The manuscript, “Integrated Catchment Classification Across China Based on Hydroclimatological and Geomorphological Similarities Using Self-Organizing Maps and Fuzzy C-Means Clustering for Hydrological Modeling” by Niu et al., introduces a catchment classification method, which combines the SOM for climate zone classification and the FCM for further classification based on topographic, soil, vegetation, and topological features for catchments. After classification, 10 watersheds in 5 classification groups were chosen to compare their within-group similarity and inter-group distinction in hydrological signatures (flow duration curve). 

Though the topic of this manuscript fits the scope of HESS, I have some concerns about the study, which are commented below. Therefore, I suggest that the manuscript to be revised with major revisions at this moment.

General comments

1. The novelty of the study is unclear to me. Is this the first study of fuzzy classification for small and medium-sized watersheds in China? Compared with other classified results, what are the major differences (not methodology) or improvements, such as hydrologic signatures? This should be elaborated in the Introduction and Discussion sections. 
2. The structure of the paper needs to be reorganized. The Results section contains many texts that should be moved to Methods and Discussion. For example, Line 302-306 for how the optimal number of clusters is chosen should be moved to the Methods explaining FCM; Line 407-498 for the flow duration curve and hydrologic signature looks like a great point that should be moved to the Discussion section. Overall, the current organization, having discussions inside Results, makes the manuscript long and disruptive to read. The manuscript should be organized more neatly, where the Results should focus on presenting numbers, while moving and consolidating interpretations and implications in the Discussion.   
3. The validation of classification is only performed for 10 watersheds in all entire China, which I think is insufficient. Based on the Figure 6, there are many same-class watersheds that are fairly distant from each other. However, the current selection of watersheds, though the similarity of FDC in each class is shown,  might be insufficient to support the conclusion, as these watersheds in same classes are too spatially close to each other. Therefore, I am wondering how the similarity of FDC would be if watersheds that are more spatially distant are chosen for evaluation.  
4. The application of the study is not thoroughly discussed. The section 4.1 focuses more on the advantages of the probabilistic approach of FCM over hard-boundary classification ones, and the potential of improving regional hydrological modeling. However, the potential of 1) transferring model parameters from calibrated to ungauged watersheds and 2) estimating floods under various design storms based on the similarity of flow duration curve could be discussed, and can improve the novelty and value of the research.
5. I think the references are not in the required style of HESS (https://www.hydrology-and-earth-system-sciences.net/submission.html#references)    

Specific comments

1. Line 69: “an indisputable fact” looks like a strange statement. Do you mean machine learning is now widely used for regionalization studies?
2. Line 102-103: need to add references.
3. Line 117-126: The organization of this paragraph needs improvement. The six indices should be stated before reasoning why they are selected. It would make the flow more logical, rather than making readers wonder what indices are chosen (line 119, three indices but not stating what they are).
4. Line 174: reference for FCM?
5. Line 178: “may be the most” to “is a”
6. Line 207: reference for Penman-Monteith equation
7. Line 214: What are the average size and the range of catchment sizes? 
8. Figure 2: Though the interpretations of hexagons values are provided, I still don’t quite get the physical meanings of these plots and wonder how they should be interpreted spatially on maps (if a basemap can be added, it would be helpful). In the figure’s caption, briefly explain the legends and how readersshould interpret the figure.  Also, line 276-285 should be moved to Methods.
9. Line 304: What is the AP algorithm? I don’t think this was mentioned in Methods, and should add the reference
10. Line 369: Two questions here: 1) For soil & veg characteristics, the second PC has an eigenvalue of 0.91. Why do you choose this PC below one? 2) Improve the writing: Instead of using semicolons, clearly state which class (topographic, soil & veg, and tolological) you are discussing first (For topographic, XXX. For soil & veg, XXX.), then state how each PC is correlated to the input indices.
11. Line 449: space between class and (Li et al)
12. Figure 7: some recommendations: 1) List each site/catchment's classification in the line charts, and 2) maybe consider another color scheme to present the variation range. The grey colors are hard to differentiate. Also, briefly describe the ranges in the legend within the caption for people to understand.
13. Line 495: correct the citation
14. Figure 8: What will FDCs look like if using discharge in mm/day (normalized by drainage area)? I am wondering this because these watersheds vary significantly in drainage size, and normalizing discharge by size may allow for expanding the validation to more gauged watersheds.
15. Line 517: The statement, “leading to errors”, should be more evidence-based. What specific errors could inaccurate classification result in? What consequences/risks will these errors cause? Provide references of previous studies showing so.
16. Line 546: Is this a possible reason causing the challenge that some watersheds are hard to be classified with one dominant group?

Overall Evaluation

The manuscript proposes a two-stage catchment classification framework in which both stages integrate Self-Organizing Maps (SOM) and Fuzzy C-Means (FCM) clustering. In the first stage, the SOM–FCM approach is employed to classify climatic zones across China based on hydroclimatological indicators. Ten representative watersheds were selected to assess intra-group similarity and inter-group distinctiveness in hydrological signatures, demonstrating the feasibility and robustness of the proposed framework for hydrological regionalization.

It presents interesting and meaningful findings, offering a valuable contribution to understanding catchment similarity patterns across China through the integration of hydroclimatological and geomorphological attributes using unsupervised learning techniques.

I think it is well structured and orginazed, but it still need a lot of modification to achieve the publications. Some major and minor comments are made for improving the quality of this study. I suggest it can be accepted after minor revision.

Major Comments

1. The Results section should focus on reporting experimental outcomes and data interpretation only. Methodological explanations, such as the description of the Affinity Propagation (AP) algorithm (Line 304), should be moved to the Methods section for better structural consistency.
2. Please justify the representativeness of these catchments or consider expanding the validation dataset to reinforce the robustness of your conclusions.
3. Please include a paragraph discussing the limitations of the proposed method and outline possible future research directions to improve applicability and generalization.
4. I think you need check the English grammar and sentence carefully. Please revise the English description of your manuscript.
5. Please carefully review and standardize all references to conform with the HESS reference style guide.

Minor Comments

1. Line 276: When describing the SOM neuron grid (19×22), briefly justify why this specific grid size was selected (e.g., based on data dimensionality, heuristic optimization, or quantization error minimization).
2. Figure 2: The description of the SOM component planes is clear but would benefit from a short explanatory note in the figure caption clarifying the color scale meaning (e.g., “red indicates high values, blue indicates low values”).
3. Ensure consistent use of “” or “Figure” throughout the manuscript according to the journal’s style guide.
4. The transition between Section 3.1.3 (FCM clustering results) and Section 3.2 (Results of catchment classification) is abrupt. Consider adding a bridging sentence such as:“Based on the derived climate clusters, we further classified catchments with similar landscape attributes within each climate region.”
5. Lines 460–471:The sentence “The flow regime in climate region II presented multiple peaks following multiple peaks in precipitation in June and July during the same period.” is ambiguous. Please revise or clarify its intended meaning.
6. Line 494:The expression “Additionally, catchments with large spatial distances are capable of exhibiting similar hydrological characteristics (Maduwang and Daiying, year)” needs revision for clarity and proper citation formatting.
7. Please revised your figure and make it clearly, specially the size and format of figures.