How Do Satellite Precipitation Products Affect Water Quality Simulations? A Comparative Analysis of Rainfall Datasets for River Flow and Riverine Nitrate Load in an Agricultural Watershed

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript nitrogen-3263110 concerns an in-depth comparative analysis of three rainfall datasets in predicting flow and nitrate transport within the Tar-Pamlico coastal watershed (North Carolina, USA) using the SWAT+ model. The results prove significant changes in flow and nitrate load predictions depending on the rainfall dataset used. I think the results reported in the manuscript could be useful for the scientific community involved in hydrological modelling and nitrate transport, but after having carefully read it, I suggest accepting it with “minor revisions”.

Main comment: I suggest sharing in the manuscript the results from the discussions of the results. In this present form the reader does not understand the difference between your results (description of data, their organization, etc.) with respect to the critical analysis of how your results were treated.

Main comment: please underline in-depth, by adding several sentences in the new paragraph “discussion”, how your study improved the application of the SWAT+ model for the assessment of nitrate transport for different rainfall scenarios.

Minor comments and suggestions

Figure 1: add a small map of USA with the location of your case study area.

Reword the conclusions by using the bullet point approach.

Author Response

Response to reviewer-1:

 

 

Comment 1: The manuscript nitrogen-3263110 concerns an in-depth comparative analysis of three rainfall datasets in predicting flow and nitrate transport within the Tar-Pamlico coastal watershed (North Carolina, USA) using the SWAT+ model. The results prove significant changes in flow and nitrate load predictions depending on the rainfall dataset used. I think the results reported in the manuscript could be useful for the scientific community involved in hydrological modelling and nitrate transport, but after having carefully read it, I suggest accepting it with “minor revisions”.

 

Response 1: Thank you for your thoughtful and constructive comments on my manuscript. Based on your valuable feedback, I have carefully revised the manuscript to address the points raised. I believe these changes have improved the clarity and overall quality of the paper. Your positive response and recommendation for minor revisions are greatly appreciated, and I am hopeful that the revised version will meet your expectations.

 

 

Comment 2: Main comment: I suggest sharing in the manuscript the results from the discussions of the results. In this present form the reader does not understand the difference between your results (description of data, their organization, etc.) with respect to the critical analysis of how your results were treated.

 

Response 2: Thank you for your insightful comment. To address this, I have added further details to distinguish between the presentation of results and the critical analysis of these findings. Specifically, I have included a deeper discussion on how the results from each rainfall dataset (ERA5, IMERG, and gridMET) impact the flow and nitrate load simulations. The manuscript now clearly outlines not only the organizational aspects of the data but also offers a critical evaluation of how these datasets influence the interpretation of nitrate transport and flow predictions. These additional details aim to provide the reader with a more comprehensive understanding of the implications of the results and how they align with or diverge from expectations based on dataset characteristics.

 

 

Comment 3: Main comment: please underline in-depth, by adding several sentences in the new paragraph “discussion”, how your study improved the application of the SWAT+ model for the assessment of nitrate transport for different rainfall scenarios.

 

Response 3: Thank you for your valuable feedback. In response, I have added a new paragraph focusing on the flow-nitrate relationship, which provides a detailed discussion of how different rainfall datasets influence both flow and nitrate transport in the watershed. This discussion is now fully integrated with the results, offering a clearer connection between the data presented and the critical analysis of the findings.

 

 

Minor comments and suggestions

Comment 4: Figure 1: add a small map of USA with the location of your case study area.

 

Response 4: Thank you for your comment. I would like to inform you that the North Carolina state map is already provided in Figure 1. Additionally, I have added more elements to the figure, including visual representations of the three rainfall datasets (ERA5, IMERG, and gridMET) and SWAT+ simulations (in addition to figure 2).

 

 

Comment 5: Reword the conclusions by using the bullet point approach.

 

Response 5: Thank you for your suggestion regarding rewording the conclusions using a bullet-point approach. While I understand that bullet points can provide a concise summary of key findings, there are multiple ways to structure a conclusion. For this paper, I prefer to maintain a paragraph format because it allows for a more cohesive narrative that ties together the complex interactions between rainfall datasets, flow dynamics, and nitrate transport.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript deals with the prediction of nutrient (nitrate) runoff. Soil and Water Assessment Tool Plus 14 (SWAT+) is adopted to compare three rainfall datasets, i.e., ERA5, IMERG, and gridMET, and their influence on flow and nitrate load predictions in the Tar-Pamlico watershed, located in eastern North Carolina.

The paper is an original contribution, and the tests on real case study (i.e., Tar-Pamlico watershed, in eastern North Carolina) strengthen the research.

The article complies with the aims and is is of interest for the readership of the Nitrogen journal.

English language is clear, the presentation is good; anyway, I have detected some criticisms in the text that should be properly addressed.

Authors can benefit from the comments below to improve their paper. These have to be accomplished before manuscript acceptance.

 

 

Title

Title is appropriate.

 

 

Abstract

The abstract is concise and reflects the content of the article. It summarizes the main outcomes of the study.

 

 

Keywords

Ten relevant keywords are provided.

 

 

Introduction

Aims of the study are properly clarified in the Introduction. Relevant references are included.

Lines 31-33: Concerning relevant studies addressing hydrological and nutrient dynamics, Authors are recommended to include, among others, the following reference recently published on a MDPI journal:

-        Innovative and Reliable Assessment of Polluted Stormwater Runoff for Effective Stormwater Management. Water 16(1):16, https://doi.org/10.3390/w16010016.

Lines 70-75: The authors should clarify why it is interesting/better to explore a monthly rather than a daily scale for flow and nitrate transport modeling.

Lines 88-91: Spatial and temporal distribution of nitrate loads are affected by the spatial and temporal distribution of rainfall. Also the description of the areal distribution of rainfall is important to improve hydrological modeling.

 

 

 

Materials and methods

This section is clear and adequately detailed.

Authors are encouraged to adopt SI units.

The provided figure 1 is clear and necessary. The examined rivers could be profitably included in the map.

Lines 142-143: Since model calibration is crucial for the reliability of model outcomes (i.e., flow and nitrate loads), Author should provide more detail on the adopted calibration procedure.

 

 

Results and discussion

This section is clear and it follows a logical sequence.

Discussion is consistent with the results. It is supported by some relevant references.

Lines 182-184: Author should specify the time period/periods considered for the comparison between grid-MET, IMERG, ERA5 and observed rainfall data. Were only seasonal data compared?

Are the annual flow values ​​(m³/s) of the different years from 2003 to 2019 always higher for IMERG and lower for grid-MED in different locations of the Tar-Pamlico river basin?

Are there experimental data available on nitrate concentrations in the watershed under consideration? Author should consider to promote a future monitoring campaign in the watershed.

Please, replace in the section “Km” with “km”: k lowercase. “N” is the symbol of nitrogen. Does “N” mean nitrate or nitrogen? Please, check.

 

 

Limitations and Future Research Directions

Future research directions should explore sensitivity analysis and uncertainty quantification in modelling when assessing flows and nitrate loads

 

Conclusions

Conclusions seem reasonable and are supported by the results.

 

 

References

One references is recommended in order to enhance “Introduction” Section. It concerns the importance of taking into account the dynamics of pollutants in wet-weather runoff for effective design and management of stormwater control measures Apart from this reference, based on my knowledge, no important reference is missing.

 

Author Response

Response to reviewer-2:

 

 

Comment 1: The manuscript deals with the prediction of nutrient (nitrate) runoff. Soil and Water Assessment Tool Plus 14 (SWAT+) is adopted to compare three rainfall datasets, i.e., ERA5, IMERG, and gridMET, and their influence on flow and nitrate load predictions in the Tar-Pamlico watershed, located in eastern North Carolina.

 

Response 1: Thanks for taking time out from your busy schedule to review my manuscript. Your assessment is correct. I used 3 rainfall datasets to analyze its impact on nitrate load simulations.

 

 

Comment 2: The paper is an original contribution, and the tests on real case study (i.e., Tar-Pamlico watershed, in eastern North Carolina) strengthen the research.

 

Response 2: Thanks for finding this study impactful. Your recognition means a lot to this work.

 

 

Comment 3: The article complies with the aims and is is of interest for the readership of the Nitrogen journal.

 

Response 3: Thanks for your complement.

 

 

Comment 4: English language is clear, the presentation is good; anyway, I have detected some criticisms in the text that should be properly addressed.

 

Response 4: Thanks for the complement. I have revised the manuscript thoroughly to make the paper structure better.

 

 

Comment 5: Authors can benefit from the comments below to improve their paper. These have to be accomplished before manuscript acceptance.

 

Response 5: Thanks for providing comments on this work. 

 

 

Comment 6: Title: Title is appropriate.

 

Response 6: Thanks for finding the title appropriate. To improve it even further, I changed the title to:  How do satellite precipitation products affect water quality simulations? A Comparative Analysis of Rainfall Datasets for River Flow and Riverine Nitrate Load in an Agricultural Watershed

 

 

Comment 7: Abstract: The abstract is concise and reflects the content of the article. It summarizes the main outcomes of the study.

 

Response 7: Thanks for the complement.

 

 

Comment 8: Keywords:Ten relevant keywords are provided.

 

Response 8: Thanks for your comment. 

 

 

Comment 9: Introduction: Aims of the study are properly clarified in the Introduction. Relevant references are included.

 

Response 9: Thanks for your comment. I added additional references.

 

 

Comment 10: Lines 31-33: Concerning relevant studies addressing hydrological and nutrient dynamics, Authors are recommended to include, among others, the following reference recently published on a MDPI journal:Innovative and Reliable Assessment of Polluted Stormwater Runoff for Effective Stormwater Management. Water 16(1):16, https://doi.org/10.3390/w16010016.

 

Response 10: Thanks for your comment. I added the suggested reference.

 

 

Comment 11: Lines 70-75: The authors should clarify why it is interesting/better to explore a monthly rather than a daily scale for flow and nitrate transport modeling.

 

Response 11: Thanks for your comment. The details are added in the methodology and results & discussion part. It is challenging to calibrate the model for nitrate at daily scale, and also data limitation is a big aspect. On top of that the policies we simulate using these models are at annual average scale, so having monthly temporal resolution is not-required considering scope of the study.

 

 

Comment 12: Lines 88-91: Spatial and temporal distribution of nitrate loads are affected by the spatial and temporal distribution of rainfall. Also the description of the areal distribution of rainfall is important to improve hydrological modeling.

 

Response 12: Thanks for your comment. I added these details in section 3.1. 

 

 

Comment 13: Materials and methods: This section is clear and adequately detailed.

 

Response 13: Thanks for your comment and finding the materials and methods section appropriate.

 

 

Comment 14: Authors are encouraged to adopt SI units.

 

Response 14: Thank you for your comment. I have made an effort to primarily use SI units throughout the manuscript, especially in the supplementary information. However, in some instances, I have included other units, particularly where they are more commonly used or understood by the local audience in North Carolina and the broader U.S. Given the regional focus of the study, I aimed to balance the use of SI units with units that might be more familiar to practitioners and policymakers in this area.

 

 

Comment 15: The provided figure 1 is clear and necessary. The examined rivers could be profitably included in the map.

 

Response 15: Thanks for your comment. I enhanced figure 1 even more.

 

 

Comment 16: Lines 142-143: Since model calibration is crucial for the reliability of model outcomes (i.e., flow and nitrate loads), Author should provide more detail on the adopted calibration procedure.

 

Response 16: Thanks for your comment. This study utilized the SWAT+ model from my previous publication. However I added the details in framework (Figure 2)

 

 

Comment 17: Results and discussion: This section is clear and it follows a logical sequence.

 

Response 17: Thanks for your comment and finding this section adequate.

 

 

Comment 18: Discussion is consistent with the results. It is supported by some relevant references.

 

Response 19: Thanks for your complement.

 

 

Comment 20: Lines 182-184: Author should specify the time period/periods considered for the comparison between grid-MET, IMERG, ERA5 and observed rainfall data. Were only seasonal data compared?

 

Response 20: Thanks for pointing it out. I added the details in section 3.1 and 3.2. Also in figure 2.

 

 

Comment 21: Are the annual flow values ​​(m³/s) of the different years from 2003 to 2019 always higher for IMERG and lower for grid-MED in different locations of the Tar-Pamlico river basin?

 

Response 21: Thanks for your comment. Mostly yes, but study discussed more about annual average values from 2003 to 2019.

 

 

Comment 22: Are there experimental data available on nitrate concentrations in the watershed under consideration? Author should consider to promote a future monitoring campaign in the watershed.

 

Response 22: Thanks for your comment. We have nitrate concentration data available at Washington, NC for significant time period.

 

 

Comment 23: Please, replace in the section “Km” with “km”: k lowercase. “N” is the symbol of nitrogen. Does “N” mean nitrate or nitrogen? Please, check.

 

Response 24: Thanks for your comment. I revised the units accordingly. 

 

 

Comment 25: Limitations and Future Research Directions: Future research directions should explore sensitivity analysis and uncertainty quantification in modelling when assessing flows and nitrate loads

 

Response 25: Thanks for your comment, I revised the section accordingly.

 

 

Comment 26: Conclusions: Conclusions seem reasonable and are supported by the results.

 

Response 26: Thanks for finding the conclusion section reasonable. I made few changes to make it more impactful. 

 

 

Comment 27: References: One references is recommended in order to enhance “Introduction” Section. It concerns the importance of taking into account the dynamics of pollutants in wet-weather runoff for effective design and management of stormwater control measures Apart from this reference, based on my knowledge, no important reference is missing.

 

Response 27: Thanks for suggesting the reference, I added it in the revised version. Thanks for your time and all your comments.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been significantly improved following the recommendations of the Reviewers; all my concerns have been addressed and convincingly justified by the Authors.