This manuscript describes a novel spatially distributed ecohydrologic model ECOTWIN. This model links hydrology with nutrient and isotopic modules. The capability has great potential to improve our understand the terrestrial and aquifer ecosystem interactions. Although similar capability has been attempted in several other studies with different focus such as dissolved organic carbon (DOC), the inclusion of lateral flow with nutrient and carbon cycle is still a challenge in most spatially distributed (eco)hydrological models.

There are some missing information in the current manuscript, such as (1) groundwater level; (2) resolution for evaluation; (3) chemical reaction. See details below. The presentation of some information can be improved, especially some figures are too small to be read. 

Distributed, do you mean spatially distributed? 

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Sentence stoped abruptly without giving reasons.

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What is  “somehow”

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Where is 81% come from? Reference needed

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On disk/memory change to online and offline coupling

Figure 1. Is soil zone part of vadose zone? 

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What if the water table is above the last soil layer?

Equation 16:

If the pond is not next to the river channel, why or how it contribute to the channel flow?

Similarly, in equation 18, if the terrestrial grid cell is far away from the river channel, how does the vadose zone contribute to the channel flow?

Figure 2:

Does not have coordinates. Such as longitude and latitude information.

What are the particular reason why these 17 watersheds were chosen. 

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Does the model need spin-up to reach steady state? 

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How do you consider the upstream - downstream dependency in discharge, some watersheds are connected based on Figure 2.

Figure 4:

Show line/color meaning in the Figure as well. Also missing time step information.

The subfigures are too small to read. Maybe move some into SI and only show a few here. 

Figure 5. Is the time series watershed averaged? If so, the caption should mention that. 

Figure 6:

The GRACE datasets resolution might be too coarse for comparison; there need to be some justification somewhere. 

Check other spatial products as well, maybe a table listing all produce with original and resample resolution for reference.

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Integrated C++ framework: C++ is a programming language, not a framework. 

Reviewing of the manuscript open for discussion ‘EcoTWIN 1.0: A Fully Distributed Tracer-Aided Ecohydrological Model Tracking Water, Isotopes, and Nutrients’ by Songjun Wu et al. submitted to Geoscientific Model Development (Manuscript ID: egusphere-2025-3941).

The authors have developed/improved and ecohydrological model with advanced traits, tracking water, isotopic and nutrient fluxes. The reviewer considers that this model development work is worthwhile and beneficial to the modeling community and fall within well with the journal topic. There are a couple of issues that the reviewer suggests for the authors to refer, and they are listed in detail as follows:

1. Almost all figures need to be replotted. Here I give some examples. Fig. 1: The texts are too small to read, and the subtitles need add some explanations for the figure, so that the reader is easy to follow it; Fig. 2: the coordinates need to be added, and some important landmarks or rivers information should better be added; the texts are to small to be identified; Fig. 3: the color for the color should be more identifiable, rather than orange and blue two colors; Fig. 4: the time series figures give no information to the reviewer; similar issues also for Fig. 5-7, and the reviewer will NOT list all issues for the figures. The authors should check them carefully and revise them all.
2. The structure of this manuscript may be adjusted. Part 4.1 and 4.2 may be moved under the section of 3 Model calibration and validation? This should be one section to discuss the model advantages of accuracy compared with previous models? For example, providing specific skill metrics values.
3. Provide some explanations for the reason why these 17 catchments are selected for this study.
4. What is the difference between model calibration and validation? The reviewer could not quite understand herein, it seems the difference is defined by the different variables that are choose for the model-to-data comparison. Please explain it.
5. More skill metrics should be defined and used for model performance. For example, Root Mean Square Error, Correlation Coefficient, Mean Value Difference etc. to evaluate the model performance. Then, give the statistics and compare them with previous other models.
6. Some small mistakes or errors. Table 1, the table better not crossing two pages. Line 378, add the equation number and the meaning of it (e.g., what does l mean in the left of the equation). Be consistent use KEG or KGE, and define it and explain it (e.g., what are the value ranges, and the corresponding performance, excellent, good, normal, poor etc.)
7. Line 451: ERA5 reanalysis products equal to observations? Please double check it. Line 531-532: ‘However, it increases severely increase the ……’ Delete one of the ‘increases’. Line 694: the first letter of the word ‘mediterranean’ should be initialized.

The authors have responded some of my concern while it is still not clear to me what changes and improvement have been made during this round. The responses are either vaguely stated in the response letter or could not find the revisions in the updated manuscript. Additionally, the differences between the observations and ERA5 data are still not well explained. Most figures still need significant improvement. A much clear point-to-point response letter and revised manuscript should be added.