The Long-Term Detection of Suspended Particulate Matter Concentration and Water Colour in Gravel and Sand Pit Lakes through Landsat and Sentinel-2 Imagery

Round 1

Reviewer 1 Report

Reviewer’s Report on the manuscript entitled:

Long-term detection of SPM concentration and water colour in gravel and sand pit lakes through Landsat and Sentinel-2 imagery

The authors investigated physical and optical properties in a large sample of pit lakes located in the Po River basin (Italy) during 1990-2021, using a combined approach of Landsat and Sentinel-2  imagery and traditional limnological techniques. They showed that Landsat and Sentinel-2 have a great potential for SMP assessment. I found the topic and results generally interesting. The presentation, figures and literature review can be further improved. Some additional clarifications on the methods applied need to be presented as well. Please see below my comments.

Title. Please replace SPM by its full name. Please avoid using abbreviations in the title. The title could be improved.

The abstract has too many details but lacks the main findings and purpose of the study. Why is this study important and who may benefit from it should be clearly mentioned in the end abstract.

Introduction. Please add the following review article on SPM by Walch et al.

https://doi.org/10.1016/j.watres.2022.118655

Lines 107-115. Please use the present verbs here only not past verbs. Please also use bullet points to highlight the main contributions of this study.

Figure 1. Please include the geographic latitudes and longitudes for this map.

Line 133. Please do not start a sentence with a number.

Line 160. The authors could take advantage of the Google Earth Engine (GEE) platform for all the data processing. Are the authors aware on GEE? Please elaborate.

Line 170. Cloud contamination is a main problem in optical imagery. How did the authors insure cloud free images? Did they use any cloud masking algorithms. Please elaborate. See for example, the first article that I suggested below describing this for the ecoregions of Italy.

Line 217. Please explain how you resampled the data.

Line 222. How is this ROI selection done? Please add the details.

Figure 2. Since the x-axis and y-axis have the same value range, I suggest having the plots demonstrated as square panels not rectangle. Please reproduce the figures. Please do not stretch it vertically to avoid the texts to be stretched. Likewise for Figure 3. Please also enlarge the font size of both texts and numbers in this figure.

Discussion Section.

Ghaderpour et al. in [https://doi.org/10.1016/j.jag.2023.103241] studied the impact of climate change on land cover in Italy and Po river basin. It shows how the land cover types have changed over time due to climate change and anthropogenic activities. I suggest the authors discuss their results and potential influence on the water quantity and quality of the Po river.

Formetta et al. in [https://doi.org/10.3390/hydrology9100163] studied the Po river seasonal and annual fluctuations in a regional scale. Please also comment of the relation between river flow reduction, anthropogenic activities, and SPM concentration.

Please add an acronym table at the end of the manuscript listing all the acronyms used in the manuscript. Please also check to ensure that all the abbreviations are defined they first time they appear in the manuscript and be consistent with their style.

Finally the manuscript can benefit from a flowchart in the method section that shows inputs, outputs, and methods used.

Thank you for your contribution

Regards,

There are some grammar/punctuation/typos that need to be checked and corrected.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This paper offers a comprehensive examination of the spatial and seasonal variability of Suspended Particulate Matter (SPM) concentrations and water color (λ_dom) in pit lakes, utilizing a combination of remote sensing data from Landsat and Sentinel-2 images and in-situ measurements. The authors effectively utilize ACOLITE and WISP-3 tools to correlate SPM concentrations and λ_dom with satellite imagery, providing a novel perspective on the environmental dynamics of these lakes, particularly in the context of the Po River basin. The findings that SPM concentrations and λ_dom are highest along the Po River and decrease in smaller pit lakes provide critical insights into the impact of quarrying activities on aquatic ecosystems.

It is evident that the authors have dedicated significant effort to this study; however, I would like to offer some comments and suggestions for the authors to contemplate:

1. The level of detail in the abstract concerning statistical data might be streamlined. While the robustness of the data is commendable, such granularity is perhaps better suited for the results section of the paper.

2. I have noted an inconsistency in the terminology used in the abstract and the keywords. The term "Suspended Particulate Matter" (SPM) is used in the abstract, while the keywords list "suspended particulate matters." I recommend that the authors ensure consistency in the terminology used throughout the manuscript to maintain professional integrity and clarity.

3. In discussing the introduction of pit lakes (PLs), it would be beneficial to briefly mention the specific impacts these activities have on ecosystems, including potential threats to wildlife habitats. This could provide a more comprehensive understanding of the significance of PLs in environmental dynamics.

4. It would be advantageous to augment the discussion by delineating the merits of employing remote sensing technology as opposed to traditional methodologies. Highlighting the cost-efficiency, temporal efficacy, and extensive spatial coverage of remote sensing underscores its pivotal role in facilitating expansive environmental surveillance and stewardship.

5. In the introduction, it would be valuable to briefly compare the Landsat and Sentinel satellites, emphasising Landsat's long-term data record for historical trends and Sentinel's high-resolution and spectral capabilities for detailed, frequent monitoring, thereby justifying their combined use in analysing SPM in PLs. However, I have not seen any relevant information about Landsat and Sentinel.

6. In the “2.1. Study area”, include a brief rationale for why these particular geographical areas were chosen, beyond being well-distributed and representative. For instance, are there specific characteristics or impacts associated with these areas that are relevant to the study?

7. In the “2.1. Study area”, expand on the methodology used to identify and categorize the PLs. How were active, ceased, and doubtful PLs differentiated? What criteria were used?

8. In the “2.1. Study area”, it might also be beneficial to discuss the relevance of the study period (1990 to 2021). Why was this time frame chosen, and what is its significance in the context of PLs and quarrying activities?

9. In the “2.2. Processing of satellite images”, I noticed that while the selection of Landsat-5, Landsat-7, and Sentinel-2 satellites is well-justified for their comprehensive temporal coverage, there is no mention of the known issue with Landsat-7 data, the Scan Line Corrector (SLC) failure, which leads to striping in the images. This is an important aspect to address, as it could significantly affect the quality of the data and the subsequent analysis. Could you please clarify how the study accounted for the SLC-off gaps present in Landsat-7 images post-May 2003? It would be valuable to detail the specific approaches or algorithms used to mitigate this issue.

10. In the “2.2. Processing of satellite images”, I observed that while you have specified the total number of satellite images used (375), there is no detailed breakdown of the image distribution across the different satellite platforms or the temporal distribution of these images. For thoroughness and reproducibility, it would be beneficial to readers if you included a table or summary that outlines the number of images acquired from Landsat-5, Landsat-7, and Sentinel-2, respectively, along with the time frames these images cover.

11. In the “2.3 Field campaigns and validation”. While the methodology section is well-detailed, it may be improved by including a brief rationale for the choice of statistical metrics used for validation. Why were these particular metrics chosen, and how do they contribute to the reliability of the validation process?

12. In the “2.3 Field campaigns and validation”. It's important to detail how the ACOLITE-processed images were further analyzed post-processing. What specific steps were taken to ensure the accuracy of SPM concentrations after accounting for potential errors in pixel identification?

13. In the “3.1. Satellite data validation”. Explain any potential methodological reasons for the overestimation by the ACOLITE-processed Landsat images and the low R² value at 740 nm for Sentinel-2.

14. In the “3.3. Impact of quarrying activity and precipitation”. Elaborating on the methodology used to calculate the mean SPM concentrations before and after quarrying activities, to ensure the reader understands the process and can trust the results.

15. In the Figure 6. What specific software or tools were utilized to process the satellite imagery and extract the Suspended Particulate Matter (SPM) concentration data? What specific software or tools were utilized to process the satellite imagery and extract the Suspended Particulate Matter (SPM) concentration data?

16. In the “4.1. Reliability of remote sensing for PLs water quality assessment”. A more in-depth explanation of the recalibration process for the SPM_Nechad2010 algorithm could be provided, including how the overestimation was identified and corrected.

17. In the “4.2. Assessment of PLs water quality”. Discussing the methodological challenges and potential biases in the analysis could provide a more critical perspective on the results.

18. In the “4.3. Impact of quarrying activity and precipitation”. It may be beneficial to discuss the limitations of using satellite imagery for assessing the impact of quarrying and precipitation on SPM concentrations, such as the resolution of images or the timing of satellite passes relative to quarrying events.

 Moderate editing of English language required

Author Response

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Reviewer 3 Report

Line 36: give the min and max for both SPM and lambda dom. It will give an idea to the reader about the min-max range for both active and dormant PLs.

Why have you not considered a parameter which directly shows biological activity and correlated it with SPM and lambda dom i.e. BOD or COD?

Line 396 or the section 4 should come way before, after the introduction. I felt it was missing in your paper and was waiting to read detailed justification about the chosen two parameters.

Line 409-418 are repeated info given earlier in paper.

Line 430 needs to be removed. It is clear in other fields of research: re increased suspension of dissolved organic matter, just cite a appropriate research paper here.

Line 432-444, is more detailed explanation and should be kept. I read pretty much the same thing earlier in paper.

Line 503-509 is wishful thinking when budgets are being cut everywhere. Chose a different future direction scenario.

Author Response

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Reviewer 4 Report

The paper is well written and organized, allowing for a clear interpretation of all the work made by the authors.

My main concerns are:

1)      A lack of detail about the two methods used for SPM  and λdom computation.

2)      Why Landsat 8 data were not used. In my opinion, taking into account that possible overlapping S2 and OLI8/9 should have occurred during the measurement campaigns, it would have represented an added value for the work.

3)      The contribution to Rrs from the lake bottom. I do expect that pit lakes usually show shallow waters and hence this contribution should be considered.

Author Response

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Round 2

Reviewer 1 Report

Dear authors,

Thank you for addressing my comments and improving your manuscript.

Regards,

Please carefully proofread your manuscript before publication for any potential editorial issues.

Author Response

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Reviewer 2 Report

1.       Given the significance of this issue, I would like to kindly request that you provide sample Landsat-7 images of the study areas. This would help to substantiate the claim that the SLC-off gaps did not notably affect the small lakes under investigation, particularly those located at the centre or edges of the Tile. Your effort in providing these images would be invaluable in addressing any concerns related to data quality and ensuring the robustness of the subsequent analysis. It would also enhance the transparency and replicability of your research, which are essential aspects of scientific inquiry.

2.       While I appreciate the inclusion of ACOLITE software in the processing of satellite imagery as noted in Figure 6's caption, I've noticed that there's a lack of detailed explanation regarding the derivation of Suspended Particulate Matter (SPM) concentrations in water bodies. Could you please provide an expanded description of the methodology used in ACOLITE to map out SPM concentrations?

Moderate editing of English language required

Author Response

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Reviewer 4 Report

Dear authors,

I'm not convinced by your reply to my comments, especially for the question about why you did not use L8/L9. Spatial resolution issues (i.e. 30 m) are the same as those you faced using L5-L7 before 2015, but the integration between L8/L9 and S2 would have added more value to your research, starting from the difference in the results you achieved with atmospheric correction.  I think this point should be at least considered in the discussion. 

Author Response

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