Towards improving subgrid surface flow hydrograph estimation using the relative surface connection function as a connectivity indicator

A major challenge in present-day hydrological sciences is to enhance the performance of existing distributed hydrological models through a better description of subgrid processes, in particular the subgrid connectivity of flow paths. The relative surface connection function (RSCf) was proposed by Antoine et al. (2009) as a functional indicator of overland flow connectivity. This function performs better compared to previous connectivity indicators and it can be potentially integrated in hillslope or watershed models as a descriptive function of subgrid overland flow dynamics. Nevertheless, several issues remained to be addressed, which was the subject of the present research. First, it was as yet unknown how changes in scale affect the RSCf and whether it can be extrapolated to other scales. We found that both scale effects and border effects affect the RSCf at the plot scale and a minimal scale to study overland flow connectivity was identified. The RSCf showed a great potential to be extrapolated to larger scales. Secondly, it was also unknown how the RSCf is affected by surface roughness and slope and whether these effects can be predicted. Results showed that the characteristic parameters of the RSCf are greatly influenced by surface roughness and slope. Based on a simple rectangular conceptualization of surface roughness, predictive equations relating the RSCf in function of slope and roughness were established. Finally, it was still unknown how the RSCf evolves after erosion processes and to what extent changes in the RSCf reflect changes in overland flow generation. The RSCf showed important changes in terms of both maximum depression storage and shape. These changes were found to be highly correlated to the delay and the rate of increase of the hydrograph, flow velocities continuity and erosion energy.