The Diurnal Variation of L-Band Polarization Index in the U.S. Corn Belt Is Related to Plant Water Stress

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Please refer to the attachment.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Few spelling/incorrected usage mistakes.

Author Response

Repeated ‘is important’. Thank you for noticing this, correction made at Line 57.

Why put ‘plant water stress’ in italic, is there some special meaning? We used italics here to
show that this term was being defined. We have removed the italics and added some text
that has the same effect. Please see changes near Line 66.

In addition, it’s better to give an example of spatiotemporal distribution map of both data to let
readers know the general pattern of these two observations over the study area. We appreciate this
suggestion, and have added Figure 5, a map of the spatial distribution of SMAP and ESI
data. Please see changes near Line 221 for more information on Figure 5.

As for the data, please add more detailed information of the used TB and ESI, for instance,
their spatiotemporal resolutions? Is there any spatiotemporal gap and quality flag of each of the two
datasets? How about the RFI in the TB observations? How were these data preprocessed? Along
with the addition of Figure 5 to show spatial resolution, some text around Line 229 has
been added to discuss temporal resolutions and data preprocessing methods. When RFI
was an issue, the quality flag in the SMAP data took a bad value, in which case the TB
observation was not used.

Since hypotheses are proposed in L90-98, I don’t think it’s necessary to hypothesize something
at this place, facts and previous research progress should be mentioned here instead. We decline to
make any changes at Line 70 because this is our overall hypothesis and later we state the
specific hypotheses that we will test. We have already stated facts and discussed previous
research.

Fig. 5, if the year of 2019 was excluded, how about to leave a breakpoint in this figure to avoid
a misunderstanding of the fluctuation of ESI. Thank you for this suggestion. There is now a
break in the lines representing ESI during 2019 in Figure 6.

L276, ‘. . . 0, μ = 0.068357, is. . . ’, from the statistical results presented in Table 2 and 3, keeping
to three decimal places seems enough for the value μ. Means in Tables 2, 3, and 4 are now
reported to 3 significant digits.

Figure 6, since this Section is Result, why sourced a reference here? Readers may wonder
whether this is an original result? Figure 7 is an original result. The citation referred to the uncertainty calculation of brightness temperature, which has been moved to the body text
of the results section.

 

 

Reviewer 2 Report

Comments and Suggestions for Authors

The paper investigates the relationship between the L-band Polarization Index (PI), derived from satellite remote sensing data, and plant water stress in the U.S. Corn Belt. The authors hypothesize that the L-band PI, which is a measure of the difference between vertically and horizontally polarized brightness temperatures, can reveal changes in vegetation water content related to plant water stress. The study compares PI data from NASA’s Soil Moisture Active Passive (SMAP) satellite with the Evaporative Stress Index (ESI) during the growing season. The review has the following comments.

1. While the focus on the U.S. Corn Belt is appropriate for the study, the results may not be generalizable to other regions with different crop types or environmental conditions.

2. The paper mentions the limitations imposed by the SMAP satellite's overpass times, which may not align with the maximum and minimum vegetation water content. This could potentially affect the accuracy of the diurnal variation analysis.

3. The paper assumes a certain level of expertise in remote sensing and plant physiology, which may make it less accessible to readers from other disciplines.

4. The figures presented in the paper serve the purpose of illustrating the key findings related to the L-band Polarization Index (PI) and its correlation with plant water stress. However, the simplicity of the figures might not fully exploit the richness of the data collected. In the context of a scientific paper that aims to provide a comprehensive analysis, the figures could be enhanced to offer a more detailed and nuanced interpretation of the results.

Author Response

The figures presented in the paper serve the purpose of illustrating the key findings related
to the L-band Polarization Index (PI) and its correlation with plant water stress. However, the
simplicity of the figures might not fully exploit the richness of the data collected. In the context of a
scientific paper that aims to provide a comprehensive analysis, the figures could be enhanced to offer
a more detailed and nuanced interpretation of the results.
Thank you for your suggestion on the inclusion of more visualization of PI. We have
added a histogram of the diurnal change in PI, Figure 9 and accompanying text on Line 345.

 

While the focus on the U.S. Corn Belt is appropriate for the study, the results may not be gen-
eralizable to other regions with different crop types or environmental conditions. We acknowledge
that an analysis of L-band polarization index may not be appropriate for the estimation of
water stress for other land covers or climates.

 

The paper mentions the limitations imposed by the SMAP satellite’s overpass times, which may
not align with the maximum and minimum vegetation water content. This could potentially affect
the accuracy of the diurnal variation analysis. We agree that early afternoon is likely a better
observation time window for diurnal maximum PI, however this observation is currently
unavailable at a frequent overpass period.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

At the first place, I am afraid that the authors did not respond to all my comments. Only limited point-by-point responses I spotted in the revised version as well as uploaded materials.

I went through the manuscript again. Yes, it is a complete work, and the structure, results and discussion are presented orderly and understandably. However, I still have a major concern that may, in my opinion, relate to the scientific contribution of the current study to community, which deserves to be paid attention to.

 

Major concern:

I have reported this issue last time, but the authors did not respond. So I would like to raise the point again, i.e., the strength and new findings of this investigation need to be more highlighted.

Because there is a lack of detailed review addressing vegetation status observed by passive microwave remote sensing, especially over various plant water conditions. This is more relevant and vital to the topic of the manuscript. The authors argued in the point-by-point response that ‘have already stated facts and discussed previous research’. However, these stated progress (L30-L68) are mainly focused on the mechanisms of vegetation physiology and phenology. Obviously, they show less support to the research objective, since the objective is related to some hypotheses addressing the relationship between TB behaviors with vegetation status. Therefore, at least in the Introduction Section, the authors must provide a review on previous efforts to prove or deduce what questions are unexplored or debatable that can lead to the research goal and the hypotheses, instead of using a rough citation ‘[1-9]’. On this basis, readers can be better informed what is the improvement of the current study and to get new ideas. 

Comments for author File: Comments.pdf

Author Response

Please see our response to your comments (Reviewer 1) in the attached document.

Author Response File: Author Response.pdf