INCORPORATING FIELD MEASUREMENTS OF FLOW AND SOLUTE TRANSPORT IN A FULLY INTEGRATED, SURFACE-SUBSURFACE CATCHMENT MODEL

Fully integrated, surface-subsurface hydrological codes allow for coupled simulation of both surface and subsurface processes. Studies coupling surface and subsurface water flow in fully integrated codes are increasingly common; however, coupling surface-subsurface solute transport has received much less attention. This work demonstrates how observations of stream flow and dissolved organic carbon (DOC) discharge from a wetland provided guidance on the development of a 3D, fully integrated catchment model. A sensitivity analysis is performed on the model to evaluate a range of transport conditions, including the surface-subsurface interface condition (e.g. advective exchange only, advection plus diffusion, advection plus full mechanical dispersion) and subsurface dispersivities. The catchment model compares well to measured solute discharge at the catchment outlet, and reproduces the observed trend of an increasing proportion of discharge from the wetlands in total stream flow, with increased stream discharge. Additionally, the model captures hysteretic behaviour between DOC concentration and stream discharge. This hysteretic behaviour was observed in the field data, although the characteristics of the model’s hysteresis loops (slope and distinction between wetting and drying) are somewhat different to the observed results. The study shows that the transport condition at the surface-subsurface interface affects the internal dynamics of solute transport within the catchment differently than dispersion within the subsurface. The addition of solutes to the fully integrated catchment model can help identify where the model may differ from actual catchment processes, however the model results are highly non-unique and produce different distributions of solute within the catchment despite agreement with solute fluxes at the catchment outlet. This work highlights the importance of obtaining and respecting field data when modelling surface-subsurface solute transport in fully integrated codes to help constrain the modelled solute transport solution.