HESS Opinions <q>Catchments as meta-organisms &ndash; a new blueprint for hydrological modelling</q>

Savenije, Hubert H. G.; Hrachowitz, Markus

doi:https://doi.org/10.5194/hess-21-1107-2017

Articles | Volume 21, issue 2

https://doi.org/10.5194/hess-21-1107-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Observations and modeling of land surface water and energy...

https://doi.org/10.5194/hess-21-1107-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 21, issue 2

Opinion article

|

22 Feb 2017

Opinion article |

| 22 Feb 2017

HESS Opinions Catchments as meta-organisms – a new blueprint for hydrological modelling

Hubert H. G. Savenije and Markus Hrachowitz

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Final revised paper (published on 22 Feb 2017)
Preprint (discussion started on 29 Aug 2016)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of the Opinion paper “How to make our models more physically-based”', Anonymous Referee #1, 27 Sep 2016
- AC1: 'Hydrological models accounting for living organisms', Hubert H.G. Savenije, 18 Oct 2016
RC2: 'Conceptual model or physically based model - this is not the question!', Erwin Zehe, 07 Oct 2016
- AC2: 'Hydrological models as living organisms', Hubert H.G. Savenije, 27 Oct 2016
RC3: 'Referee Comment HESS opinions: "How to make our models more physically-based"', Remko Uijlenhoet, 27 Oct 2016
- AC3: '“Modelling catchments as living organisms”', Hubert H.G. Savenije, 30 Oct 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by Editor and Referees) (08 Dec 2016) by Marc Bierkens

AR by Hubert H.G. Savenije on behalf of the Authors (22 Dec 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Dec 2016) by Marc Bierkens

RR by Anonymous Referee #1 (02 Jan 2017)

RR by Remko Uijlenhoet (17 Jan 2017)

RR by Erwin Zehe (03 Feb 2017)

Suggestions for revision or reasons for rejection

I congratulate Prof. Savenije to the revision of his opinion paper as well as to his very well-reasoned response to my assessment of his first manuscript. The author did a tremendous job in addressing my key points, particularly also with respect to better discuss the complementary merits and weaknesses of process based models and conceptual models.

Pattern formation during non-stationarity (ever flowing) environmental conditions and biota, which partly engineer the environment to build and sustain their niche are indeed cardinal challenges to both our theories and models. I particularly like the inspiring way how Prof. Savenije connects these points to the ground breaking insights of Jim Dooge (published more than 30 years ago) and his related reflections on the promise of energetic optimality. The latter might provide a key to connect and unify states based mechanistic thinking (Newtonian) and co-evolutionary (Darwinian) thinking and models. The analogy of a catchment to meta-organism is more than a well-chosen metaphor – it is a blueprint for a new model paradigm. In this respect I wonder whether a title like “Catchments as meta-organisms – a new blueprint for hydrological modelling” might not better reflect the content of this paper.
We need indeed dynamic model structures to cope with pattern formation and evolving/changing landscape characteristics which balance necessary storage, recharge and drainage. In this respect I acknowledge that the conceptual approach of a dynamic root zone provides an interesting alternative to the use of landscape evolution models the in eco-hydrology community. In a conceptual model framework it is indeed straight forward to speculate about suitable concepts for these issues in an ad hoc manner; more importantly the value of these concepts may be empirically tested within a predictive modelling exercise. In case of success we learn in a diagnostic sense that ecosystems optimize their root zone storage to survive a drought of a certain return period. From this finding we may further postulate that this might reflect an energetic tradeoff, as the plant has to perform work to grow roots. In this respect it is interesting to refer to the recent work of Hildebrandt et al. (2016), which provides evidence that plants minimize their energy expenditure during root water uptake.

In line with the authors I think that we will not make progress towards models which may deal with emergence and non-stationary catchments, when sticking too much to the established continuums approaches (which are also empirical), as they treat soil hydraulic properties as constant, the plant phenological cycle as invariant and which neglect kinetic energy in soil water flow. In fact soil properties and phenological cycles are dynamic states of the catchment as a meta-organism and kinetic energy of soil water is maybe the key to include preferential flow into our equations. Nevertheless, we may use physically based models either to test dynamic model structures/macropore systems and a dynamic phenology in a similar ad hoc fashion within an a posteriori predictive exercise (Loritz et al. 2016). The real challenge is to improve our models such, that we can deal with emergence in a predictive a priory manner, instead of showing that this is helpful after the fact. In this context it is interesting to acknowledge that the alternative route to success can also imply a more rigid physical treatment, instead of conceptualization. A good example is the direct numerical simulation of dune formation by Kidanemariam and Uhlmann (2014), which is based the least small of assumptions. Last not least I ‘d like to admit that the interesting concept of two water worlds the authors refer to is not so much out of the box, in fact it is a straightforward implication of soil physics and the soil water retention curve (Zehe and Jackisch, 2016).

Overall this work became an excellent and highly inspiring reflection of the main challenges environmental systems pose to environmental modelling and I see now reasons to hold this back from publication.

Erwin Zehe

References
Hildebrandt, A., Kleidon, A., and Bechmann, M.: A thermodynamic formulation of root water uptake, Hydrol. Earth Syst. Sci., 20, 3441-3454, doi:10.5194/hess-20-3441-2016, 2016.
Loritz, R., Hassler, S. K., Jackisch, C., Allroggen, N., van Schaik, L., Wienhöfer, J., and Zehe, E.: Picturing and modelling catchments by representative hillslopes, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-307, in review, 2016.
Kidanemariam, A. G., and Uhlmann, M.: Direct numerical simulation of pattern formation in subaqueous sediment, Journal Of Fluid Mechanics, 750, R2 10.1017/jfm.2014.284, 2014.
Zehe, E. and Jackisch, C.: A Lagrangian model for soil water dynamics during rainfall-driven conditions, Hydrol. Earth Syst. Sci., 20, 3511-3526, doi:10.5194/hess-20-3511-2016, 2016.

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ED: Publish subject to technical corrections (06 Feb 2017) by Marc Bierkens

AR by Hubert H.G. Savenije on behalf of the Authors (08 Feb 2017) Author's response Manuscript

Short summary

The natural environment that we live in is the result of evolution. This does not only apply to ecosystems, but also to the physical environment through which the water flows. This has resulted in the formation of flow patterns that obey sometimes surprisingly simple mathematical laws. Hydrological models should represent the physics of these patterns and should account for the fact that the ecosystem adjusts itself continuously to changing circumstances. Physics-based models are alive!