the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Routing stemflow water through the soil: a dual labelling approach with artificial tracers
Markus Flury
Jérôme Latron
Pilar Llorens
Abstract. Few studies have explored the stemflow double-funnelling phenomenon, although subsurface flow along roots and macropores plays a significant role in determining hydrological responses in forested catchments. In this study, a stemflow experiment on Pinus sylvestris L. (Scots pine) used artificial tracers to view and quantify the preferential flow of stemflow water after infiltration into the soil. Forty-one litres of water labelled with enriched deuterium and Brilliant Blue FCF were applied at a flow rate of 7 L h-1 to the stem of a pine tree, which corresponds to stemflow caused by about 50 mm rainfall. TDR probes were distributed around the tree trunk and in depth profiles to measure high-resolution volumetric water content. One day after the stemflow discharge, soil pits were dug in the different cardinal directions and at varying distances from the tree. Photographs were taken for imaging analysis to quantify preferential flow metrics. Soil samples were taken from the different profiles to analyse dye concentrations and isotopic compositions. We found that stemflow infiltrated through an annulus-shaped area around the tree base. We observed a heterogenous spatiotemporal soil moisture response to stemflow and the occurrence of shallow perched water tables around the tree trunk. Dye staining demonstrated that stemflow infiltrated primarily along the surface of coarse roots and also through macropores. The dye coverage was less extensive close to the soil surface and increased with depth and with proximity to the tree trunk. Lateral flow was also observed, mainly in the shallow soil layers. A set of metrics demonstrated the prevalence of preferential flow. Deuterium and Brilliant Blue FCF concentrations correlated with each other significantly. The concentrations decreased at increasing distance from the tree trunk, indicating dilution and mixing with residual soil water. Macropores, coarse roots (living or decayed) and perched water tables produced a complex network regulating the preferential flow. Our results suggest that stemflow heavily affects soil moisture distribution, and thus also groundwater recharge and surface runoff. Our study provides insights into the physical processes controlling stemflow belowground funnelling and improves our understanding of forest-water interactions.
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Juan Pinos et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2022-382', Alejandro Gonzalez Ollauri, 15 Dec 2022
Dear authors,
I found your study very interesting and sound. I particularly liked your experimental desing and the approaches followed to showcase the results. Well done! I think your study will be a good contribution to the field of stemflow and will animate discussion.
However, I am suggesting some major revisions of the text to help convey the message more clearly and to contextualise the study a bit better. I am also suggesting to slightly tone down some of the claims and to stress/clarify the limitations to the study. You will find my comments and revision suggestions in the attached document. I hope you do not find my comments too harsh, as it was my itnention to be as constructive as possible. You will also see that in some sections of the manuscript I am just highlighting text without providing any comments. I encourage the authors to pay attention to these and rephrase the text as appropriate.
I look forward to reading the revised version.
Best wishes,
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AC1: 'Reply on RC1', Juan Pinos, 10 Feb 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2022-382/hess-2022-382-AC1-supplement.pdf
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AC1: 'Reply on RC1', Juan Pinos, 10 Feb 2023
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RC2: 'Comment on hess-2022-382', Anonymous Referee #2, 28 Dec 2022
General comments:
The manuscript by Pinos et al. was interesting and pleasant to read. The study is methodologically sound and the manuscript is comprehensive. The results enrich the scientific discourse and fit very well within the scope of HESS. The figures are descriptive and well-chosen.
Despite the solid status quo, I think the manuscript would benefit greatly from major revisions on the discussion and related sections as described below. Furthermore, I address specific text passages under "specific comments". I hope that my input and investment will motivate the authors to put the finishing touches on their work, and I look forward to the revised version of the manuscript.
Title: The title would benefit from rephrasing toward being more precise by naming the main outcome of the study.
Abstract: The abstract does not yet do justice to the manuscript. It should not only contain the most important information, but also highlight the strengths of the study. Instructions for a good abstract can be found, for example, in "How to construct a Nature summary paragraph" and in Jean-luc Doumont’s work (e.g. https://www.principiae.be/pdfs/UGent-X-003-slideshow.pdf). I recommend to tackle the revision of the abstract as the last step to benefit from the improvement of the discussion.
Discussion: The discussion overall appears somewhat unfinished and incoherent, and the main messages are not clear. Basically, the necessary elements are contained, but the discussion lacks elaboration of the key process-oriented findings. In addition, the discussion of methodological aspects could also be improved by not only evaluating the methods’ success, but by highlighting and framing observations based on methodological strengths and weaknesses. This could take the authors farther away from a methodological and case study. In this regard, the authors should also rethink the structure of the discussion, which in the current version is oriented toward result groups. Instead, they could formulate stronger contextual subsections, for example on the three main findings (messages), and then summarize the results that support those messages. A mixture of contextual and methodological subsections is also conceivable (however, the most important content-related insights should be clearly recognizable). The restructuring would also allow for more meaningful subsection headings, which I recommend (as for the manuscript title).
Conclusion: As the conclusion depends on the discussion, it also needs additional work. See in the specific comments for more detail.
Specific and technical comments:
- 8: Start with the motivation instead of the research gap.
- 12: “50 mm of rainfall”
- 21: “A set of metrics” sounds somewhat diffuse. Either specify or be more general (e.g. “analyses”)
- 24: Delete “heavily”
- 32: “Stemflow…” This sentence is difficult to read and the wording should be improved. In such cases, I will only write "rephrase" in the following.
- 37: “overland, preferential or subsurface flow” This combination is odd to me. Please reconsider what your point is.
- 42: Naming something a key topic, more citations should be given to prove that.
- 47: “Several experimental techniques both invasive and non-invasive…”
- 47: “the double-…” As this study is about the belowground part of double funneling, I find using the term in such context confusing and imprecise. Please keep this in mind throughout the manuscript.
- 76, 77: “catchment”
- 80: delete “throughout the year”
- 80: “spring and autumn being the wettest season”
- 88: delete “respectively” (“…stand density is x and basal area is y.”)
- 89: “representative” is a statistical term which should rather not be used out of statistical context. If it was indeed a statistical process leading to the selection of the tree, describe this process. Otherwise, I recommend using “exemplary” instead. Some words on the selection of the tree would be interesting also in that case and make the matter more clear.
- 109: The geometrical details of the installation remain unclear. Why are the depth intervals different? How were sensors installed vertically at depth?
- 112: “measure” instead of “sample”
- 124: delete “what was done”
- 136: Please check with HESS if this is the way they want online sources to be cited.
- 148: delete “closer to the tree”
- 149: delete “throughout the day”
- 151: delete “roughness”
- 154: replace “, and then transported to the laboratory” by “for transport”
- 156: The justification is unnecessary because it is obvious and can be omitted. If desired, an abbreviated version can be retained in subordinate clause. Either way, the procedure should always be given first and then the reason, not the other way around.
- 159: What about the used camera? Since the photos had to be digitized, was it not a digital camera?
- 167 and following: I would prefer using correct mathematical symbols and equations, meaning that one metric can not be symbolized by several letters (e.g. As instead of TSA). In any case, however, the quantities used in equations should be consequently printed in italics also outside of the equations.
- 200: “extracted by cryogenic extraction” repetition, rephrase
- 206: “on the area…” very diffuse formulation, maybe just too complicated, rephrase
- 207: “no relevant…” What does that mean, what are the implications for this study? No change in soil physical properties does not mean that no differences in hydrological processes will occur or nothing interesting will happen below this depth?
- 209: One steel cylinder sample per depth interval and profile?
- 220: Outliers are statistically defined. “Obvious outliers” do not exist. Please give more detail about the process.
- 223: Why was Rosetta chosen? There has been much discussion about the problems of PTFs and work invested on how to improve them. Using them, this should be addressed, especially when choosing maybe not the best suited PTF.
- 233: Please use WRB instead of US SoilTax. When using a classification system, do not forget the reference.
- 235: “As…” rephrase
- 238: Please elaborate which metrics were attained using Rosetta and which measured directly and also discuss the values within this context.
- 245: Please refer also to the amount/intensity of applied stemflow and the tree and soil characteristics it refers to in order to compare in an informed way.
- 246-247: Please, here also, give more details about the studies.
- 251: It would be great to have some measure of variance to being able to characterize and compare.
- 258: “earlier” instead of “rather”
- 259: Please set the maximum difference in SWC in relation to pre-stemflow values.
- 264 ff.: Pleas give absolute additional to relative values.
- 270: "…driven by throughfall rather than stemflow during a stemflow dye tracer experiment using natural rain events" or similar - give detail about the study!
- 270: “Metzger…” Compare also Metzger et al. (2021) on stemflow infiltration areas - maybe that study would fit better here.
- 270: “during rainfall events” Not during, but after rainfall. I think there is no explicit information about SWC during rain events, only about SWC increase, which was a little higher in the vicinity of tree stems depending on event size. Also, "vicinity" is defined as <1 m, which differs from the measure you seem to apply here (not defined explicitly).
- 272: “because…” This is a good thought. Maybe you can find more references to compare stemflow infiltration with and without throughfall and go into more detail here.
- 276: “Thus,…” This is also an interesting point, which you should set in relation to the results of the other studies you mentioned above and the root architecture of their studied trees (being deciduous hardwood species). You should also check for other studies on (Scots) Pine and compare their results for water redistribution in the soil (e.g. Carlyle-Moses et al. 2018?).
- 280: “SWC” Pre-stemflow, post-stemflow, and the difference? Please be more specific.
- 281: “For instance,…” This would be a safe indicator for bypass flow, yet it depends on the place of infiltration (did water infiltrate directly above the measured location? Otherwise, it could also be a stronger lateral flow component in the deeper soil (due to what)? Please elaborate more on this.
- 283: “(1) preferential flow” This now reads like a summary, but was not mentioned before. It is especially interesting as you applied stemflow artificially and thus regularly around the tree stem. If you want to make this point, more information/discussion is needed.
- 284: “(2) preferential flow” In this point, it is unclear to me if this refers to infiltration (into the soil) or percolation (within the soil) or a mix of both. Especially, "infiltration...due to...coarse roots" sounds odd, because the roots are within the soil and not at the surface. Or do you refer to where the stem and the soil meet? Please rephrase to be more clear.
- 286: “water table” It would be nice to have a reference here as well.
- 290: “in…piezometers” How is “in” meant here?
- 294: “SWC” Please again also give absolute numbers.
- 306: “Figure 8…” This reads bumpy as beginning of the section as well as only the sentence itself. I would recommend to begin the section with a statement (the most important and a rather general statement previewing what the section is about). A reference to the figure in brackets is enough.
- 307: “Trend” Is it actually a (statistical) trend? Was a regression analysis made? If not, that would be great.
- 307 (“The distribution…”) – 313: I find this paragraph too long and the statement rather trivial. The flow mainly taking place laterally due to the low (non-existent) slope inclination can be said in one sentence and does not need to be compared with studies located on stronger slopes, because this is comparing apples with oranges.
- 316: Make clearer that these are pre-defined terms for those readers who skipped the methods section. You can use quotes and/or references.
- 319: Please here refer to the thought I mentioned above: the location of the water input in horizontal reference to the measurement location. It is an important observation that the soil water flow directly below the input is more regular.
- 329 ff: “Increased…” This is an interesting thought referring to a potential ecological meaning and fate of stemflow. As the ecological aspect is often unclear in stemflow studies (except for double-funneling in more arid climates), I think it would be worthwhile to discuss this effect in more detail.
- 331: “the soil matrix” The matrix... or the next-smaller pore fraction only. This is pretty much a question of continuity.
- 334 ff: This paragraph is a bit lost here and should go up to where I commented to have just this discussion. Also, there is room for differentiation here: Especially preferential flow does not only depend on the root network architecture, but also on soil physical and chemical properties (which themselves also are affected by tree species) and initial wetness.
- 361: “The double…” You strongly refer to the double funneling phenomenon in the introduction and now shortly pick it up again here, but it is totally missing in the discussion. You want to work on your red thread. Chose you focus and signal words (can be double funneling or other) and make sure it is represented in all chapters relevant to content (abstract, introduction, discussion. conclusion).
- 366: “has proven to be” instead of “is”
- 370 ff: The conclusion should wrap up the study in summarizing the most important aspects of the discussion and bringing them together to an overall outcome. This is neglected here, there is rather an unambitious "the method worked" and "further work is needed". I think you can really strengthen your paper by putting more effort into the conclusion (ideally after revising the discussion).
- 376: “Further studies are needed” I would avoid this wording, it sounds a bit hackneyed.
- 376 ff: The outlook is ok content-wise, but it reads a bit sloppy and more like a brainstorming. Please rephrase in a more focused, coherent and professional way.
- 379: “hot topic” too much!
- 570: Figure 1: Avoid red and green colored symbols within one figure for reasons of accessibility. The symbols could logically build on each other. The axis names could be more informative (e.g. “local x-coordinate”). All figures: Different units of length are used (mm, cm, m). It would be great to choose only one and use it consequently. It does not make sense to use a mm scale when the scale of interest is much larger.
- 570-571: “photograph” instead of “picture”, “stemflow application” instead of “stemflow set-up”
- 683: Figure 7: The color code of “no water level detected” is too hard to differentiate from the lower parts of the water level color scale. “No data” should be simply left plane and does not need to appear in the legend (it is more intuitive and matches the area in the middle and around the circles, where there is also implicitly no data).
- 712: Figure 8: As cardinal directions are given directly in the figure as headings (the columns of the panel), it would be consistent to also give the profile distance in the figure (the rows of the panel) instead of in the figure caption. To make it less noisy, x-axis labels and names should be deleted where they are unnecessary (just like with the y-axis).
- 750: Figure 10 would be even more consistent and self-explanatory if the first y-axis and the deuterium label in the legend would be in red (as the symbols, and as is for the brilliant blue). Maybe choose a slightly darker red in order to improve readability.
Citation: https://doi.org/10.5194/hess-2022-382-RC2 -
AC2: 'Reply on RC2', Juan Pinos, 10 Feb 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2022-382/hess-2022-382-AC2-supplement.pdf
Juan Pinos et al.
Juan Pinos et al.
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