In-situ unsaturated zone stable water isotope ( 2 H and 18 O ) measurements in semi-arid environments using tunable o ff-axis integrated cavity output spectroscopy

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Interactive comment on "In-situ unsaturated zone stable water isotope ( 2 H and 18 O) measurements in semi-arid environments using tunable off-axis integrated cavity output spectroscopy" by M. Gaj et al. Received and published: 25 June 2015 General comments

Anonymous Referee #1
The study presents soil isotope depth profile data of a semi-arid region in Namibia measured in-situ via soil gas probes attached to an OA-ICOS. The manuscript is well written and structured as well as easy to read. The study is in the scope of the journal and I recommend the presented study for publication in HESS after revising the submitted manuscript based on the suggestions of the review process.
The introduction section is well written and the relevant processes and fundamentals C2241 Interactive Comment Full Screen / Esc

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Interactive Discussion Discussion Paper of stable isotope hydrology for this study are addressed. However, I suggest shorten the introduction since a lot of basic information are given in extent.
The presented sampling approach and the methods used are well described. However, the data poses more questions than the authors can answer. The amount of samples taken from other water cycle components (e.g. rainfall, groundwater) is rather limited which makes it hard to interpret the measured soil isotope profile data on its own. More frequent rainfall isotope data would be helpful in this regard to answer the following question: What is the driver of the isotopic composition in the presented soil profiles? Given the fact that rainfall and groundwater isotope data are very limited, a more detailed discussion about the vegetation influence on soil water isotopes would be desired. It would further be desirable if the authors could make better use of the spatio-temporal isotope variability rather than just presenting depth profiles, which is nothing new.
The authors show soil moisture data for each profile. Did they correlate soil moisture versus isotopic signatures to proof the minor impact on the development of the isotope soil profiles in their study area?
When comparing the new in-situ method to the widely used cryogenic vacuum extraction technique, differences especially for isotope data of the upper soil profile were measured which could not be satisfactorily explained. Could these differences in the top soil be attributed to method issues rather than to kinetic processes? Could the new in-situ method after all be limited in terms of low water contents (<5%) as the differences between the cryogenic vacuum extraction and the in-situ approach occurred within the very dry top 15 cm of the profiles (<2% soil moisture) and enriched isotope values were likewise measured in the upper soil profile? For instance, Wassenaar et al. (2008) admitted a limitation of their method, which is likewise based on vaporequilibrium measurements in this regard (not suitable for water contents <5%). The authors addressed this issue in its own section but still, the question remains why almost inverse isotope results were obtained from cryogenic extraction and the in-situ  (Fig. 4) and further isotope results of which extraction method (in-situ vs. cryogenic extraction) are more reliable in the end. Finally, the authors should include a small paragraph on future research directions in the study area or further applications of the in-situ approach.

Specific comments:
Generally, the text should be checked for punctuation. C2244