Spatial and Temporal Pasture Biomass Estimation Integrating Electronic Plate Meter, Planet CubeSats and Sentinel-2 Satellite Data

Round 1

Reviewer 1 Report

Authors worked on pasture biomass estimation using optical satellite imagery based NDVI and ground measurements using electronic raising plate meter (RPM). They also determined the minimum number of ground measurements of RPM and walking patterns to achieve good accuracy. The overall paper is technically sound and well written. Findings of this paper may be used in agricultural decision making, However more work need to be carried out prior to deployment of this work.

I have few suggestions given as follows..

Line 39: Please add the y-axix label in the estimation graph, and also rewrite the caption, as sentinel-2 is not the only satellite source authors used in their work.

Line 112: Please add GPS model used and its accuracy

Section 2.2.1 Satellite Images: Please add some details about the image registration of all collected satellite images, as the misregistration leads to biased calibration, regressions and/or estimation.

Line 160-163: “Paddocks were divided into four sectors having around 60 plate readings (sectors corresponding to transects 1-2, 3-4, 5-6 and 7-8 in Figure 2), and the average NDVI and the kg DM/ha (derived from the calibrated RPM) of those sectors were 163  calculated.”

Please explain why paddocks were divided into four sectors and the average was taken? By averaging, we may loose the spatial variability and thus affects on estimation.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The article by Gargiulo et al. links the frequency and spatial sampling pattern of electronic raising plate meter (RPM) measurements to the accuracy of pasture biomass estimates. The authors further evaluate the use of multi-sensor optical Earth observation data for upscaling the RPM measurements.

I think the authors do demonstrate the value of Earth observation data as a tool for scaling up sparse spatial and temporal RPM measurements acquired at the point-scale. This, thus, potentially provides both a cost-effective and objective means of farmers being able to produce estimates of pasture biomass. In the present form, however, I would reccommend major revisions to this manuscript. The main issue for me is that the justification for testing different “Walking Patterns” when acquiring readings (sampling) is unclear. Surely, unless you have prior knowledge of the spatial variability in biomass across the field/paddock, the best strategy would be to apply a regular grid approach. A point also worth discussing is using the Satellite imagery data to produce maps (e.g. EVI, NDVI) of a given field to identify areas that appear different then building a more efficient sampling approach based on these locations.

Some of the background in the Introduction section is unclear and I would recommend revising some sentences, I have highlighted some of these in the specific comments. Some of the text in the Methodology could be more concise. In particular, I would recommend the author(s) follow the some examples of how Sentinel-2 and Planet CubeSat are describe in previous Remote Sensing publications.

Some specific comments:

Lines 42-44: Consider revising sentence.

Lines 46-47: Why would accurate measurements of pasture availability increase milk production? Surely it depends on what one would do with the measurements.

Lines 42-48: Throughout this paragraph I am not clear on what is meant by “utilization of pastures”, do the authors mean to make more productive? Please clarify.

Lines 50-51: Please clarify what is meant by “compressed height”.

Line 54-56: Sentence might need revising, please check.

Line 56-57: Minimum number of RPM readings? Please clarify.

Line 57-59: Is this error the difference between the RPM estimated pasture biomass and a more accurate measure of biomass? Also, I am not clear on what is meant by up to 90-100 readings, is this per season? This is unclear and needs more detail about this past study.

Line 59-61: Again, I am not clear on the number of readings. What if there were less readings but more RPM instruments covering a given paddock, would that reduce error?

Line 61: designed specifically to address what question?

Line 65: Is this not just a case of positioning the RPM based on GPS coordinates? Is this really considered a recent advancement?

Line 70: Please clarify that cloud cover is only an issue for optical satellite data.

Line 71: This is not always the case that saturation occurs with high plant density when using index-based methods. It depends on the bands used, for instance the red-edge bands are sensitive to chlorophyll but do not always saturate with high values.

Lines 75-78: Consider revising this sentence, seems like a list and not clear what the point is.

Line 89-91: As before, I am not clear on what the number of reading are related to, number of RPMs (or points locations) or number of readings throughout the season? For instance, you could have lots of readings at one location, but this would not characterise the biomass for the entire paddock.

Figure 1: Consider the use of a map inset. Would finer scale mapping be available to better resolve the study area? The large red dot seems untidy.

Table 1: At this point it is not clear why the cloud data is relevant.

Line 111-114: These two sentences in the methods appear to contradict each other. Please check.

Figure 2: Aerial image scale bar appears a bit untidy. Please consider changing to have regular from 0 intervals (e.g. 100 m). Also, what do the numbers mean in the walking patterns, sample point number? Please clarify in the figure caption.

Line 128: ESA have several locations, please remove “(Paris, France)”

Line 130: Is the weight really relevant?

Line 131: The spatial resolution is not 10 m for all bands (red-edge is 20 m). You could say Sentinel-2 has a spatial resolution of up to 10 m.

Line 132: Please include the URL to the open access hub (or include as a citation).

Lines 135-137: Why did you visually inspect and use the cloud cover tool?

Line 127-150: Consider breaking this paragraph up, it will read better. Also, a summary table of the number of Sentinel-2 and Planet imagery used in the analysis might be nice.

Line 142: Again, weight relevant?

Line 158: Why not apply the +/- 2 day criteria to the Planet data.

Line 186-187: “… compared with previous research” What previous research? Citation?

Line 189: I think it would be useful to provide these “standardised equations” and others if possible.

Line 221: Please use “Sentinel-2” here and elsewhere.

Figure 3: In the linear regression plots please include intercept value.

Results section: Please consider the use of sub-headings throughout the Results section.

Line 293: Please consider using Thomas et. al. here

Lines 290-305: This reads like a review of past research, it would be good if the authors draw some more comparison to these past studies and the present research.

Line 308: “…front or back…” of paddock, please re-consider terms here.

Line 309-311: I guess the point of sampling in the middle would be to avoid edge effects.

Lines 336-337: If the spatial resolution of Cubesat Planet is cm-scale then Planet images have a greater resolution than the 10 to 20 m satellites you discuss in the previous study.

Lines 345-347: Unless I am mistaken, Planet CubeSat data is not freely available for commercial use(?) If so, it would be prudent to make the point this would have an impact on farmers cost-benefit analysis for using the data.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

A brief summary

This research focused on the comparison of RPM, Planet and Sentinel-2 satellite data for the estimation of pasture biomass. Research objectives were fulfilled and the manuscript is written well. However, the research part regarding remote sensing should be elaborated more in the Discussion and Conclusion sections, as it is currently deficient. Many terms regarding remote sensing and GNSS in the manuscript should be checked and corrected.

Broad comments

The Introduction is written very well and covers the necessary aspects of the research. Material and Methods and Results sections are written well, with some segments that require clarification. Current parts of the Discussion are fine, but there should be more emphasis on remote sensing. Since your study area consisted of two 1.1 ha paddocks, you should discuss the potential application of UAVs in future work, as it eliminates problems with cloud cover and temporal resolution, which referred to in the Introduction. This would offer much more possibilities in my opinion compared to the radar images that you mentioned in the discussion. You should also discuss the impact of relatively small study area size on the application of Sentinel-2 images. Based on your research, what could be different in the implementation of Sentinel-2 in a larger area? Again, since this is the Remote Sensing journal, you should expand conclusions regarding applied satellite images.

Specific comments

Lines 23 and 65: You might want to replace GPS with GNSS here, as it became more frequently utilized globally.

Lines 68-72: Please split this sentence into two shorter ones to improve readability.

Line 84: Please replace “spectral indices” with “vegetation indices” to make it more exact. You can also then remove the text inside brackets.

Figure 1: The bottom part of the figure should be modified so the readers can observe the locations of two experimental paddocks and their relative position. This part shows very little new information compared to the upper part and is not justified. My recommendation is to merge the upper part of Figure 1 and the upper part of figure 2 into a study area figure.

Table 1: These observations need more clarification in terms of data source and, more importantly, the relationship of the time periods of cloud cover observations and of Sentinel-2 imaging of the study area. Cloud cover is variable during the day and this should be clarified.

Line 112: “With GNSS receiver” might be more accurate.

Line 114: “Coordinates”.

Line 127: One of “free” and “open-source” terms is redundant. I suggest removing “free”.

Line 131: Spatial resolution of 10 m only for four bands. You should clarify that in the text.

Lines 133-135: It is enough to write about the bottom of atmosphere reflectance.

Line 187: Please add a reference to this research.

Line 221: Please write “Sentinel-2” instead of “Sentinel” in the entire manuscript.

Figures 5 and 7: The font size of the text in figures should be larger for better readability.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have responded to my previous comments and have applied updates to the manuscript accordingly, which I appreciate. I would, however, have expected a more detail reponse to my comments. It would be good to report the specific text changes, along with corresponding line number, that were applied to accomodate my comments.

Author Response

Response by the authors to Reviewer 2 comments (2nd revision)

Point 1: The authors have responded to my previous comments and have applied updates to the manuscript accordingly, which I appreciate. I would, however, have expected a more detail response to my comments. It would be good to report the specific text changes, along with corresponding line number, that were applied to accommodate my comments.

Response 1: We appreciated the reviewer's comments and we apologize for not having included the lines references in our previous cover letter. For avoidance of doubt, please find below all the comments made by Reviewer 2 in his/her first revision, as well as our specific responses (i.e. the one previously submitted) but this time clearly indicating what lines were changed in the revised version. 

Response by the authors to Reviewer 2 comments (1st revision)

Note: as indicated above, the following are our responses to the comments made by the reviewer in his/her first revision. We have included them here again with all the reference to line numbers for each change made in the text.

Point 1: The main issue for me is that the justification for testing different “Walking Patterns” when acquiring readings (sampling) is unclear. Surely, unless you have prior knowledge of the spatial variability in biomass across the field/paddock, the best strategy would be to apply a regular grid approach.

Response 1: Acknowledged. We clarified and explained with a higher level of detail how the RPM is used and why different walking patterns are applied.  As suggested, a potential strategy would be to use a standard grid approach; however, in practice, farmers use more convenient patterns (the ones tested in the study) to minimize labor (this was explained in lines 71-76). The use of walking patterns was also clarified in some other segments of the discussion (lines 310-316 and 346-348)

Point 2: A point also worth discussing is using the Satellite imagery data to produce maps (e.g. EVI, NDVI) of a given field to identify areas that appear different then building a more efficient sampling approach based on these locations.

Response 2: Acknowledged. This is an interesting concept that could be applied in future work. We included this in the discussion in lines 346-348.

Point 3: Some of the background in the Introduction section is unclear and I would recommend revising some sentences, I have highlighted some of these in the specific comments.

Response 3: Acknowledged. We made significant changes throughout the whole introduction. We clarified concepts such as pasture utilization (line 46) or the minimum number of readings (lines 64-65) and provided a detailed explanation of the RPM use (lines 55-61), among others. Most of the details are provided in Point 5 responses.

Point 4: Some of the text in the Methodology could be more concise. In particular, I would recommend the author(s) follow the examples of how Sentinel-2 and Planet CubeSat are described in previous Remote Sensing publications.

Response 4: Acknowledged. We used previous research to improve and be more concise in the description of both satellites (lines 134 to 156)

Point 5: specific comments:

Lines 42-44: Consider revising sentence.

Response: Acknowledged and changed this sentence (lines 45-48)

Lines 46-47: Why would accurate measurements of pasture availability increase milk production? Surely it depends on what one would do with the measurements.

Response: Acknowledged. Changed this sentence and explained that accurate measurements allow better pasture allocation, and then higher pasture utilization (which increases milk production) (lines 49 to 51)

Lines 42-48: Throughout this paragraph I am not clear on what is meant by “utilization of pastures”, do the authors mean to make more productive? Please clarify.

Response: Acknowledged and defined the concept “pasture utilization” in the second sentence of the paragraph. (line 46)

Lines 50-51: Please clarify what is meant by “compressed height”.

Response: Acknowledged and included a detailed explanation of how compressed height is measured (lines 55 to 59)

Line 54-56: Sentence might need revising, please check.

Response: Acknowledged and changed (lines 61 to 63)

Line 56-57: Minimum number of RPM readings? Please clarify.

Response: Acknowledged and defined this term in lines 64-65

Line 57-59: Is this error the difference between the RPM estimated pasture biomass and a more accurate measure of biomass? Also, I am not clear on what is meant by up to 90-100 readings, is this per season? This is unclear and needs more detail about this past study.

Response: Acknowledged. We previously defined the meaning of “RPM readings” and also changed this sentence to provide a more precise explanation of what error means (lines 64 to 68)

Line 59-61: Again, I am not clear on the number of readings. What if there were less readings but more RPM instruments covering a given paddock, would that reduce error?

Response: Acknowledged. Provided an explanation of how the RPM is used and the definition of RPM readings (lines 64-65)

Line 61: designed specifically to address what question?

Response: Acknowledged and changed the sentence to clarify that no studies were specifically designed to evaluate which is the minimum number of readings required that minimize error (lines 70-71)

Line 65: Is this not just a case of positioning the RPM based on GPS coordinates? Is this really considered a recent advancement?

Response: Acknowledged and removed the segment of the sentence that referred to recent advancements (lines 76 to 78)

Line 70: Please clarify that cloud cover is only an issue for optical satellite data.

Response: Acknowledged and clarified this (lines 82-85)

Line 71: This is not always the case that saturation occurs with high plant density when using index-based methods. It depends on the bands used, for instance the red-edge bands are sensitive to chlorophyll but do not always saturate with high values.

Response: Acknowledged and clarified this by explaining that saturation only occurs with specific vegetation indices (lines 84-85)

Lines 75-78: Consider revising this sentence, seems like a list and not clear what the point is.

Response: Acknowledged and changed this sentence to improve readability (lines 87-90)

Line 89-91: As before, I am not clear on what the number of reading are related to, number of RPMs (or points locations) or number of readings throughout the season? For instance, you could have lots of readings at one location, but this would not characterise the biomass for the entire paddock.

Response: Acknowledged and provided an explanation in previous points (lines 64-65)

Figure 1: Consider the use of a map inset. Would finer scale mapping be available to better resolve the study area? The large red dot seems untidy.

Response: Acknowledged. We included a map inset and merged this with part of Figure 2 to provide better detail of the study area (another reviewer also suggested this) (line 116)

Figure 2: Aerial image scale bar appears a bit untidy. Please consider changing to have regular from 0 intervals (e.g. 100 m). Also, what do the numbers mean in the walking patterns, sample point number? Please clarify in the figure caption.

 Response: Acknowledged. We improved scale bars (merged this map with Figure 1) and clarified the meaning of the numbers in Figure 2 caption (line 116)

Table 1: At this point it is not clear why the cloud data is relevant.

Response: Acknowledged. We replaced this with a table (Table 1) showing the number of satellite images available, discarded due to high cloud cover and utilized in the study (with details per regrowth and satellite type) (line 157)

Line 111-114: These two sentences in the methods appear to contradict each other. Please check.

Response: Acknowledged and changed this sentence to avoid contradiction (119 -126)

Line 128: ESA have several locations, please remove “(Paris, France)”.

Response: Acknowledged and removed (line 134-135)

Line 130: Is the weight really relevant?

Response: Acknowledged and removed this (line 138)

Line 131: The spatial resolution is not 10 m for all bands (red-edge is 20 m). You could say Sentinel-2 has a spatial resolution of up to 10 m.

Response: Acknowledged and provided an explanation of the differences in spatial resolution between bands (lines 139-141)

Line 132: Please include the URL to the open access hub (or include as a citation).

 Response: Acknowledged and included the URL in the text (line 137)

Lines 135-137: Why did you visually inspect and use the cloud cover tool?

Response: Acknowledged. Images were visually inspected to ensure that the results of the filter tool were accurate. However, there was no disagreement between both methods. We removed the visual inspection segment. (lines 142-143)

Line 127-150: Consider breaking this paragraph up, it will read better. Also, a summary table of the number of Sentinel-2 and Planet imagery used in the analysis might be nice.

Response: Acknowledged and included Table 1 (lines 134-157)

Line 142: Again, weight relevant?

Response: Acknowledged and removed (lines 148)

Line 158: Why not apply the +/- 2 day criteria to the Planet data.

Response: Acknowledged. We did this to give the best “opportunity” to each satellite. The criteria were to avoid penalizing either Sentinel-2 with additional missing data points (if RPM readings were taken on the same day); or Planet with differences in the number of days between acquisition and ground measurements (clarified this in lines 164-168).

Line 186-187: “… compared with previous research” What previous research? Citation?

Response: Acknowledged and removed this sentence as it was unclear. Details of the comparison with previous research are provided in this paragraph (section  2.3.4) and in the two tables included (2 and 3).(lines 195 – 207)

Line 189: I think it would be useful to provide these “standardised equations” and others if possible.

Response: Acknowledged. We included Tables 2 and 3 containing the standardised equations evaluated with a high level of detail. (lines 207 and 207)

Line 221: Please use “Sentinel-2” here and elsewhere.

Response: Acknowledged and replaced throughout the whole manuscript (lines 155, 164, 165, 204, 232, 260, 267, 268, 270, 271, 350)

Figure 3: In the linear regression plots please include intercept value.

Response: intercept was set to zero to avoid negative biomass values that would affect the results (explained in lines 212)

Results section: Please consider the use of sub-headings throughout the Results section.

Response: Acknowledged and included subheadings (lines 225, 235, 246, 258, 276)

Line 293: Please consider using Thomas et. al. here

Response: Acknowledged and changed accordingly (line 296)

Lines 290-305: This reads like a review of past research, it would be good if the authors draw some more comparison to these past studies and the present research.

Response: Acknowledged and changed it to provide a better comparison with our results (lines 292-307)

Line 308: “…front or back…” of paddock, please re-consider terms here.

Response: Acknowledged and replaced these terms by “borders” of the paddock (line 313)

Line 309-311: I guess the point of sampling in the middle would be to avoid edge effects.

Response: Acknowledged. This is an interesting concept; we include it as part of the discussion (lines 310-313)

Lines 336-337: If the spatial resolution of Cubesat Planet is cm-scale then Planet images have a greater resolution than the 10 to 20 m satellites you discuss in the previous study.

Response: Acknowledged, and I apologize for the mistake. Extremely thankful for picking this up. Changed this sentence accordingly (lines 341 -343)

Lines 345-347: Unless I am mistaken, Planet CubeSat data is not freely available for commercial use(?) If so, it would be prudent to make the point this would have an impact on farmers cost-benefit analysis for using the data.

Response: Acknowledged and thankful for this suggestion. We included a segment referring to this in the discussion (lines 358-360)

Reviewer 3 Report

The authors made an improvement in the manuscript regarding my previous comments but I still have some suggestions listed below. My concern with their responses to reviewer comments is that they are not supported by the exact line range containing the improved text. Since they did not make an effort to do that, the reviewing process is unnecessarily more difficult. Also, I can not know, based on this pdf, which parts of the manuscript are changed.

Figure 1: You made an improvement regarding a study area figure, but all parts of the figure should be clearly visible. Therefore, coordinates must be made with a larger font size. Also, a scale bar and north arrow in the bottom part are very poorly visible, so I suggest that you do it in white color.

Lines 368-369: 1 m of temporal resolution? This is unclear. I believe that you meant spatial resolution, and then this resolution is up to 1 cm. The Discussion part regarding UAV should be expanded and supported with some relevant references as a lot of interesting research on the subject was published recently.

Author Response

Response by the authors to Reviewer 3 comments (2nd revision)

Point 1: The authors made an improvement in the manuscript regarding my previous comments but I still have some suggestions listed below. My concern with their responses to reviewer comments is that they are not supported by the exact line range containing the improved text. Since they did not make an effort to do that, the reviewing process is unnecessarily more difficult. Also, I cannot know, based on this pdf, which parts of the manuscript are changed.

Response 1: We appreciated the reviewer's comments and we apologize for not having included the lines references in our previous cover letter. For avoidance of doubt, please find below all the comments made by the reviewer in his/her first revision, as well as our specific responses (i.e. the one previously submitted) but this time clearly indicating what lines were changed in the revised version.

Point 2: Figure 1: You made an improvement regarding a study area figure, but all parts of the figure should be clearly visible. Therefore, coordinates must be made with a larger font size. Also, a scale bar and north arrow in the bottom part are very poorly visible, so I suggest that you do it in white color.

Response 2: Acknowledged. We increased font size of the coordinates, and added a white background to the scale bar and north arrow which improved significantly the visibility in the figure (line 116, Figure 1)

Point 3: Lines 368-369: 1 m of temporal resolution? This is unclear. I believe that you meant spatial resolution, and then this resolution is up to 1 cm. The Discussion part regarding UAV should be expanded and supported with some relevant references as a lot of interesting research on the subject was published recently.

Response 3: Acknowledged and apologies for this mistake. This referred to temporal resolution and was removed. Additional discussion regarding the use of UAV was included (lines 370-382). Highlighted text in lines 363-365 refer to a segment that was rearranged within the paragraph to improve readability.

Response by the authors to Reviewer 3 comments (1st revision)

Note: as indicated above, the following are our responses to the comments made by the reviewer in his/her first revision. We have included them here again with all the reference to line numbers for each change made in the text.

Point 1: Material and Methods and Results sections are written well, with some segments that require clarification. Current parts of the Discussion are fine, but there should be more emphasis on remote sensing.

Response 1: Acknowledged. We improved multiple segments of Material and Methods and Discussion. We explained some concepts with a higher level of detail and were more concise on others. As suggested, we emphasized the use of remote sensing. For example, we discussed the limitations of Sentinel-2 (lines 355-360), the opportunity to use UAV and other ground-based sensors to improve the estimations (lines 370-382) and the possibility of building NDVI maps to do more efficient sampling (lines 346-348) among others.

Point 2: Since your study area consisted of two 1.1 ha paddocks, you should discuss the potential application of UAVs in future work, as it eliminates problems with cloud cover and temporal resolution, which referred to in the Introduction. This would offer much more possibilities in my opinion compared to the radar images that you mentioned in the Discussion.

Response 2: Acknowledged and extremely grateful for this suggestion. We discussed about the limitations of radars and the potential to UAV, and also the opportunity to integrate this with other data sources (lines 370-382)

Point 3: You should also discuss the impact of relatively small study area size on the application of Sentinel-2 images. Based on your research, what could be different in the implementation of Sentinel-2 in a larger area? Again, since this is the Remote Sensing journal, you should expand conclusions regarding applied satellite images.

Response 3: Acknowledged. We included a discussion of the limitations of using Sentinel-2 in small paddocks (lines 355-358)

Point 4: Specific comments

Lines 23 and 65: You might want to replace GPS with GNSS here, as it became more frequently utilized globally.

Response: Acknowledged and replaced  (lines 25, 77, 121 and 337)

Lines 68-72: Please split this sentence into two shorter ones to improve readability.

Response: Acknowledged and split this sentence in two (lines 85-85)

Line 84: Please replace “spectral indices” with “vegetation indices” to make it more exact. You can also then remove the text inside brackets.

Response: Acknowledged and replaced throughout the whole manuscript (lines 84 and 96-97)

Figure 1: The bottom part of the figure should be modified so the readers can observe the locations of two experimental paddocks and their relative position. This part shows very little new information compared to the upper part and is not justified. My recommendation is to merge the upper part of Figure 1 and the upper part of figure 2 into a study area figure. 

Response: Acknowledged. Merged both figures and used a map inset (this was also based on another reviewer’s suggestion) (line 116)

Table 1: These observations need more clarification in terms of data source and, more importantly, the relationship of the time periods of cloud cover observations and of Sentinel-2 imaging of the study area. Cloud cover is variable during the day and this should be clarified.

Response: Acknowledged. As this did not provide relevant information at this stage, we replaced this with a table showing the number of satellite images available, discarded due to high cloud cover and utilized in the study (Table 1) (line 157)

Line 112: “With GNSS receiver” might be more accurate.

Response: Acknowledged and replaced (line 121)

Line 114: “Coordinates”.

Response: Acknowledged. Replaced the word “coordinates” for “position” (line 123)

Line 127: One of “free” and “open-source” terms is redundant. I suggest removing “free”.

Response: Acknowledged and removed the word free (line 134)

Line 131: Spatial resolution of 10 m only for four bands. You should clarify that in the text.

Response: Acknowledged and provided an explanation of the differences in spatial resolution between bands (138-141)

Lines 133-135: It is enough to write about the bottom of atmosphere reflectance.

Response: Acknowledged and changed (lines 135-136)

Line 187: Please add a reference to this research.

Response: Acknowledged and provided details of the comparison with previous thorough this paragraph and in tables 2 and 3 (lines 206 and 207)

Line 221: Please write “Sentinel-2” instead of “Sentinel” in the entire manuscript.

Response: Acknowledged and changed throughout the manuscript (lines 155, 164, 165, 204, 232, 260, 267, 268, 270, 271, 350)

Figures 5 and 7: The font size of the text in figures should be larger for better readability.

Response: Acknowledged and increased font size in both figures (Figure 5 line 257 and Figure 7 line 273)