|Review „Form and function in hillslope hydrology: In situ identification and characterization of flow-relevant structures” (revised version).|
By C. Jackisch et al.
Overall, the revised version is much improved. The authors have put a considerable effort in getting a clear structure such that the findings are not only presented side by side but follow a red thread that is obvious to the reader. Due to the improvement, the value of the large amount of presented data and results is now much more evident.
Nevertheless, there are still issue to be resolved to bring the manuscript into a form ready for publication. These aspects are listed below.
Terminology: Much of the manuscript deals with the distinction between structure(s) and heterogeneity. However, the actual meaning of these notions is not precisely defined (see for example in the abstract L. 11 – 12, p. 24, L. 25 - 26). As I read the manuscript, structure refers to flow paths (for fast transmission of water (and potentially solutes)) that connect through a large fraction of the flow domain while overall heterogeneity is considered to have only local connectivity. To me it seems essential to distinguish explicitly between situations where such flow paths are caused either by specific pore structures that differ from the other parts of the pore space (e.g., root channels, worm burrows) or whether the flow paths emerge as a results of locally heterogeneity in macroscopically homogeneous soils (see for example (Roth 1995)). Only in the first case, there is in principle the possibility to detect flow structures by local measurements (e.g., on soil samples) because one can infer (potential) connectivity based on the type of pore structure that is identified (e.g. a worm burrow).
In the context of this field experiment, this distinction leads to question whether or not observed flow paths at the plot or hillslope scale can be related to the existence of types of pore structures that are different (at a local scale) from the rest of the pore space. If such structures existed this could help to potentially generalize the findings also for other locations.
If I am right with my view, the authors should make the differences between what they call structure and heterogeneity more explicit and be also more specific in what others have already published on this issue. If my interpretation was wrong, there would also be a need for clarification.
p. 2, L. 3 – 4: Sentence too generic.
p. 2, L. 5 – 12: Too general, without insight into the topic of this paper. I cannot find the link between form and function in Wittgensteins Tractatus. So why to mention?
p. 2, L. 15: What needs to be revised?
p. 2, L. 16: Flurys experiments were not at the catchment scale but carried out in different soil types.
p. 2, L. 21 – 25: This paragraph is clear and helpful.
p. 4, L. 11: These responses are functions, not form, aren’t they? If not, what do you actually mean by functions as compared to form?
p. 5, L. 31: Why is it not Fig. 2?
p. 7, L. 15: Provide the dates of the experiments.
p. 7, L. 28: Sentence is not clear.
p. 7, L. 29: Why should this time be only depend on the duration of the input signal and be independent of the size of flow domain (imagine the same input duration for a small system where the input leads to steady-state across the entire system as compared to a large one where most of system has not yet been influenced by the input at the time of ending the input)?
p. 8, L. 2: Provide details of the quick sampler.
p. 8, L. 14 – 15: Skip it if you don’t use it for the paper.
p. 10, L. 12: Give the date of the experiment.
p. 10, L. 15: What is facilitated?
p. 10, L. 18: Before or after the natural rainfall? Please provide a schedule where all relevant events are clearly indicated.
p. 10, L. 29: The modeling was not introduced earlier nor is it mentioned later on. I suggest that you briefly compare the prior predictions (based on your prior knowledge of the system) to the actual results. Please also describe the prior assumption about the flow structure you implemented. This provides a very nice opportunity to demonstrate how you gained insight from the experimental work. You should do so even in case that the prior predictions were rather different from the actual outcomes – this would not be interpreted as a weakness (see for example (Holländer, Blume et al. 2009)). Did you consider the natural rainfall for the modeling?
p. 12, L. 27: How was the grid generated?
p. 13, L. 19: Why is this rainfall event only mentioned here?
p. 13, L. 26: According to Fig. 3 in the companion paper the irrigation is indicated after the runoff event. Please clarify.
p. 13, L. 29 – 31: The procedure is not clear. How can two similarity attributes show increasing similarity? Why is a decrease in similarity attributed to the irrigation? This part is essential to this paper but is rather obscure. It might help to have a figure in this manuscript where all relevant events are depicted on a time line and all relevant comparisons for this similarity calculations are demonstrated. Mathematical notations/equations might also facilitate the understanding.
p. 14, L. 5: I do not fully agree with this statement. Upon inspection, it seems to me that the Ks range is clearly the largest in the topsoil with a pronounced range decrease with depth. The data suggest that at the surface there are samples with very high and with very low values (e.g., due to macropores on the one hand, and due compaction on the other hand, respectively). A similar range decline seems to be related to texture.
p. 14, L. 27: Sentence not clear.
p. 16, Fig. 5: The profiles of the T and B locations seem to differ with regard to gravel content and depth of the periglacial material: what are the implications?
p. 16, L. 7: what is the positive bias in the analyses?
p. 17, L. 2: Please mention the recovery explicitly. It is almost hidden in Fig. 7.
p. 19, L. 1: What is inconsistent about the distributions?
p. 20, Fig. 9: I like these figures. The right panel indicates that the response of water content is faster than that of solute transport pointing to the displacement of pre-event water. This aspect should be explicitly discussed in the manuscript.
p. 20, L. 6: Does this remain unclear at that stage or after considering all of the results. Please clarify.
p. 20, 3.3.2: This section is difficult to follow all of the temporal comparisons. A figure with the time line would help.
p. 21, L. 1 – 2: Why should be a difference between connected flow paths and the irregular network of inter-aggregate pores (see (Roth 1995))?
p. 22, Fig. 9: How can one understand this figure without considering the natural rainfall?
p. 22, L. 1: What’s the overall response here?
p. 24, L. 4 – 5, 10: You say the properties were not expected for this soil texture but you provide an explanation, which is not very peculiar (network of inter-aggregate pores). So, is this pore network different from that of soils of similar texture?
p. 24, L. 13 – 14: Sentence not clear.
p. 24, L. 18: Why should all flow paths identified by their soil moisture response be stained? It seems that you confuse water flow and solute transport (see comment above on Fig. 9).
p. 24, L. 20: What are the point-sampling related issues?
p. 24, L. 22: Sentence not clear.
p. 24, L. 25 – 26: See comment above on terminology.
p. 24, L. 28 – 29: This sentence is rather obscure.
p. 25, L. 4 – 6: It would be useful to refer to some literature.
p. 25, L. 10: Which threshold?
p. 25, L. 17: Where can one see these discrepancies?
p. 25, L. 25: This is actually one of the essential questions here: are the flow paths due to structures which differ from other parts of the pore space in their properties. You suggest that they actually do, however such flow path may also emerge from local heterogeneity (see comment above).
p. 27, L. 30 – 31: What are these new ways?
p. 28, L. 4: As mentioned above, I have serious doubts that Wittgensteins work on formal logic really helps in the context of this manuscript.
p. 28, L. 5: This sentence is actually at the heart of the issue and I suggest you bring this aspect up already in the Introduction. Furthermore, there are ramification to what you conclude that should be discussed in more detail. You criticize – based on your findings - the concept of investigating separately relevant soil structures and their hydrological functions/responses. You suggest that irrigation experiments and time-lapse GPR measurements could solve this problem. While this holds true locally, your argument suggests a major problem for (hillslope) hydrology, which is that of generalization and spatial extrapolation. The rational why to investigate structure and functions at least partially separately is that (physical) structures are generally much more stable in time and relatively easy to observe as compared to hydrological functions. Hence, observations of structures allow for estimates about hydrological responses without carrying out for example irrigation experiments at all sites. If you claim that such a separation does not make sense your claim also entails the message that flow experiments are necessary everywhere to make statements about hydrological functions. I suggest that you discuss this aspect in more depth.
p. 29, L. 20: Conclusive with regard to what?
I think the manuscript has the potential for an interesting contribution to HESS but still needs to address a number of issues as listed above. Some of them actually got only visibly because the current version is much more readable than the original one.
Holländer, H. M., T. Blume, H. Bormann, W. Buytaert, G. B. Chirico, J.-F. Exbrayat, D. Gustafsson, H. Hölzel, P. Kraft, C. Stamm, S. Stoll, G. Blöschl and H. Flühler (2009). "Comparative predictions of discharge from an artificial catchment (Chicken Creek) using sparse data." Hydrology and Earth System Sciences 13: 2069-2094.
Roth, K. (1995). "Steady state flow in unsaturated, two-dimensional, macroscopically homogeneous Miller-similar medium." Water Resources Research 31: 2127 - 2140.