Using a long time-series of precipitation, stream, branch, and lysimeter water isotope data, the authors explore seasonality and age fractions of water within each pool over a two-year period in a we tropical montane ecosystem in Peru.  They find that tree xylem water were the most isotopically variable reflecting more recent precipitation inputs, whereas lysimeter water at 1m and stream water are isotopically stable indicating older stored water.  I found this study very interesting and novel in that it includes all these pools within one study, allowing a more complete understanding of how water was moving within this ecosystem.  In temperate ecosystems, isotopic studies have indicated that trees use less mobile, older water within the soil that generally originated from wet winters.  However, in this wet tropical forest, trees are using recent precipitation, such that once precipitation percolates through to 1 m (lysimeter data), the seasonal variation is gone, indicating a long temporal mixing of the water at 1 m depth, and in the stream water.  I found the data and presentation convincing.  I particularly liked the various isotopic metrics they calculated to illustrate the water dynamics: seasonality, young water fraction and new water fraction.  Overall, I think this work will be a strong contribution to the literature, and provide ideas for similar studies in other ecosystems.

I had a lot of comments and clarifications that I think would make a clearer and cleaner story.  In addition, I think more details on soil moisture, water table depth, and precipitation amounts would help make their case in many instances.  Please see the detailed comments directly on the pdf.  

The paper “The seasonal origins and ages of water provisioning streams and trees in a tropical montane cloud forest” by Burt et al. presents an assessment of indices of young and new water fractions of tree, soil and stream water as well as indices of the seasonal origin of these water pools in a tropical forest catchment in the Andes of Peru. Even though I find the paper generally well written, I consider that some parts of the methods and discussion sections could be further developed to support the findings of the study. Given the relevance of the paper for the isotope ecohydrology of the understudied Andean region, I consider it is a manuscript suitable for publication in HESS after some points described below are implemented in the manuscript. 

General comments

Soil water isotopic data: In most figures the authors show soil water isotopic data without accounting for differences due to landscape position. However, the signals are different between those positions (valley vs hillslope). I suggest to consistently report the soil water isotopic composition results separately, as they show differences useful for interpretation. Please also note throughout the manuscript that soil water for isotopic analysis was collected at 1 m depth, as the variability in the depth and position across the landscape where soil water was collected can cause important differences in the observed isotopic signals. I also encourage the authors to check out papers in which this has been observed in other tropical montane catchments in the northern Andes (e.g., Mosquera et al., 2016; Mosquera et al; 2020; Lahuatte et al., 2022) to put their work in context and support some of their inferences yielded from the presented data.

Description of study site: The interpretation in the discussion section would benefit if the paper would include a more complete description of the study site (e.g., land use, the areal distribution, depth and properties of soils, and the vegetation characteristics such as root depth and leaf area index). Please expand the section with as much information as available, and use for interpretation.  

Specific comments:

L40-41: replace “twice monthly” in line 40 by “every two weeks” at the end of the sentence in line 41

L44: It is important to highlight throughout the manuscript the depth at which soil water was collected (i.e., 1 m; L156), as the isotopic signal would likely be different if soil water would have been collected at different depths. Perhaps it would be even worth noting that soil water was collected below the root zone.

L49: please add “in this study” after “2 years old”, as that time step is specific for the presented monitoring setup and not general for the method applied.

L55: I think that the importance of the study lies on the fact that it was carried out in a humid environment, as opposed to drier environments where this type of research has been carried out before. Consider specifying this “HUMID tropical montane cloud forest”

L122: is the reported elevation the mean catchment elevation? Please specify

L145: please discuss whether the spatial variability in the selection of trees at different distance from the streams could have influenced (or not) in the interpretation of the presented tree water isotopic data.

L164-167: the reader would really appreciate a plot of the rainfall amount and stream water level data measured at the study site to put the finding in context in relation to the site hydrometeorological conditions. I strongly suggest the authors to include a plot of these data in the manuscript for facilitate interpretation.

L164 and L166: report the brand, model and accuracy of the tipping bucket used to measure rainfall amount, and the accuracy of the type and accuracy of the logger used to measure water level.

L184-187: Since the samples were analyzed in different laboratories, were some replicate samples analyzed using both instruments to correct for potential analytical discrepancies? If yes, please explain; if not, please discuss the implication in the manuscript.

L188-190: was potential contamination in water samples assessed (e.g., using the ChemCorrect software)? If yes, please explain how this was accounted for and report the number of reanalyzed or discarded samples; if not please discuss the potential implication of contamination if the reported isotopic signals, particularly for tree water data.

L200: I think the correction was only carried out for xylem water samples? Please update if needed.

L210-211 values >+1 and <-1 are reported in Fig. 4. Please describe what they mean.

L237: I believe it should be Eqts. 4-6 instead of 4-7. Please update if needed.

L264: not sure what “cloud waters” means (e.g., fog, drizzle, or the combination of both). Please specify. Also, isn’t the isotopic composition of cloud water actually enriched as it plots above the LMWL? Clarify if needed.

L270: I suppose the corrected data is shown in the paper figures. If so, please specify in the caption and/or legend of the figures. I would also be useful for the reader to see how the correction changed the measured isotopic composition in supplementary material for reference.

L306: there is no fig S2 in the manuscript of supplementary material. Please revise and correct as needed.

L317: specify the geomorphic position where samples were collected.

L387-388: I am not convinced that sampling frequency could cause the observed attenuation in soil water isotopic data. I think this relates to the soil water being sampled at 1 m depth only. I think that the papers suggested in the general comment on this subject above could help to better interpret and support the presented findings.

 L396-397: I am not entirely convinced of this potential explanation of the differences in the isotopic composition of soil water. I think this could be related to differences in the water storage capacity of soils at different geomorphic positions across the landscape as previously observed in other montane Andean catchments (e.g., Lazo et al., 2019). This is why reporting the properties, areal extent and depth of soils across the catchment is important for interpretation.

L425: what about ecophysiological processes (e.g., the possibility of foliar water uptake due to the influence of fog/cloud water at the study area)? Please expand the discussion considering this point.

L512: replace “high” by “strong”

L519: this is likely because the lysimeters were placed below the root zone. This is why I strongly suggest that the sampling depth for soil water must be reported consistently throughout the manuscript.    

Fig 2: Classify soil water isotopic composition into different locations across the landscape. E.g., the text in the caption of Fig 4 clearly indicates this, and I think it would be valuable to specify this in all figures where this is not clear.

Fig 3. Same as in Fig. 3

SUGGESTED REFERENCES TO CONSIDER

Lahuatte, B., Mosquera, G.M., Páez-Bimos, S., Calispa, M., Vanacker, V., Zapata-Ríos, X., Muñoz, T., Crespo, P., 2022. Delineation of water flow paths in a tropical Andean headwater catchment with deep soils and permeable bedrock. Hydrol. Process. 36, e14725. https://doi.org/10.1002/HYP.14725.

Lazo, P.X., Mosquera, G.M., McDonnell, J.J., Crespo, P., 2019. The role of vegetation, soils, and precipitation on water storage and hydrological services in Andean Páramo catchments. J. Hydrol. 572, 805–819. https://doi.org/10.1016/J.JHYDROL.2019.03.050.

Mosquera, G., Crespo, P., Breuer, L., Feyen, J., Windhorst, D., 2020a. Water transport and tracer mixing in volcanic ash soils at a tropical hillslope: a wet layered sloping sponge. Hydrol. Process. 34, 2032–2047. https://doi.org/10.1002/hyp.13733.

Mosquera, G.M., Célleri, R., Lazo, P.X., Vaché, K.B., Perakis, S.S., Crespo, P., 2016b. Combined use of isotopic and hydrometric data to conceptualize ecohydrological processes in a high-elevation tropical ecosystem. Hydrol. Process. https://doi.org/10.1002/hyp.10927.