9. Conclusions
In order to better understand internal catchment behavior, without the shortcomings of ‘‘virtual experiments’’, we adopted an approach that extracts information from real data in a more efficient way than is traditionally done. Our methodology is based on a combination of the
‘‘top-down’’ approach to model development, which is a framework for understanding catchment behavior based on data interpretation, and a ‘‘multiobjective’’ approach to model evaluation, which is based on the consideration that multiple measures of performance are needed to properly
extract information from the data. The modeling started with a basic model structure
applied to the Alzette catchment in Luxembourg. Subsequently,further refinements of model conceptualization were introduced and evaluated, initially in a lumped and then in a spatially distributed mode. We determined that model performance is particularly sensitive to the description of the state of wetness of the catchment. This may seem trivial, but we showed that the improved wetness strongly depends on the process of interception and on the distribution
of model internal states in conjunction with distributed rainfall input. These results are of interest to ongoing discussions on which there is little consensus to date. In fact, the interception process, although accounting for an important component of the water balance, is often neglected in modeling application, particularly in relation to hydrograph simulation. Regarding the spatial heterogeneity of rainfall, while theoretical studies with artificial data
show that it may have a considerable impact on catchment discharge, most applications using real data support an opposite conclusion.
Our results contribute to the debate on the relative merits of lumped versus distributed models, showing that fast catchment response benefits more from distributed modeling than slow catchment response. Processes depending on a large stock to flux ratio (e.g., groundwater flow, transpiration, percolation) do not require information on the spatial distribution of rainfall, whereas fast processes, such as interception and surface runoff do. This is due to the damping effect of the basin, which filters the space-temporal variability of the input signal and is larger for slow processes than for fast processes.