Using Enhanced Gap-Filling and Whittaker Smoothing to Reconstruct High Spatiotemporal Resolution NDVI Time Series Based on Landsat 8, Sentinel-2, and MODIS Imagery

Round 1

Reviewer 1 Report

Good afternoon, dear authors.

I am extremely grateful to review a very valuable, in my opinion, article.

The problem raised by the authors is extremely relevant and arises among researchers from various regions of the world. The authors' approach to solving this problem is extremely interesting.

In general, the article has more advantages than disadvantages. There are several comments on the article, the correction of which will significantly improve the quality of the article.

1. Lines 409-501. The sample program code is not working, it is impossible to open the link and check it. Provide a working link to familiarize yourself with the program code.

2. Line 481. Probably an error in specifying the degree of the number. Also check the fonts and design rules (lines 506-606, line 5, etc.).

3. Figure 2. It is not clear what the signatures mean – a, b, c, d, e, f.

4. Why in Figure 4, 5 does the NDVI value change from 0 to 1, and not from -1 to 1?

5. In Figure 1, give the position of the research area on the map of China. In Figure 1a, check the legend. If you believe the legend, do you have 6 objects related to Chongzuo City? Or should it be one object?

6. Give a more detailed explanation of the use of formula 1. I don't understand how you accounted for the "t" variables. Give specific examples of calculation using this formula in the response to the reviewer.

The article can be accepted after the elimination of comments.

Author Response

Dear Reviewer,

We would like to thank the anonymous reviewer for providing an opportunity to revise the manuscript. The comments and suggestions of the reviewer are all valuable and very helpful. We have studied them carefully and have made revisions to improve the manuscript. Revised portion are marked in red in the manuscript and the main corrections and additions have been uploaded as an attachment for the reviewer's consideration.

Best regards,

Authors

Author Response File: Author Response.docx

Reviewer 2 Report

This study presents an algorithm for reconstruction of 30 m NDVI time series. The topic is interesting, but the novelty is not significant.

Using spatial similarity between satellite time series for filling missing values has been widely used in the community. The contribution of this study is not clear.

Line 62-63. A large number of studies on vegetation phenology used SG filter to smooth satellite time series. The statement that “it cannot simulate the vegetation phenology change curve” is not a disadvantage of SG filter.

Line 67-69. The selection of WS filter does not mean that SG filter was substituted.

Line 75-77. Fitting of upper envelope of NDVI time series is optional for AG method.

I suggest the authors provide an objective appraisal on existing algorithms.

Line 85-86. Confusing.

Line 150-156. This study used only NDVI, and it can be calculated with 250 m MOD09Q1. Hence, the more bands of MOD09A1 product is not an advantage in this study.

Section 3.2 Enhance Gap Filling. Many similar schemes have been proposed.

Line 291-298. Results should only contain your own results but not citations.

Figure 6. Is it meaningful to reconstruct NDVI time series of open water and buildings?

Author Response

Dear Reviewer,

We would like to thank the anonymous reviewer for providing an opportunity to revise the manuscript. The comments and suggestions of the reviewer are all valuable and very helpful. We have studied them carefully and have made revisions to improve the manuscript. Revised portion are marked in red in the manuscript and the main corrections and additions have been uploaded as an attachment for the reviewer's consideration.

Best regards,

Authors

Author Response File: Author Response.docx

Reviewer 3 Report

EGF-WS methodology is used to arrange NDVI time series data. Then, some performance metrics are used to evaluate application results. The subject is very important and the study is valuable in terms of climate change & water resources management but the novelty of the study is not sufficiently emphasized. Some suggestions and comments to the authors are presented below:

1. The flowchart of the suggested methodology should be given by more branches and in detail in Figure 3. Thus, the readers can easily follow the application procedures.

2. Literature part is looking weak. Give new and last updated examples from literature about “drought indices” as

doi.org/10.1080/02626667.2021.1934473

doi.org/10.3390/rs15051297

3. The performance metrics part is very important for the evaluation of application results. For this aim, more performance metrics as NSE, RSR (Ratio of RMSE to the standard deviation of the observations), MAE, R-squared etc. from literature can be calculated additionally RMSE, AD, Edge. Then, they can be given in a table. Because of the importance of the assessment of results, these metrics are widely used in the literature, especially in the forecasting studies.

4. Some statistical properties as coefficient of variation, confidence intervals, distribution characteristics, min and median, etc. of used dataset should be given in a table.

5. Spatial efficiency metric (SPAEF) can be used to compare two raster maps for different cases.

6. Conclusions part can be improved in the paper. Here is presented in a general concept.

7. What is the novelty of the paper? It should be emphasized in the paper.

Author Response

Dear Reviewer,

We would like to thank the anonymous reviewer for providing an opportunity to revise the manuscript. The comments and suggestions of the reviewer are all valuable and very helpful. We have studied them carefully and have made revisions to improve the manuscript. Revised portion are marked in red in the manuscript and the main corrections and additions have been uploaded as an attachment for the reviewer's consideration.

Best regards,

Authors

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The article can be accepted in present form. 

Author Response

Dear Reviewer,

Thank you very much for reviewing my manuscript and for your positive evaluation. I am delighted to learn that the article can be accepted in its present form.

I would like to express my gratitude for your valuable feedback and support throughout the review process. Your insights and suggestions have contributed to improving the quality of the manuscript.

Once again, thank you for your time and consideration.

Sincerely,

Best regards,

Authors

Author Response File: Author Response.docx

Reviewer 2 Report

My major concern is the novelty of the study. Implication of the algorithm on GEE is not a real novelty. The contribution of the study to the community should be clearly illustrated.

In the introduction, the statement of some algorithms should be improved. for example, Line 77-79. “Typically, the NDVI time-series reconstructed by time-function fitting methods are close to the upper envelope of the crop growth curve, resulting in an overestimation of NDVI during crop transplantation.” Are there any studies supporting this claim? Why function fitting methods are close to the upper envelope? Fitting the upper envelop of VI time series is optional in TIMESAT.

More citations about interpolation of VI time series using spatial similarity are needed.

Figure 6. if reconstruction of NDVI time series of buildings and water bodies is meaningless, why did the authors decide to keep them in figure 6?

Author Response

Dear Reviewer,

We would like to thank the anonymous reviewer for providing an opportunity to revise the manuscript. The comments and suggestions of the reviewer are all valuable and very helpful. We have studied them carefully and have made revisions to improve the manuscript. Revised portion are marked in red in the manuscript and the main corrections and additions have been uploaded as an attachment for the reviewer's consideration.

Best regards,
Authors

Author Response File: Author Response.docx

Reviewer 3 Report

I suggest accepting the manuscript. The authors carefully revised the paper by answering each comment from the last round. There is only a small correction in the paper. The references and citations must be checked in the paper. Some of them are improper.

Author Response

Dear Reviewer,

Thank you very much for your positive evaluation and valuable suggestion on our manuscript. We sincerely appreciate your time and effort in reviewing our work.As per your suggestion, we have carefully checked the references and citations in our paper to ensure their accuracy and appropriateness. Any improper references and citations have been corrected to maintain the quality and credibility of the research.

Once again, we would like to express our gratitude for your valuable feedback and support.

Sincerely,

Best regards,

Authors

Author Response File: Author Response.docx