Potential of L- and C- Bands Polarimetric SAR Data for Monitoring Soil Moisture over Forested Sites

Round 1

Reviewer 1 Report

The paper "Potential of L- and C- bands polarimetric SAR data for monitoring soil moisture over forest sites" was, for this reviewer, a pleasure to read.  The paper is well thought out, well presented, and its conclusions are well supported by sound statistical analysis.

There are some minor suggestions here to perhaps improve the final copy:

1.  The last sentence of the abstract is a little terse and a little confusing.  Which of the polarimetric parameters contribute to soil moisture estimation?

2.  The unit "m^3 / m^3" is used repeatedly throughout the text.  The reviewer, who is not familiar with satellite soil moisture estimation, had to look up what the unit meant.  Perhaps a footnote to give the reader just a little background on this odd-looking unit and its interpretation?

3.  Line 91, 92:  Should be "coherent" and "incoherent", not "coherence" and "incoherence".  

4. Since there is little standardization in SAR circles, a definition or a figure showing what incidence angle is would be helpful (for me, it is the adjacent angle to grazing, but not everyone defines it as such).

5.  Lines 231-234:  It states that there are "two main differences", but it appears that you've only define one of the differences in the text?

6.  The instruments used for ground truthing (like the Hydra probe or the pin profilometer) should be described in a little more detail, including their overall accuracies.  These accuracies should be taken into account in the statistical analysis (at least notationally).

7.  Line 83, "DBH" is used before it is defined.  Not defined until Line 258.  VWC is never spelled out in the text.

8.  Table 3, should be "Trembling Aspen", capitalizing the first letters of the proper noun.

9.  The variable "alpha" is used twice, once as a polarimetric parameter and the other as a confidence level in the p-test.  You should consider some way to distinguish these (subscripts for each variable?) so as not to confuse the reader.

10.  The figures are extremely well presented.  They are not cluttered and they contain a lot of useful information.  However, Figure 10, the gray is a little light.  Perhaps a slightly darker gray?

11.  In Figure 10, the R^2 values seem low for F1, F2, and F3.  Are they low because the y-axis is a logarithmic scaled axis?

12.  The conclusions are well stated and supported by the analysis.

The paper was a pleasure to review.  Good work!

 

Author Response

Please find attached the response to reviewer #1

Author Response File: Author Response.pdf

Reviewer 2 Report

General Comment:

The manuscript investigates the potential of PolSAR signatures to monitor soil moisture over SMAPVEX12 forested sites. The authors first developed relationships between L-band UAVSAR and C-band RADARSAT-2 parameters and soil moisture over the forested site to achieve this objective. The ground measurements of soil and vegetation parameters collected during the SMAPVEX12 have been used for the analysis. Further, the developed relationships were used to identify the optimal parameters for soil moisture monitoring over forested sites. The authors performed qualitative analysis on the temporal profiles of L- and C-bands for linear and circular backscattering coefficients at different polarizations and incidence angles. A comparative analysis is also presented for four different forest sites having varying biomass densities. The authors revealed that L-band SAR (linear and circular backscattering coefficients) has a higher potential for soil moisture monitoring over forested sites as compared to C-band. They also suggested that C-band SAR has no potential to monitor soil moisture even for the low biomass site.

All in all, it’s a sound scientific contribution in a comprehensive way that provides useful information on the use of SAR for reliable monitoring of soil moisture over forest regions while presenting a comparative study between L-band and C-band SAR, different polarization, incidence angles, and biomass densities. These comprehensive studies are seldom being shown, especially over forest land cover, and should be of interest to readers of the journal “Remote Sensing”. Many studies on the use of SAR over agricultural, grassland, and shrubland are available with their accuracy analysis. However, the investigations over forest land cover are limited, currently. Thus, the study is interesting. I believe such studies could be immensely helpful in effectively utilizing the high spatio-temporal L-band SAR observations (e.g., upcoming NISAR missions) for accurate soil moisture retrievals over forest regions.

The manuscript is written nicely with a thorough and relevant presentation of the latest review of the literature. The results are presented very well, and discrepancies are evaluated with scientific fairness. I do not see any major flaws in the science part. However, a few minor issues are listed below (specific comments), which should not be difficult to address to improve the manuscript further.

Specific Comments:

 - L113: “…identify the optimal parameters for soil moisture monitoring over forested sites…”- The authors used “correlation coefficient” as the only criterion for identifying optimal parameters. Is only “correlation coefficient” statistics good enough to select the optimal parameters? How about involving “variance or standard deviation” characterization in the selection of the optimal parameters? Here my concern is that “correlation” statistics alone can fail to characterize “bias” in the dataset.

- Section 5.2 “Linear regressions…”- What quality control was applied to the data used in the manuscript before developing Linear regressions? This must be detailed.

- Section 6.2 “Regressions analyses” – I feel Regressions analyses is not an appropriate heading under “Result Discussion”. Since the authors used regression for analyzing the potential of both L- and C- bands polarimetric SAR signals for soil moisture monitoring, I suggest providing the heading in that relevance.  Further, modify the subheadings accordingly.

- L16: “correlation coefficients (ρHHVV, φHHVV)”-  Usually “correlation coefficient” is being represented as “R” or “r”, the given symbols (ρHHVV, φHHVV) corresponding to “correlation coefficients” are confusing. Besides, it is not specified what “ρ” and “φ” represents.

- L16  “pedestal height” – please provide a symbol/Abbreviation for the term “pedestal height”, if used in the manuscript.

- Line 67-68: statements “At C-band, the radar signal is affected by the structure… incidence angle and the polarization [19]” do not have many links with other statements of the paragraph. Try mixing these statements in the middle of the paragraph for better understanding.

- L82: “…L- and C- bands fully PolSAR signatures…” – here it is mentioned “fully PolSAR signatures”, whereas in Line 10, it is mentioned “dual-frequency Polarimetric Synthetic Aperture Radar (PolSAR)? Are they both the same, or is there some difference?

 - Why are the filed IDs mentioned as “F1, F2, F3, and F5”? Is there any specific reason? If not, replace “F5” with “F4”.

- Table2: The values of “RMS” for UVSAR and RADARSAT-2 have very less deviation under all four sites. Similarly, “correlation length” has less deviation except for Field F2. Can authors provide a potential reason for a higher value under UVSAR (correlation length =22.75)  as compared to RADARSAT-2 ((correlation length =10.5)?

 - L366: “mean values of the backscattering coefficients” Mean value over what? Mean of the backscatter within Field or grid cell? Please specify for clarity.

 - L367: “They will be used to analyze…” It is unclear what will be used to analyze – if it is “developed linear regressions”, please specify for clarity.

 - L371: “correlation coefficient (R)” – the symbol used for correlation “R” here is different from the symbol used in the abstract section “r” in L20-23. Are these representing different meanings? If not, then please check through the manuscript.  

 - Figure 5. The given figure looks faded; I request to enhance the quality of this figure. I also suggest providing the subplot Id (i.e., a and b) within the subplots (upper left/right corner of the subplots) 

 - Figure 4: replace “…soil moisture over…” with ““…soil moisture measurements over …”. If the author explains the abbreviations such as “F1_SM_USDA”, “F1_SM_MAFRI” (presented in figures) in the caption, that will be helpful to readers for better understanding.

 - Figure 6: It looks polarization angle is missing in the figure. Please mention if any.

 - Figure 7 and Figure 8. What are the error bars represent? Please specify.

 - Table 10. “ρHHHV “ – what the symbol “ρ” mean- its not defined anywhere in the manuscript.

 

 

 

 

 

Author Response

Please find attached the response to reviewer #2

Author Response File: Author Response.pdf