WEC-C: a distributed, deterministic catchment model -- theory, formulation and testing

WEC-C is a distributed, deterministic, catchment-scale water flow and solute transport model containing a number of innovations not present in comparable models. For example, it allows for the imposition of catchment topological changes resulting from land uses such as surface mining. In addition. it features preferential vertical flow in the vadose zone modelled using a dual continuum approach. Numerically, it differs from many existing models in that it employs a direct linkage between the vertical and lateral solvers of its split- solver scheme and, due to its use of explicit solvers, is stable regardless of the form of the soil moisture characteristic curves. The WEC-C model framework is a rectangular grid of uniform cell size in the lateral plane combined with a system of soil layers, of variable thickness, in the vertical direction. Within this structure, the governing equations for flow and transport are discretised and solved. All parameters are defined locally in each computational cell so that all available data on catchment variability can be incorporated directly into the model. This paper describes the formulation of WEC-C, which is based on operator splitting with first- order accurate solvers for both the vertical and lateral flow and transport models. WEC-C was subjected to four stringent, synthetic tests designed to evaluate its accuracy by comparison with available analytical and numerical solutions. These showed that there were scale issues associated with the model, and that induced numeric dispersion of solutes could be significant. Nonetheless, it is suggested that WEC-C is still useful as a distributed catchment model.