Simplified Modelling of Coupled Surface-Groundwater Transport Using a Subcatchment Mass Balance Approach

Abstract

Catchment models based on steady-state mass balances enable rapid assessment of contaminant fluxes and concentrations in rivers. However, such models often focus on surface drainage, without taking groundwater into account. This paper presents a novel steady-state mass-balance catchment model that includes groundwater. The model incorporates a conceptual reservoir under each surface subcatchment, with lateral subsurface exchanges between adjacent reservoirs and vertical exchanges between the reservoirs and the surface drainage network. This leads to an easily solved coupled algebraic system of equations. The approach is demonstrated for nitrogen in a meso-scale catchment in New Zealand. Exchange coefficients were extracted from a full groundwater model, while recharge sources were obtained from separate hydrological and leaching models. Other parameters such as decay coefficients were determined through calibration. Although the exchange coefficients are generated from a detailed groundwater model, alternatives such as simple groundwater models or phreatic contours could be used instead. The effective decay parameters were different from what was expected, which is partly due to the model structure (for example, the assumption of complete mixing in each reservoir), but may also be due to input uncertainty. The applications demonstrated the successful deployment of a novel, simple, fast-running and flexible coupled surface-groundwater model.