based distributed hydrological model simulating complex surface– subsurface flow and transport interactions is presented. The subsurface component is modeled by the three-dimensional Richards equation for flow and the classical advection- dispersion-reaction equation for transport, solved using finite element/finite volume tech- niques. The surface model is based on a path-based (rill flow) diffusion wave equation for both flow and transport, solved using a Muskingum-Cunge scheme. The path-based paradigm, together with Leopold and Maddock scaling relations for hydraulic parameter- ization, allow the same surface model to be used for both overland and channel dynamics. A novel approach for resolution of the interactions of water across the land surface, based on a boundary condition switching algorithm, is extended to the solute flux exchanges. The use of a high resolution finite volume scheme for the advective component of subsur- face transport introduces minimal numerical diffusion even in the absence of physical dis- persion. An application to the Abdul and Gillham sandbox experiment [1] is presented to illustrate the abilities of the model and to demonstrate the influence of surface–subsurface diffusive exchanges on the tracer dynamics of this particular system.