|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.
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.