On the implementation of the surface conductance approach using a block-centred surface–subsurface hydrology model

•The surface conductance (SC) approach is explored for rainfall partitioning in a block-centred code.•A discrepancy arises between saturation of the uppermost node and the head reaching the land surface.•Previous research suggests the exchange interface be represented by the uppermost half-cell in MODHMS.•Our work suggests that the previous recommendation prevents accurate simulation of rainfall partitioning.•Adding a thin surface layer improves model accuracy with minimal increase in run time. In physically based catchment hydrology models, dynamic surface–subsurface interactions are often represented using the surface conductance (SC) coupling approach. Guidance on SC parameterisation within block-centred codes is limited, and common practice is to express the SC coefficient as the quotient of the vertical saturated hydraulic conductivity and the half-cell thickness of the uppermost layer. This study evaluates the implementation of the SC approach utilising a popular block-centred, surface–subsurface hydrology model (MODHMS) to simulate one-dimensional infiltration experiments under Hortonian conditions. Results show that defining the SC coefficient based on a half-cell thickness of the uppermost subsurface cell inhibits accurate prediction of infiltration rates (qe) and the time to initiate surface runoff (tro) for the adopted rainfall–runoff scenario. Increasing the SC coefficient independently of the grid allows for accurate simulation of qe, but not tro. The addition of a thin layer at the surface is shown to improve model accuracy substantially, such that qe and tro approach those obtained using an equivalent mesh-centred model (i.e. where the surface and upper subsurface nodes are coincident). Whilst the addition of a single thin layer in block-centred codes allows improved prediction of surface–subsurface interaction, it does not provide a surrogate for fine discretisation throughout the subsurface that is necessary for accurate simulation of unsaturated zone flow. This study offers guidance on the implementation of the SC approach in a block-centred code and demonstrates the importance of systematic testing of parameters (that are otherwise calibrated) in physically based surface–subsurface hydrology models.