Landsat-7 ETM+, Landsat-8 OLI, and Sentinel-2 MSI Surface Reflectance Cross-Comparison and Harmonization over the Mediterranean Basin Area

Round 1

Reviewer 1 Report

The paper's main contribution is the use of remote sensing to analyze vegetation dynamics and phenology in the Mediterranean area. It compares surface reflectance data from different satellite sensors and formulates adjustment equations to align the reflectance values. The integration of these datasets is feasible through the application of band-wise regression corrections. The discrepancies in reflectance values lead to corresponding variations in the estimation of biophysical parameters, such as NDVI.

 

1. **Literature Review:** The paper could benefit from a more comprehensive literature review that situates its contributions within the broader field of remote sensing and vegetation dynamics. This would help readers understand the novelty of the research.

 

2. **Study Area Justification:** The choice of the Mediterranean area as the study region is interesting, but the rationale behind this choice could be better explained. This would provide context and justify the relevance of the study.

 

3. **Reflectance Value Alignment:** The paper could delve deeper into the implications of aligning Sentinel-2 reflectance values with those of Landsat-7 or Landsat-8. A more detailed discussion on this would help readers understand the significance of this process.

 

4. **Discrepancies in Reflectance Values:** The paper should elaborate more on how the discrepancies in reflectance values lead to variations in the estimation of biophysical parameters. This would clarify the impact of these discrepancies on the overall results.

 

5. **Methodology Details:** The methodology section could provide more details on the data collection process from the Landsat-7 ETM+, Landsat-8 OLI, and Sentinel-2 MSI satellite sensors. This would enhance the replicability of the study.

 

6. **Adjustment Equations Validation:** The paper could include more information on the validation of the adjustment equations formulated. This would strengthen the credibility of the results.

 

7. **Band-wise Regression Corrections:** The paper could benefit from a more detailed explanation of how the band-wise regression corrections were applied. This would help readers understand the technical aspects of the study.

 

8. **Statistical Analysis:** The paper should provide more information on the statistical methods used to analyze the data. This would ensure the robustness of the findings.

 

9. **Limitations:** The paper could include a section discussing any limitations encountered during the experimental process. This would provide a balanced view of the research.

1.The paper could benefit from a more detailed explanation of how the band-wise regression corrections were applied.

2.The paper should provide more information on the statistical methods used to analyze the data.

3.The paper could include a section discussing any limitations encountered during the experimental process.

Author Response

The authors would like to express their gratitude to reviewer 1 for the helpful suggestions that have significantly improved the quality of their work. A general revision of the English language has been carried out throughout the entire paper as suggested in the assessment sheets.

1. **Literature Review:** The paper could benefit from a more comprehensive literature review that situates its contributions within the broader field of remote sensing and vegetation dynamics. This would help readers understand the novelty of the research.

A.1 Following the reviewer's suggestion, the authors have added a more comprehensive literature review concerning the contribution of this study to the field of remote sensing and vegetation dynamics. They have cited studies that have addressed satellite integration and their applications, while also specifying the existing gaps in the literature regarding the integration of different satellite products. These changes can be found in the Introduction paragraph (lines 72-78) and the Discussion paragraph (lines 543-566).

2. **Study Area Justification:** The choice of the Mediterranean area as the study region is interesting, but the rationale behind this choice could be better explained. This would provide context and justify the relevance of the study.

A.2 The authors express their gratitude to Reviewer 1 for the suggestion, and they agree with the importance of highlighting the motivations behind selecting the Mediterranean basin area. As a result, in the introduction, lines (98-106) have been added to provide support for the uniqueness of this area as mentioned in lines (87-97), and in the "2.1 Study area" paragraph.

3. **Reflectance Value Alignment:** The paper could delve deeper into the implications of aligning Sentinel-2 reflectance values with those of Landsat-7 or Landsat-8. A more detailed discussion on this would help readers understand the significance of this process.

A.3 The authors acknowledge the importance of emphasizing the alignment between the reflectance values of the considered satellites. For this reason, appropriate modifications have been made to the introduction paragraph (lines 71-74/98-101) and the conclusions (lines 540-566) to underscore the significance of obtaining a greater quantity and frequency of available data, which is often hindered by cloud cover and shadows, to analyse phenomena such as the effects of climate change and land-use changes. These changes serve to highlight the points discussed in lines 45-70 and 78-86.

4. **Discrepancies in Reflectance Values:** The paper should elaborate more on how the discrepancies in reflectance values lead to variations in the estimation of biophysical parameters. This would clarify the impact of these discrepancies on the overall results.

A.4 Following the reviewer's suggestion, the authors acknowledge the importance of underlining the discrepancies in biophysical parameters estimation, for this reason, they added in lines 560-563 a sentence that clarifies the impact of reflectance value discrepancies.

5. **Methodology Details:** The methodology section could provide more details on the data collection process from the Landsat-7 ETM+, Landsat-8 OLI, and Sentinel-2 MSI satellite sensors. This would enhance the replicability of the study.

A.5 Following the reviewer's suggestion, the methodology's structure has been appropriately modified to ensure maximum elements for study replicability. For this purpose, a paragraph titled "Google Earth Engine" has been inserted on line 167 to provide further details about the platform used for data extraction (lines 158-168). Additionally, the specifications of the products used have been added for each sensor (lines 179/186/194-195). Lastly, more information about the sampling method used has been added (lines 227-237).

6. **Adjustment Equations Validation:** The paper could include more information on the validation of the adjustment equations formulated. This would strengthen the credibility of the results.

A.6 The authors express their gratitude to the reviewer for the suggestion. All information regarding the validation of the formulated equations is presented in the paper, both for the test set and the training set. Concerning the test set, the validation of the obtained results is well-documented through the Mean Difference and RMSD values reported before and after adjustment. Specifically, Table 5 shows the reflectance values' differences between MSI and OLI bands before adjustment, with asterisks indicating the significance of the t-test with a p-value > 0.05. Table 7 presents the same statistics after adjustment, following the application of regression models' obtained coefficients. This information is also available for the MSI vs ETM+ comparison in Tables 9 and 11 and for the OLI vs. ETM+ comparison in Tables 13 and 15. Additionally, the same information is provided for the test set in Appendix A (Tables A.1-A.6). For each sensor pair comparison conducted on 30% of the sampled data, Mean Difference, RMSD, and t-test significance values are available before and after harmonization.

7. **Band-wise Regression Corrections:** The paper could benefit from a more detailed explanation of how the band-wise regression corrections were applied. This would help readers understand the technical aspects of the study.

A.7 Following the reviewer's suggestion the authors added a more detailed explanation of how the band-wise regression correction was applied in lines 289-293.

8. **Statistical Analysis:** The paper should provide more information on the statistical methods used to analyze the data. This would ensure the robustness of the findings.

A.8 The authors express their gratitude to the reviewer for their valuable suggestions. All information regarding the statistical methods used in this study is described in detail from lines 268 to 311. A clarification on the parameters of Eq.2 has been added at lines 301-303. The authors believe that these descriptions provide a clear understanding of the methodologies, complementing the results of the analyses. The authors understand the importance of clear and comprehensive communication regarding statistical analyses. To ensure clarity, the authors would be delighted to provide any additional details or specifications if there are specific aspects that the reviewer would like the authors to elaborate on.

9. **Limitations:** The paper could include a section discussing any limitations encountered during the experimental process. This would provide a balanced view of the research.

A.9 Following the reviewer's suggestion the limitations encountered during the experimental process have been included in lines 600-603.

Summary

R#1.1. The paper could benefit from a more detailed explanation of how the band-wise regression corrections were applied.

A. Following the reviewer's suggestion, a more detailed explanation of how band-wise regression corrections were applied was added in lines 289-293

R#1.2. The paper should provide more information on the statistical methods used to analyze the data.

A. Regarding the information about the statistical methods used, the authors have described them in detail from lines 268-311. The authors recognize the significance of clear and comprehensive communication concerning the statistical analyses and remain available to provide further details should the reviewer desire clarification on specific aspects.

R#1.3. The paper could include a section discussing any limitations encountered during the experimental process.

A. Following the reviewer's suggestion, the authors acknowledge the importance of including the limitations encountered during the experimental process. To address this, a clarification has been added at lines 600-603.

Author Response File: Author Response.pdf

Reviewer 2 Report

In the manuscript entitled “Landsat‐7 ETM+, Landsat‐8 OLI, and Sentinel‐2 MSI surface reflectance cross‐comparison and harmonization over the Mediterranean basin area.”, authors compare the difference in reflectance values of Landsat-7 Landsat-8 and Sentinel-2 satellite missions in the Mediterranean Basin study area. The study was carried out in Google Earth Engine, extracting information for the three satellite catalogues for the years 2017-2020. The results of the study analyses provide coefficients that can be used to harmonize the spectral bands of the ETM+, OLI and MSI surface reflectance products for landscape studies in the Mediterranean basin area.

 

The paper is well written, with a comprehensive introduction, the methodological and results part properly divided into paragraphs. The main aspects to be improved are: 1. sample selection; 2. discussions in relation to other similar studies conducted in other parts of the world.

 

Following are the requirements for authors.

 

Please, remove the full stop at the end of the title.

 

Lines 49-65. As a general suggestion, consider combining several short paragraphs (one to three sentences) into one.

Line 62. Please, consider to add a presentation (it is fine also a very short one) of Landsat and Sentinel satellite missions.

Lines 78. Add a space after the full stop.

Lines 97-98. Specify what type of pairwise comparison was performed. I assumed a comparison between the same wavelengths or bands. The end of sentences “sensor values” is misleading

Figure 1. Please add the grid values of the N and E coordinates. The coordinates are shown at line 117 and help to locate the study area extension.

Lines 190-192. How was the sample selection made? Was any stratification performed (biogeographic area, coverage, etc.)? Was spatial distribution in the study area of the samples ensured? Finally, is there any reason to justify the chosen sample size?

Line 216. Specify the name of first author for [31]. Please, made the same in all the manuscript.

Line 235. ODR is already presented. Please, made a check il all abbreviations.

Line 237, add capital letter.

Eq.2. Please, provide the specification of all parameters in the equation.

Figure 2. Please, try to increase the image quality.

Table 7. Please, check the caption. At line 316 a full stop is missing.

The tables layout is not consistent

Line 407-408. Is this section about OLI-ETM+. Please, check all the paragraph with name and values.

Line 461. Sentinel-2

Lines 471-472. Please rewrite this sentence in the context of the study.

Lines 498-506. Add a more comprehensive discussion of the findings in the literature. Are the results obtained in other parts of the world comparable to those in your study? Please discuss the reasons for similarities or differences with your own experience. What differences are found for the Mediterranean basin remains poorly defined.

Lines 518-519. Please, provide a more accurate description of GEE.

 

Author Response

The authors extend their gratitude to Reviewer #2 for the invaluable opportunity to review the paper and for providing valuable insights that have significantly improved the quality of their work.

R#2. Please, remove the full stop at the end of the title

A. The authors would like to express their gratitude to the reviewer for the helpful suggestion. As the reviewer's recommendation, the period at the end of the title has been removed.

R#2. Lines 49-65. As a general suggestion, consider combining several short paragraphs (one to three sentences) into one. R2. Line 62. Please, consider to add a presentation (it is fine also a very short one) of Landsat and Sentinel satellite missions.

A. The authors sincerely appreciate the valuable suggestion provided by the reviewer. The sentence has been modified accordingly to reflect the change, and a brief description of satellite missions has been provided to introduce what will be presented in paragraph “2.2 Satellite data”.

R#2. Lines 78. Add a space after the full stop.

A. The missing space has been added as suggested.

R#2. Lines 97-98. Specify what type of pairwise comparison was performed. I assumed a comparison between the same wavelengths or bands. The end of sentences “sensor values” is misleading.

A. Following the reviewer's suggestion, the authors have modified the indicated sentence to clarify that the comparison was applied between the surface reflectance values of the bands (Lines 116-118)

R#2. Figure 1. Please add the grid values of the N and E coordinates. The coordinates are shown at line 117 and help to locate the study area extension.

A. Following the reviewer’s suggestion Figure 1 was accordingly modified adding the grid values of N and E coordinates.

R#2. Lines 190-192. How was the sample selection made? Was any stratification performed (biogeographic area, coverage, etc.)? Was spatial distribution in the study area of the samples ensured? Finally, is there any reason to justify the chosen sample size?

A. The authors have followed the referee’s suggestions and provided a more specific clarification about the sampling methods utilized. They have added a description of the sample selection, specifying the type of stratification and spatial distribution used (Lines 227-235). As stated in the text, the authors opted for a random sampling of points uniformly distributed among the land cover classes. They did not use a specific stratification method, as they aimed to capture the entire variability of reflectance values in the Mediterranean basin study area, characterized by a specific set of biogeographical and bioclimatic features, as described in the paragraph "2.1 Study area". To ensure a fair distribution of points among the land cover classes, the authors utilized the ESA “WorldCover” map. A limitation, as noted in the lines (599-602), was the availability of cloud-free and shadow-free images. To address this issue and meet the requirement for spatially well-distributed points and clean images for all three sensor comparisons, the number of sampling points was gradually increased to 1500.

R#2. Line 216. Specify the name of first author for [31]. Please, made the same in all the manuscript.

A. Following the reviewer’s suggestion the name of the first author was added to the whole manuscript.

R#2. Line 235. ODR is already presented. Please, made a check il all abbreviations.

A. Following the reviewer's suggestion, the authors have modified the indicated sentence using ODR abbreviations.

R#2. Line 237, add capital letter.

A. Following the reviewer's suggestion, the authors have added a capital letter.

R#2. Eq.2. Please, provide the specification of all parameters in the equation.

A. Following the reviewer's suggestion, the parameters of Eq.2 were specified.

R#2. Figure 2. Please, try to increase the image quality.

A. The authors have followed the referee's suggestions and attempted to improve the image quality. Although the difference with the previous image may not be particularly noticeable, the procedure carried out for downloading the images from the Matlab software ensures the maximum resolution possible for this type of image.

R#2. Table 7. Please, check the caption. At line 316 a full stop is missing.

A. Following the reviewer's suggestion, the authors have modified Table 7 layout and caption.

R#2. Line 407-408. Is this section about OLI-ETM+. Please, check all the paragraph with name and values.

A. Following the reviewer’s suggestion, the authors have corrected the name and values of this section.

R#2. Line 461. Sentinel-2

A. Following the reviewer’s suggestion, the authors have corrected Sentinel -2 names.

R#2. Lines 471-472. Please rewrite this sentence in the context of the study.

A. Following the reviewer's suggestion, the sentence has been rephrased to better suit the context of the study (Lines 513 – 517)

R#2. Lines 498-506. Add a more comprehensive discussion of the findings in the literature. Are the results obtained in other parts of the world comparable to those in your study? Please discuss the reasons for similarities or differences with your own experience. What differences are found for the Mediterranean basin remains poorly defined.

A. The authors have followed the referee's suggestions and have provided more comprehensive elaboration in the discussion section of the paper. As advised, they have highlighted the similarities and differences with existing studies in the literature, paying particular attention to how this work addresses the gap present in the literature. These improvements are evident in lines 543-567.

R#2. Lines 518-519. Please, provide a more accurate description of GEE.

A. The authors acknowledge the need to emphasize the role of GEE in this study and have followed the referee’s suggestions. Therefore, a more detailed description of the potential of GEE has been added to the conclusion of the paper (lines 586 – 598). Additionally, the structure of the "Materials and Methods" section has been reorganized to include a subsection titled "2.2.1 Google Earth Engine." This addition provides more information about the platform and better addresses the journal topic "Google Earth Engine Applications for Monitoring Natural Ecosystems and Land Use."

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you for taking all my suggestions into consideration. The work is improved and more comprehensive for readers