The Future of Snowpack Drought in the Upper Colorado River Basin (USA)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Major revision. The manuscript is timely. It proposes an interesting integration of dendrohydrology with an SSP5-8.5 precipitation projection to contextualize future snow-season drought in the UCRB. The workflow is promising, but several methodological clarifications and validation/uncertainty points are needed before the quantitative conclusions can be relied.

Abstract

1. please avoid sentences starting with “we” and instead use impersonal/passive constructions.
2. Also, at Line 18, previous studies are mentioned; if you explicitly state how this study differs from previous work in the Abstract, readers will better understand the added value.
3. Finally, the term “dendrohydrology” is used in the keywords; if this definition is used, an explanatory sentence should be added somewhere in the Introduction, because many hydrologists may not be familiar with the term.

 

Introduction:

4. . Line 36: please delete “primarily”.
5. For the sentence between Lines 42 and 44, please add a reference, since SWE is defined there.
6. Instead of the question sentence at Line 61, please write an academic statement emphasizing the importance of climate-model future projections.
7. The same suggestion applies to the question sentence between Lines 68 and 69.
8. Line 86: please rewrite the sentence in the passive voice.
9. The novelty needs to be stated more explicitly at the last pharagraph. the paper is not reconstructing SWE directly (as in prior work) but reconstructing ONDJFM cumulative precipitation at a single site and then integrating with an SSP5-8.5 series Please make this distinction unambiguous

 

Materials and Methods

10. The analysis is conducted at a single station location (Kendall R.S.) using the nearest grid point from the downscaled product. Please add a short justification (or limitation) on how results at this point should (and should not) be generalized to the studied basin.
11. why 5- and 10-year windows are selected, please explain.
12. For Figure 1, adding a Digital Elevation Model (DEM) is important to better interpret snow-derived water; this would also help evaluate how representative the datasets are for the basin, especially for the green points outside the watershed.
13. Since elevation is a key control on snowfall, this could also strengthen the Discussion with additional points. Showing the main river stem and tributaries on the DEM would also be useful.
14. In the Methods, please provide full details of the selected trees, including location information, elevations, species, and relevant site characteristics (e.g., whether they are from valley interiors or near upper treeline), because selecting the appropriate tree type and setting is critical for this study.
15. Lines 140, 150, and 158: please rewrite these sentences in the passive voice.
16. Line 151: replace “our” with “the”.

 

Discussion and Conclusion

17. The Discussion section is entirely insufficient; the results should be compared with findings from other studies.
18. The key interpretive message (paleo droughts exceed the modeled ~2040s drought; future “whiplash-like” wet–dry transitions) should be supported with quantitative probability statements.
19. The Conclusions should be structured as numbered bullet points that summarize the main takeaways in generalizable statements, including limitations.
20. Lines 262 and 271: please rewrite the sentences in the passive voice.
21. Line 271: please remove the reference and move this sentence into the Discussion; references should not be included in the Conclusions.
22. please correct the reference writing for (19) "Woodhouse, C.A. A 431-Year Reconstruction of Western Colorado Snowpack from Tree Rings. J. Clim. 2003, 16, 1551–1561. 345
    https://doi.org/10.1175/1520-0442(2003)016<1551:AYROWC>2.0.CO;2."

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This MS presents a novel integration of paleoclimatic reconstructions, historical data, and future projections to assess snowpack drought in the Upper Colorado River Basin (UCRB), focusing on the Kendall R.S. site in the Upper Green River Basin. The authors reconstruct October-November-December-January-February-March (ONDJFM) cumulative precipitation using tree-ring chronologies and El Niño-Southern Oscillation (ENSO) indices (SOI and Niño 3.4), extending the record back to the early 1600s/1700s. They bias-correct the data and combine it with recent (1981–2025) and future (2026–2059) ONDJFM precipitation from a dynamically downscaled SSP5-8.5 scenario. Results highlight that projected severe droughts in the 2040s are exceeded by paleo droughts, while future wet (pluvial) periods may surpass historical ones, exhibiting "whiplash" variability. The paper is well structured. Her some comments:

Major comments

• The study reconstructs ONDJFM precipitation at a single site (Kendall R.S.), limiting generalizability to the broader UCRB. While justified by data availability, comparisons to prior regional reconstructions [9] show differences (e.g., drought timings), but the MS could better quantify spatial variability or validate against multiple sites.
• Using only the "worst-case" scenario (SSP5-8.5) may overemphasize extreme outcomes. The authors acknowledge this implicitly, but including sensitivity to other SSPs (e.g., 2-4.5 or 3-7.0) or ensemble means would strengthen projections and address uncertainty in future drying.
• Using April 1st SWE as a proxy for ONDJFM precipitation (r=0.87) is reasonable but assumes minimal melt influence pre-April, which may not hold amid earlier snowmelt trends [2]. Bias correction is applied, but details on quantile mapping implementation (e.g., distribution fits) are sparse, and potential over-correction in paleo extremes isn't tested.
• Reconstructions extend to 1600s/1700s, but validation is confined to 1940–1977 overlap. No discussion of nest models for shorter chronologies or external validation (e.g., against independent streamflow/soil moisture reconstructions [10,11]). The differing retained chronologies between RECON 1 and 2 warrant more analysis of ENSO proxy impacts.

Minor Comments

• Minor errors include "gague" (should be "gauge"), inconsistent abbreviations (e.g., "mm" vs. "millimeters"), and future dates (e.g., "2026" in copyright, "2025" in references—likely placeholders).
• Citations are relevant but lean recent (e.g., 2024–2025 papers); include more on recent UCRB studies (e.g., warming effects on evapotranspiration). Missing details on RegCM4 downscaling [7].
• Discussion is brief; expand on pluvial implications (e.g., flood risks) and "whiplash" mechanisms. Conclusions reiterate results without strong forward-looking statements (e.g., recommendations for monitoring).
• Clarify "end-year filters" as moving averages. In Table 1, specify units in regression equations (e.g., SOI is dimensionless).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript is intriguing due to its analysis and integration of manually and automatically collected data with predicted datasets and tree-ring analysis results. It is well-structured; however, some sections remain underdeveloped, making parts of the paper read more like a technical report than a fully contextualised research article. Therefore, I recommend the following revisions:

• The introduction does not sufficiently place the findings from the United States into a broader global context. While it is clear that the analysis is conducted in the U.S., the introduction should adopt a more comprehensive structure, presenting the issue at the global, regional, and local levels. This would strengthen the study's scientific framing.
• The bibliography is primarily U.S.-focused. It would be beneficial to diversify the references by including international studies to show whether similar research has been undertaken globally, thus positioning the manuscript more firmly within the international scientific context.
• The purpose of the paper should be explicitly stated. This would enhance the paper's coherence.
• The materials used are not clearly specified. I recommend including a summary table listing all the materials used. Furthermore, the methodology is not explicitly outlined. This section would benefit from a more detailed procedural explanation along with a flowchart of the methodology.
• The study area lacks sufficient detail. A more thorough description is necessary, including information on topography, climate, hydrology, and other pertinent environmental features. Moreover, the locations where data were collected and the placement of monitoring stations should be clearly explained, as these factors can significantly affect the analysis.
• Why was a single climate scenario (SSP5-8.5) selected? Was this choice made solely because it was used in previous research [9]?
• The extension of ONDJFM cumulative precipitation using April 1st SWE shows a strong correlation; however, is this relationship stable over time?
• Although April 1st SWE data extend to 2025 (including automated SNOTEL measurements), the reconstruction is calibrated only over 1940–1977. Why was it not extended until 2025? It is important to clarify whether the calibration window was extended and whether omitting recent decades could affect the interpretation of future drought severity.
• Additionally, the authors should ensure that this methodological choice does not inadvertently support a narrative suggesting the absence or insignificance of recent warming trends in influencing snowpack–precipitation dynamics.
• Frequent references to previous studies give the impression that the reader must consult earlier publications to understand the current manuscript fully. A scientific article should be self-contained. At present, it feels as if one is reading the final volume of a series without access to the earlier ones.
• The discussion section needs further development. The authors should offer a more in-depth interpretation of the results, compare them more comprehensively with existing literature, and emphasise the wider implications of their findings.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for the revisions. Please make two minor figure-formatting corrections.

1. In Line 47, “snow water equivalent” should be written in full at first mention, and the abbreviation should be introduced and used thereafter.
2. In Line 49, the definition is unnecessarily long; it should be removed and the term should be kept as “SWE.”

Best

Author Response

We sincerely thank the reviewer for the careful reading of the manuscript and for the constructive and helpful suggestions. We have addressed both comments as follows.

Comment 1:

In Line 47, “snow water equivalent” should be written in full at first mention, and the abbreviation should be introduced and used thereafter.

Response 1:

Thank you for this suggestion. We revised the first mention in the Introduction to write the term in full as Snow Water Equivalent (SWE) and used SWE thereafter.

Comment 2:

In Line 49, the definition is unnecessarily long; it should be removed and the term should be kept as “SWE.”

Response 2:

Thank you for this helpful comment. We removed the extended definition of SWE in the Introduction and retained the abbreviated term SWE thereafter, as suggested.

Reviewer 2 Report

Comments and Suggestions for Authors

All comments are well adressed

Author Response

Comment 1: 

All comments are well addressed

Response 1:

We sincerely thank the reviewer for the encouraging evaluation of the revised manuscript. We appreciate the reviewer’s confirmation that the comments have been well addressed. 

Reviewer 3 Report

Comments and Suggestions for Authors

I do not have other comments to add

Author Response

Comment 1:

I do not have other comments to add

Response 1:

We sincerely thank the reviewer for the evaluation of the revised manuscript. We appreciate the reviewer’s confirmation that there are no additional comments.