Two referees thoroughly evaluated the manuscript, and another comment was posted in the discussion. The main points were

1) Include more explanations for the spatio-temporal variability of the isotope data and differences between methods

2) More thoroughly mention and discuss limitations of the method in arid environments

3) Improve description of the new in-situ method

4) Be more specific in stating the objectives

The authors answered the comments. Most of the answers are sufficient and need to be included in the revised manuscript. Still, go carefully through the comments again, because some of them were not answered or only answered too briefly. It was e.g. referred to a reference on how error for extraction in sandy soils was considered, but the general principle should be clear from the text without reading the additional reference.
Now, I only emphasize some of the general comments that need more attention for revising the manuscript and additionally recommend improving the writing in terms of language and grammar (mainly refers to answers to the comments).

1) One of the reviewers suggested to shorten the introduction and included a more through discussion on the driving processes causing the isotope depth profiles

2) Answers on other biological processes causing isotope variability is highly speculative and not supported by any literature

3) The answer to the general referee comment #1 on isotope measurements at low water contents needs more attention. The authors argue about missing enrichment of isotope data at water content below 1%. It was further stated, that equilibrium is reached quickly but no supporting data were presented in the manuscript. Another important difference between the system presented in Wassenaar et al (2008) and this study here is, that the sampled vapor volume is not a closed systems like in a bag which needs to be considered in the discussion.

4) In my initial report before the discussion, I recommended not being too descriptive and qualitative only which was again mentioned by one of the referees. This is an important point to make your manuscript stronger.

Due to the comments requiring detailed rewriting of the manuscript, I will reconsider the manuscript after major revisions have been done.

Dear authors,

the same two referees reviewed the revised version of the manuscript. They state that still not all previous comments have been answered or implemented in the text adequately. Specifically, it includes the following main issues: (i) posed research questions not fully answered in the manuscript, (ii) little comparison with other studies in discussion, (iii) too long introduction, (iv) method comparison not addressed properly, (v) reproducibility of results low because of high standard deviations, (vi) poorly written and unclear sections (English), (vii) typos. In addition, the manuscript gets rather long and I strongly recommend focusing on the main objectives addressed.

A very last time, I give you the chance to revise the manuscript according to the recommendations. Please go carefully through the individual reviewer comments; also have a look at the previous recommendations. Many of these good recommendations are fundamental for improving the quality of your manuscript and for meeting the standard requirements for publication in HESS. After resubmission, I will make a final decision.

Regards
Christine Stumpp

The authors went back to the editor and reviewer comments (first and second round) and improved the manuscript accordingly. Now, the storyline is more focused. Beside some technical corrections (see list below), the manuscript can be accepted for publication in HESS. I recommend additionally sending the manuscript to a native speaker for a general language check before the final upload. An additional check is provided by Copernicus too (English copy-editing service; http://publications.copernicus.org/services/copy_editing_for_english.html).

I forwarded the request of the authors to the reviewers in providing their names for the Acknowledgments. The reviewers want to stay anonymous though. Nevertheless, I am convinced that they also appreciate being thanked „as anonymous reviewers“ for their good comments.


Technical Corrections:

General comment:
- write „in-situ“ in italic in entire manuscript
- check tense!
- there is no difference in “low water content” and “very low water content”. The word “very” is subjective and can be deleted in about 90% of the cases in your manuscript.
- one of the reviewers recommended to use “higher/lower” instead of “enriched/depleted”, which applies to entire paragraph and not only to the one sentence that was changed accordingly

Specific comments:
- superscript in title (18O, 2H)
- delete the second part of the title “potentials and limitations”. It is a good idea and it sounds fancy; however, I would expect something like a review or an opinion paper
- no new paragraphs but running text in abstract
- p4, ln 19-21: I do not understand these two points from reading the abstract only; besides, they are speculative not justifying to be mentioned in the abstract. Add a more general sentence, e.g. something like “for future applications, uncertainty of the in-situ method is suggested to be reduced by improving the calibration procedure (i.e. reference standards) and further studying apparent fractionation effects influencing the isotope ratios in the soil water vapor. “
- p5, ln 2: Either „water stable isotopes have been…“ or „stable isotopes of water have been….“ but not „water stable isotopes of water….“
- p6, ln8: remove comma
- p7, ln22: give Ks in m/d (not cm/d)
- p7, ln 28: add space character (9,000 m²)
- p8, ln 20: remove comma
- p10, ln 7: add comma after „in addition,…“
- p10, ln 19: add comma after „therefore,…“
- p11: do not use superscripts in the equation; change Dv to Dv (also in text); same applies to equation 6
- p 11 ln 3-5: do not use units but dimensions in brackets (e.g. not „m“ but „L“ for lengths); be consistent and give dimensions for all variables (e.g. equation 6)
- p11, ln3: why was a tortuosity factor of 0.67 chosen?
- check order of Figures in the manuscript; e.g. Figure 7 (p12, ln 28) introduced before Figure 5 (p14, ln1); by the way, the reference to Figure 7 (p12, ln28) is wrong; it is Figure 6 - still the order is not according to appearance in the text
- p12, ln 10: explain the abbreviation RMSE
-p15, ln20-22 and p16 ln1 and Table 3: please check the calculated ratios: if you want to give the contribution of transpiration to the total evapotranspiration (i.e. T/ET), it is 510mm/(510+120mm)=0,809  81%
- p16, ln15: English; “slowly increases and isotope values in the sampled vapor will get higher” instead of “might enrich itself….”
- p17, ln25: “Besides,…” not “Beside this”
- p18 ln 3: either might or can - not both
- p20, ln 11, 12, 28: add comma “Further, ….”; “Therefore, …”, “In addition,…”
- p21, ln 25-30: add one sentence why CO2 would be important for d18O analysis
- Table 3: theta:1% and theta:7% not explained; last two columns with one header only