Influence of the first-order exchange coefficient on simulation of coupled surface–subsurface flow

Presently, there is little guidance for model users on the selection of the first-order exchange coefficient (FOEC; or “conductance”) commonly used in simulating surface–subsurface interactions (e.g. infiltration). In this study, relationships between the FOEC and surface–subsurface exchange flux, surface-subsurface head difference and time to initiate overland flow are systematically explored using 1D soil column simulations with the fully integrated code HydroGeoSphere. Numerical experiments adopt five different hydrological scenarios and nine different soil profiles. Results converge on the more accurate, but sometimes more computationally intensive, continuity of pressure (COP) coupling approach as the coupling length (le) parameter within the FOEC is decreased (i.e. FOEC increased). Threshold le values that produce results converged on the COP approach vary considerably with hydrological scenario, soil type and total obstruction height (Hs; accounting for sub-grid depression storage), with most threshold le values ⩽10−2 m. Lower le values are required for infiltration under Hortonian conditions, under non-Hortonian conditions in lower permeability soils, and to capture timing of initiation of overland flow. The condition le > Hs precludes top-down saturation under Hortonian conditions. Steady-state exchange flux and time to initiate overland flow are within 0.05% and 24%, respectively, of COP results when le = Hs = 1 mm. 3D simulation of a hypothetical catchment demonstrates that the general FOEC sensitivities obtained through 1D simulation are transferrable to the 3D case. This study shows that a value of le = Hs provides an appropriate initial value for modelling applications. We suggest a FOEC parameter sensitivity assessment on a case-by-case basis to ensure adequately converged results and to avoid unrealistic model behaviour.