How Meaningful are Plot-Scale Observations and Simulations of Preferential Flow for Catchment Models?

Despite ubiquitous field observations of nonuniform flow processes, preferential flow paths are rarely considered in hydrological models, especially at catchment scale. In this study, we investigated the extent to which plot-scale observations of preferential flow paths are informative for rainfall-runoff simulations at larger scales. We used data from three plot-scale irrigation experiments in the Weierbach catchment (Luxembourg) to identify preferential flow parameterizations via a Monte Carlo simulation with HydroGeoSphere. Subsequently, we tested whether these parameter sets could be used directly to simulate the hydrological response of the Weierbach headwater with a HydroGeoSphere catchment model. The Monte Carlo simulations showed that the different depth profiles of Br-tracer observed in irrigation experiments could be reproduced when vertical preferential flow was simulated with a dual-permeability approach. However, it was not possible to identify unique parameter values for preferential flow. The direct transfer of a range of different dual-permeability parameter sets to the catchment model revealed that the variability of simulated hydrometric catchment responses (discharge and soil moisture over 18 mo) was independent of the variability among the three irrigation experiments. More importantly, the dual-permeability approach did not improve the match between simulated and observed discharge and soil moisture responses compared with the singledomain reference model, where multiple soil layers with differing hydraulic conductivities had already been implemented. This suggests that including structures that allow nonuniform lateral flow was more important for reproducing the hydrological response in the Weierbach catchment than the vertical preferential flow observed at plot scale.