Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/hess-2017-36
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Opinion article
24 Jan 2017
Review status
A revision of this discussion paper is under review for the journal Hydrology and Earth System Sciences (HESS).
HESS Opinions: The complementary merits of top-down and bottom-up modelling philosophies in hydrology
Markus Hrachowitz1 and Martyn Clark2 1Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, Netherlands
2National Center for Atmospheric Research, Boulder CO, 80301, USA
Abstract. In hydrology, the two somewhat competing modelling philosophies of bottom-up and top-down approaches are the basis of most process-based models. Differing mostly (1) in their respective degree of detail in resolving the modelling domain and (2) in their respective degree of explicitly treating conservation laws, these two philosophies suffer from similar limitations. Nevertheless, a better understanding of their respective basis (i.e. micro-scale vs. macro-scale) as well as their respective short comings bears the potential of identifying the complementary value of the two philosophies for improving our models. In this manuscript we analyse several frequently communicated beliefs and assumptions to identify, discuss and emphasize the functional similarity of the two modelling philosophies. We argue that deficiencies in model applications largely do not depend on the modelling philosophy but rather on the way a model is implemented. Based on the premises that top-down models can be implemented at any desired degree of detail and that any type of model remains to some degree conceptual we argue that a convergence of the two modelling strategies may hold some value for progressing the development of hydrological models.

Citation: Hrachowitz, M. and Clark, M.: HESS Opinions: The complementary merits of top-down and bottom-up modelling philosophies in hydrology, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2017-36, in review, 2017.
Markus Hrachowitz and Martyn Clark
Markus Hrachowitz and Martyn Clark
Markus Hrachowitz and Martyn Clark

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Short summary
Bottom-up and top-down modelling strategies in hydrology are the two endpoints in the spectrum of modelling strategies, mostly differing in their degree of detail in resolving the model domain. Given the limitations both modelling strategies face, we believe that to achieve progress in hydrological modelling, a convergence of these methods is necessary. This would allow to exploit the respective advantages of the bottom-up and top-down models while limiting their respective uncertainties.
Bottom-up and top-down modelling strategies in hydrology are the two endpoints in the spectrum...
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