See the attached document for my thoughts.

This paper presents the process and evaluation of re-programming of the legacy WaterGAP model into the new Python platform. While this study is of interest to the hydrological modeling community, the presentation of this manuscript needs to be substantively revised to highlight the scientific merit of the current study. In its current structure, I am afraid the manuscript does not meet the standard of scientific publication and cannot be accepted.

 Specific comments

1. My major concern of this manuscript is its structure and organization. The manuscript contains sections with too many (10) main sections and levels of subsections. In addition, some subsections contain overlapping information (across different main sections). This tedious structure makes the general reading of the manuscript very difficult. In addition, Section 5.1 and 5.2 contain subsections with identical titles (Architecture and new features), which should be avoided or simply merged together to reduce the level of subsections. In particular, Section 5.2.1 contains a single sentence with a reference to the Supplement and a GitHub link, which is completely unnecessary to make a separate subsection.
2. In general, the authors should either avoid using abbreviations in section titles, or spell out their full names. Examples including FAIR4RS (Section 3.1.3 and 6 title), WGHM (5.1), GWSWUSE (5.2).
3. Section 3.1.1 “Indicators for best practices in software engineering”: this section seemingly contains information that overlaps with Section 4.3.1 “Best practices for code development”, such as document and version control and automation. Please consider re-structure these parts to make the presentation more precise by, e.g. merging redundant information.
4. Figure 2: the right panel of the figure contains major user stories that are purely text. Such information can be better presented using a table or in-text description rather than a graphic presentation (figure).
5. Section 4.3 title: get rid of the period after “Code development”.
6. Section 4.3.2: narrative in this section contains many hyperlinks to GitHub repositories, which impedes the flow of the text and makes reading difficult. Authors may consider documenting these links in supplementary information (SI) and making reference to SI.
7. Table 2: A graphic presentation (e.g. bar plots) is recommended to better show the difference between the outputs of the reprogrammed and legacy software.
8. Sections 9 does not contain much in-depth discussion, but rather recap of what has been presented in previous sections. Consider either merging this into concluding remarks (Section 10).

Reviewing of the manuscript ‘The Process and Value of Reprogramming a Legacy Global Hydrological Model’ by Emmanuel Nyenah et al. (2025) submitted to Geoscientific Model Development (GMD).

This work focuses on the model development and updates to a legacy global hydrological model, after reprogramming (e.g., different structures and languages, from C/C++ to python), aiming to make this software more sustainable and accessible to model users. The reviewer considers that the authors have done sufficient efforts to develop the numerical model, while it is struggle to me is that whether this kind of article type or writing style is acceptable to the journal GMD. Several years ago, a popular storm surge modelling community suggests how to consider the modelling work aside from publication. While it is now in consistence that modeling work is as important as publications, my concern is that whether this style could be published in a scientific journal.

For a journal, the novelty is very important, and previous limitations should also be pointed out and summarized. Regarding this manuscript, one major concern to me is the structure, which includes overly 10 parts, and should be reduced significantly. This leads to a presentation of the work, instead of proposing an assumption, and solving it in a scientific way. Secondly, it should be clear what is the advantages of this new model, e.g., computational efficiency or stability? The reviewer considers that the model updates should first consider the accuracy, e.g., whether the new model improves the model simulation results. So, several case studies or model applications should be included, and model-to-data comparison of hydrological variables should be conducted and listed.

Here are additional detailed comments and suggestions:

1. L124-125: please provide reasons why this work only concerns the reprogramming of GWSWUSE and WGHM models operating at the 30 arc-minute resolution. Why do try it based on 5-arc-minute resolution?
2. Avoid using abbreviations in the subsection title, e.g., FAIR4RS, WGHM, GWSWUSE etc.
3. Some subsections are too short, and some even start with the subsection title without any contents, e.g., 5.1, 5.2 etc.
4. In Table 2, what do the values in the bracket mean?
5. Figure 5, the percentage is used for the survey results, so the reviewer suggests including the sample size or the total sample number.