Review of the manuscript ‘Evaporation, infiltration and storage of soil water in different vegetation zones in Qilian mountains: From a perspective of stable isotopes’ (HESS-2021-376)

General comment

This is an interesting study conducted in a mountain region in China. The research is based on the use of stable isotope in water to understand the main mixing processes and hydrological processes. The manuscript is logically organized and clearly illustrated. However, there are serious issues that in my opinion should prevent the publication of this manuscript in the present form. First of all, the English language is very poor, and sometimes (especially in the Introduction) it’s hard to follow the reading. Secondly, the are serious flaws in the Abstract and mostly in the Introduction that fails to reach the focal point of this work (see specific comments below). Third, the discussion is well complemented with literature references but is quite often vague and appears to be not well supported by the observations. No reference to figures and tables are reported in the Discussion and it seems that the processes explained by the Authors are based on a previous knowledge of the area and by results taken from the literature than supported by their own results. I suggest to more strictly base the inference on hydrological processes on the observed results.

In the end I suggest to resubmit this manuscript after fixing all these major issues.

Specific comments

L1-3. The title does not read well. I suggest changing into “Evaporation, infiltration and soil storage in different mountain zones” or “Evaporation, infiltration and soil storage in different vegetation zones in a mountain catchment”. Perhaps also “Evaporation, infiltration and soil storage in different mountain zones: an isotope perspective”. But there is no strict need to stress the isotopic perspective, in my opinion.

- The abstract lacks to report the main objective, or the research questions.

- The introduction suffers from different weak points and needs a severe restructuring.

1) It is not clear what different vegetation zones are, and what role they play in water exchange in the environment, and why they are important in this research.

2) The text focuses too much on the variability of the isotopic composition in vegetated environments without going deeper into the main physical processes that still need to be understood. Isotopes are just a tool, and the goal here is to understand hydrological processes with the help of isotopes, not which are the factors affecting isotopic variability.

3) Very importantly, no research gaps is put forward. We understand that studying and understanding hydrological processes in different vegetated areas is important but what is the real problem here? As a result, the specific objectives are disconnected from the rest of the Introduction and fluctuate in their own space. Moreover, what is the memory effect? Why is it important? What is not known about it? How does it fit the general story behind this paper?

L349-354. This paragraph and the related Fig. 7 fit much better in the Results than in the Discussion. I suggest restructuring this part.

L365-366. Which are the results that lead the Authors to believe this? Please, explain.

L422-424. Again, here we need some experimental evidence about this process.

Section 5.3. All this is interesting but it sounds a bit general and vague, these statements look not so related to the results, there are no references to the figures, and the reader has the impression that the Authors present their own preliminary idea that does not reflect the data. I’m happy to be mistaken here but we need to have evidence of all the described processes. Moreover, the title does not reflect the content of the section.

Minor comments and technical corrections

L58-59. D-excess is introduced here without any explanation on its formulation and physical meaning.

L96. Is this the long-term average runoff? Please, specify.

Table 1. Are the meteorological parameters averages? Over which period of time? Please, specify.

Section 3 (Methods). No explanations about the determination of the gravimetric water content are given. Please fix this issue.

L140-142. I suggest including a reference to the correction of the memory effect (e.g., Penna et al., 2012 and/or Qu et al., 2019).

Penna, D., Stenni, B., Šanda, M., Wrede, S., Bogaard, T. A., Michelini, M., Fischer, B. M. C., Gobbi, A., Mantese, N., Zuecco, G., Borga, M., Bonazza, M., Sobotková, M., Čejková, B., and Wassenaar, L. I.: Technical Note: Evaluation of between-sample memory effects in the analysis of δ2H and δ18O of water samples measured by laser spectroscopes, Hydrol. Earth Syst. Sci., 16, 3925–3933, https://doi.org/10.5194/hess-16-3925-2012, 2012.

Qu, D, Tian, L, Zhao, H, Yao, P, Xu, B, Cui, J. Demonstration of a memory calibration method in water isotope measurement by laser spectroscopy. Rapid Commun Mass Spectrom. 2020; 34:e8689. https://doi.org/10.1002/rcm.8689

L169. Please explain the role of 10 in the equation.

L176. Add “2017”.

L194: Precipitation events?

L213-220. This part can be reported in a Table or in a boxplot.

Fig. 3. Use different colours to distinguish between the different variables. Particularly, I suggest using different closures for the two isotopes and then keep them in all the other graphs.

L263? Does “deep” mean “grey”? In that case use the correct term.

L266. The diagram is normally called “dual isotope space”.

L272-274. This part can/could be moved to the Discussion.

L311. As far as I understand the plot does not show the “differences” but the raw values. Please, correct.

L385-386. Add a reference to a figure to corroborate the statement.

L409. I think the correct citation is Amin et al., 2020.

L463. Typo.

General Comments

This a potentially interesting paper, but one that needs major attention before it is suitable for publication. The paper is poorly written in places. While I have sympathy with authors having to write in a second language, which is something that I cannot do, some sections of the paper are very difficult to follow. More importantly, the sections of the paper are not well linked. It is not clear from the Introduction how the paper addresses the important issues. The same can be said about the Discussion where it is not clear how the data in this paper inform the issues being discussed. For example, runoff generation is mentioned in the introduction and appears in the general conclusions, but there is no discussion as to how the data in the paper help us understand it (there are several similar examples as well). The sections describing the data tend to be very generalised and the data description needs to be more informative. Moreover, the data need to be presented in the paper or as a supplement.

Overall, the paper needs to be rewritten so that the data are discussed in a more rigorous manner that help understand the aims. I am not convinced that it actually addresses important issues or that the aims of understanding the memory effect or runoff generation are advanced by this study.

Title: Having a title that is grammatically incorrect is not a good way to promote your research. Something like: “Evaporation, infiltration and storage of soil water in different vegetation zones in the Qilian mountains: a stable isotope perspective” would be better

Abstract

The abstract needs improvement. Abstracts are important as they are what convince the readers to look at the rest of the paper. They should convey not only what has been studied and why, but should also contain enough details so that the main conclusions are evident. This abstract needs improving, specifically:

• Be specific: “different water bodies in different vegetation zones” does not convey what you have done
• Avoid qualitative terms such as “weak” 
• Some of the sentences are unclear. I am not sure what “The water storage capacity of surface soil was weak in vegetation zones” really means as surely all the catchment is vegetated?)
• There are several grammatical and spelling errors (Nvertheless) that detract from the work.

Introduction

The introduction is also not very clear. Some of this reflects the writing style and occasional poor grammar. As well, there needs to be a much clearer explanation of the background. The explanations are vague and would not convey much meaning to anyone not working in the field. There needs to be clearer explanations and more precise terminology.

L31-33. Not very clear what you mean here

L48. “Storage” is not a transport mechanism

L50. Do you mean on the ground surface or in the near-surface part of the soils?

L48-75. This would not be readily understandable to many readers who had not worked with these types of data. It is too generally worded and needs details. This paragraph is important as it sets the framework for using the stable isotopes to understand processes

• Define that you are discussing ¹⁸O and ²H data (there are lots of stable isotopes!).
• Terms such as “makes soil water isotopes enriched” are vague. Specifically, evaporation enriches the residual water in ¹⁸O or ²H (or increases the δ¹⁸O and δ²H values)
• Likewise, “soil moisture fractionation is positively correlated with evapotranspiration but negatively correlated with precipitation”. Are you talking about the magnitude or sign?
• How significant?
• Define the d-excess (briefly)
• L63-70. Lacks detail and is unclear.

L76-78. Not clear what you mean by this. Are the water resources more unstable or are they transitioning?

L84. “Heat conditions” – do you mean temperatures?

L82-90. These are fine as general aims, but can you explain (briefly) why this is important (i.e. what are you doing that is new, what are the broader implications?). There is a disconnect here between the broad general themes in the rest of the introduction and your specific study. Also, runoff generation and the memory effect are not explicitly discussed in any depth in the paper (need to make sure that your aims are actually what you discuss).

L88. If it is important, define the memory effect and explain why we need to understand it

Study Area

This section needs referencing. Also a few more details as to the spatial variation of rainfall and temperature (I presume that the highest rainfall and lowest mean temperatures are in the mountains?)

L94. What is a “first-class tributary”?

L98-101. Probably worth reporting the Koppen Zones.

L103-106. Refer to Fig. 1.

Fig. 1. What is the inset on the left-hand map?

Data and Methods

The methods used here are standard and suitable for the project. As with much of the rest of the paper, there are a few details lacking and the explanations are not very clear.

L111-113. It would be helpful here or in Section 2 to outline what 2017 was like in terms of rainfall and temperature as these vary year-by-year. In particular, distinguish between long-term averages and values in the sampling year.

L116-122. This is rather a clunky description (not sure that you need to specify explicitly that you wrote dates on bottles). What do you mean by “four parallel samples” being also collected?

L143. It’s “permil” not “thousands”. As written, “thousandths of the Vienna Standard Mean Ocean Water (VSMOW)” is meaningless.

Section 3.2. The analysis is only part of the uncertainty. Did you perform multiple extractions on the same sample to test the uncertainty associated with that. This will undoubtedly be higher and needs to be considered.

Section 3.3.1 The line-conditioned excess is less used than the d-excess (but is potentially more informative). You should explain what it is (and define the term). The explanation “The physical meaning of lc-excess is expressed as the deviation degree between isotopic values in samples and LMWL, which indicates the non-equilibrium dynamic fractionation process caused by evaporation (Landwehr et al., 2014; Sprenger et al., 2017)” is not very clear.

Section 3.3.2. More details are needed as to where these data are derived from. Are they local data measured at the field site or interpolated estimates? Application of the Penman-Monteath equation is very data sensitive. What do you think the errors are here?

Section 3.3.3. These are based on your measurements, yes? Again, do you have estimates of uncertainties. Also, some of the techniques (eg moisture content) need more detail.

Results

This section suffers from the shortcomings of the rest of the paper. The explanations are not very clear and are often overly general. Also, I cannot see where the raw data are (no Table or Appendix); presenting the actual data is required.   

L175-178. How precise are these values (i.e. is the 1dp precision warranted)? What was the rainfall during those times?

L180-182. Not very clearly worded.

L213-220. The ranges in stable isotope values are probably more useful. Suggest that you report the range and the mean (you can omit the SD as that is less useful). Also report the number of observations, so we get an idea of how much data there is. Ideally the mean should be weighted by precipitation amount (it is not clear that that is the case, but you should be clear whether it is).

L220-224. Poorly worded.

L224-227. This isn’t really that obvious from Fig. 3. Can you report the magnitudes in the text?

L232-260. This section has too little detail in it to follow. You need to explain the data more specifically (avoid vague terms such as “depletion” or “enrichment” and report some values). More importantly where are the data? Fig. 4 is labelled as a “heat map” but seems to be the values (I think) and they are on Fig. 5. However, these also need to be in a Table somewhere.

L266-269. Speculative, can you provide a reference to show that these processes cause secondary evaporation.

L270-275. A reference would also help here

L275-276. Seems redundant as I’m not sure where else the moisture could come from.

L288-295. Again, lacks detail. It is difficult to follow these arguments when the data is discussed in very vague terms.

L298. What is “dynamic fractionation”?

L296-309. As with much of the rest of this section, I struggled to follow the details. The explanations are not clear, there are a fair number of general statements that lack detail, and a number of findings that are not obvious. For example, “Evaporation signal can easily penetrate deep soil, which made the GWC value of all sampling activities at this site lower than 20% (Fig.6)” which seems to be at odds with “With the increase of soil depth, the fractionation signal gradually weakened”.

Discussion

This section has some interesting ideas in it but it is not well linked to the data in the study. You need to show how the data that you collected informs our understanding. Some of the later part of this section is written more like an introductory literature review.

Section 5.1

L325-354. Some of this section describes the data (the observations on soil moisture) and that material belongs in the results.  

Section 5.2

This section does not link well with the results. It is difficult to follow how your data help you make these conclusions. More justification and explanations are required. Moreover, there is little discussion of processes here – how does the data help understand how processes operate? You have concentrated on discussing the isotopic variability, without using it to understand what is going on.

This is the section where you should discuss aspects such as the memory effect and runoff generation, but you do not do so.

Section 5.3.

This section reads more like an introduction. It is not clear how what you have done in this study relates to these broad general findings. As with the Introduction, you need to make a clearer link between your study and these general statements. This are all important issues, but there needs to be linkages.

Climate change in mentioned several times, but it is not clear how your study informs our understanding of its impacts. Those types of links need to be made clearer if they exist. Likewise, there are comments about groundwater recharge and runoff but no indication of how your results help understand those processes. Runoff generation was not actually discussed in the body of the paper (it appears in the introduction and the end of the discussion, but not in the discussion of the specific results).

Same comments apply to: subsurface runoff (presume that you mean interflow?); the management practices; human activities. These are topics that all appear in this section with no real link to the data in the rest of the paper.

There are also a number of superfluous details here. For example, why is mining waste (L426-428) relevant to this study.

Conclusions

As with the discussion, the links to the study are not well made. In some ways this material is less general than some of the latter parts of the Discussion (Section 5.3) and it would be worth reordering so that you have the more general ideas at the end.

General Comment

This paper presents an interesting hydrological and runoff study from the Qilian region where water and soil water samples were obtained across different climatic, topographical and vegetative conditions in order to understand the infiltration, evaporation and storage processes. The paper is well structured, but major issues need fixing as also suggested by the other referees. Overall, the English language needs to be proofread and words such as “obvious” should be avoided. The Abstract needs substantial work to emphasize the purpose/objectives of the study, describe the methods used, and to relate quantifiable results. Further discussion of the results themselves is required as well as linking the results obtained (what are the observations withdrawn from the data) to previous research. Please see additional comments below.      

Abstract  

L11-12: Is this really true? That in arid areas most of the water comes from mountains? How about low lands? And groundwater? I think this sentence is not needed.

L12: should be the “processes” not “process”

L13: “have” instead of “has”

L15: instead of “In current study” use “In this study”

L15-17: This is an important sentence that summarizes the work done. I would suggest to rewrite it being more specific to which isotopes, which types of vegetation zones and why this is needed.  

L17: Weak compared to what? Results should be quantified instead of using “weak” and “save up”

L19-21: What is the result in the paper that lead to this hypothesis? The authors need to add evidence of this instead of speculating

L22: What is evaporate strongly? How much?

L21-22: The lower elevation vegetation zones within the Mountain Grassland and Deciduous forest? Aren’t these areas at high altitude?

L25: Delete word “reasonably”

Introduction

L39-40: Soil water in the unsaturated zone from precipitation can transform into water vapour or groundwater recharge.

Line 40: Delete “Its”

Line 41: Delete “very”

Line 48: Storage is not a transport mechanism

Line 54: is it soil water profiles?

Line 56: Delete “In addition,”

Line 58: Describe what the d-excess is

Line 86-70: Delete “,and” and rewrite following sentence. It is not clear at the moment.

Line 71: Do not use “Generally speaking” in a scientific publication

Line 71: Do “wet” areas refer to tropical regions?

Line 80: can better help adapt

Line 82: “In this study” instead of “In current study”

Line 82: “,” after soil water

Line 83: Is it in four regions of different climate, vegetation and topographical conditions? As opposed to vegetation zones?

Line 85: Then, it can be clarified that this study explores how evaporation, infiltration and storage processes differ within these four regions according to the climate, vegetation and topography.

Line 89: similarly to the previous comment, are the authors restricting the analysis to only vegetation zones? I would argue that the study compares regions with varying climatic, topographic and vegetative conditions.

Study Area

Line 99: ranges between 2000m and 5000m above sea level

Data and Methods

L110: Delete “and determination”

L116: What does “parallel” mean here?

L170-172: Equation before line 170 needs reference

Results and Analysis

L175: “PET” should be written Potential evapotranspiration (PET), then the authors can use PET but it needs complete spelling the first time it is used.  

L177: I assume it is also the daily evapotranspiration? Need to make it explicit which type of evaporation

L184: Delete “generally speaking”

L191: “temperature” instead of heat

L194: 72 precipitation events? Make it explicit, where all these rainfall?

L207: Rewrite sentence to “The temperature of the studied regions was ordered as follow:”

L208: Define first what AM, CF, MG, and DF mean

L213: Do not use obviously in scientific publications, you can say what it was significantly different? Did you do any statistical analysis to conclude this? If so what please mention it in the results

L211-220. This info would be better in a table

L235: “The low temperature environment of Alpine Meadow and abundant and uniform precipitation events made the monthly mean values of δ2H and δ18O change little” how much?

L239: “Evaporation fractionation of soil water isotopes in Coniferous Forests was more intense.” More intense than what? These kind of statements need quantification.

L277-278: “With the decrease of altitude, the soil water evaporation became stronger and stronger, except soil in Deciduous Forest”. This sentence does not make sense, please rewrite and quantify stronger.

Discussion

I am in agreement with the comments of Referee 1 and Referee 2 concerning the discussion. It feels more like a summary of previous studies. The authors need to refer to the results and put them in context of previous work and how their study is contributes to that pool of knowledge.

L323-325: “The soil water storage capacity of Alpine Meadow with low temperature and rainy weather was obviously higher than that of other vegetation zones.” The authors need to explain how this conclusion is evident from their data without using words such as “obviously” referred to figure 7 for discussion.

L440-442. Fix this sentence grammatically

Conclusion

L457: Storage capacity decreased (instead of weakened)

L461: Soil “water” evaporation in spring…

L463: Is it “isotopic” instead of “isotopci”?

L463-465. This sentence needs fixing. I could not understand what it conveys.