9. Conclusions
[69] 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.
[
70] 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.
[
71] 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.
9. Conclusions
[69] 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.
[
70] 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.
[
71] 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.
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