  Hydrological models of karst aquifers are often semi-distributed, and
physical processes such as infiltration and spring discharge generation are
described in a lumped way. Several works have previously addressed the problems
associated with the calibration of such models, highlighting in particular the
issue of model parameter estimation and model equifinality. In this work, we
investigate the problem of model calibration using the active subspace (AS)
method, a novel tool for model parameter dimension reduction. We apply the
method to a newly proposed hydrological model for karst aquifers, LuKARS, to
investigate if the AS framework identifies catchment-specific characteristics
or if the results only depend on the chosen model structure. Therefore, we
consider four different case studies, three synthetic and one real case
(Kerschbaum springshed in Waidhofen a.d. Ybbs, Austria), with varying hydrotope
distributions and properties. We find that both the hydrotope area coverage and
the catchment characteristics have major impacts on parameter sensitivities.
While model parameters are similarly informed in scenarios with less varying
catchment characteristics, we find significant differences in parameter
sensitivities when the applied hydrotopes were different from each other. Our
results show that the AS method can be used to investigate the relation between
the model structure, the area of a hydrotope, the physical properties of a
catchment and the discharge data. Finally, we successfully effectively reduce
the parameter dimensions of the LuKARS model for the Kerschbaum case study
using the AS method. The model with reduced parameter dimensions is able to
reproduce the observed impacts of land use changes in the Kerschbaum
springshed, highlighting the robustness of the hydrotope-based modeling
approach of LuKARS and its applicability for land use change impact studies in
karstic systems.
