Remote Soil Moisture Measurement from Drone-Borne Reflectance Spectroscopy: Applications to Hydroperiod Measurement in Desert Playas

Round 1

Reviewer 1 Report

Introduction section is not enough as a high-quality research. Literature review is very poor. Please edit it and fix it by more literature reviews.

Figure 1 is not clear, please use a better map for this aim. And shift it to materials and method section.

Caption of Figure 2 is to much! Please use these sentences in text.

Figures 7 and 8 must bee more discussed.

Do not put figure 9 -11 continuously, each figure needs explanation separately.

Figure 12 and 13 must be discussed more, please explain them clearly.

Discussion part should be compared by other studies result.

It is not clear how the author used the Remote Sensing data! Please describe more about data processing section (which website, which software, et are used).

A high quality graphical abstract is required for better understanding of research process.

Author Response

Please see attached response to reviewers document. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Generally speaking, I do not know what is the contribution of this study that warrants a publication in “Remote Sensing”.  Specifically,

• Quantitative results are needed in the abstract.
• The introduction should be significantly improved. The existing methods/indices used from optical remotely sensed data should be extensively reviewed and specific issues with them. In addition, this is the first time when I saw a subtitle of “study area” in the introduction.
• The presentation of the field measurement was confusing and seems out of logic. The soil properties shown in Figure 4 need to clearly described (including unit) and presented before the introduction of Figure 4. The authors may consider break down Figure 4.
• The continuum-removed water index (CRWI) needs to be described here in this paper. In addition, it is confusing to read the following two statements made in the manuscript: (1) from abstract “Therefore, we developed a new, reflectance-based soil moisture index (continuum-removed water index, or CRWI)” and (2) from Section 2 (lines 166, 167), “Reflectance data were processed to calculate the continuum-removed water index (CRWI) after [12] and the WISOIL moisture index after [10].” It seems that CRWI was developed by the author(s) in a previous study [12]. This should be made very clear in the introduction. Even though [12] was mentioned in introduction, but just blended in for a general statement.
• The subtitles in the method section should be more meaningful. What do you mean by GIS method? The material presented in this section was not the core for GIS anyway.

Author Response

Please see attached response to reviewers document. 

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors describe an interesting and novel approach to quantify soil moisture using drones and spectroscopy readings. The authors demonstrate the application of their method in a desert playa ecosystem. Soil moisture across these areas is challenging to characterize using conventional soil moisture estimates (e.g., satellite soil moisture) because current estimates and models are provided in coarse resolution and they usually show gaps under extremely dry conditions. Therefore the authors develop a new soil moisture index based on a combination of multiple spectral bands that they compare with previous indexes to verify its reliability. The approach presented by the authors could have an interesting impact in soil moisture research. I recommend this paper for publication after moderate revision and clarification.  

In methods the interpolation strategy and its validation are missing. By the output I guess the authors used conventional geostatistics (e.g., Kriging).There is a large amount of literature supporting the relationship between soil moisture and topographic datasets derived from DEMs. 

 Can the authors describe the upscaling strategy and analyze the impact of using covariates (e.g., regression-kriging) using elevation data? 

In another note I feel that in the results there is a disproportion between the number of figures and text. I feel that there is a large amount of work that needs to be better described and discussed or simplified in the results section. Also the discussion section is too short and lacks discussion against previous work. There are not referenced discussions in this section. 

Finally, all presented scatter plots show two main clusters of data, please consider to confirm the results using some form of cross validation to identify the impact of the clusters in the predictive power of the new soil moisture index. 

Specific comments: 

Please add quantitative results to the abstract to avoid leaving relative to the reader the impact of the main conclusions presented in the paper. 

17 surface soil moisture from direct in situ soil moisture measurement? Please clarify  

104 - 105 are these conditions a limitation for the precise calculation of the index elsewhere?

149 cite Topp equation and state its main components

166 define the range of reflectance data

178-181 I would start this paragraph with these sentences.

197-201, 205 maybe some references could be useful here

220-221 did the authors define the right pixel size to maximize the accuracy of the index?

222-234 include in this description the spatial support of all gridded inputs 

237 define a ‘digitate zone’

260 are the authors presenting the correlation in text but showing the R2 in the parenthesis. Are the authors talking about the explained variance or simple correlation? Please indicate how the R2 is calculated. Please describe the bias of estimates. Consider to use information criteria from soil moisture community standards e.g., Gruber 2020 https://www.sciencedirect.com/science/article/pii/S0034425720301760  

262 an R2 of 40% between clay and soil moisture is a significant (and realistic) portion of the explained variance. Maybe not say that they do not correlate, but that the correlation with the soil moisture index is higher. 

342 there is an abrupt change in ideas here, please improve the connection between paragraphs. 

Author Response

Please see attached response to reviewers document. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors improved manuscript accordingly.

A graphical abstract is required for better understanding.

Discussion part should be compared by previous studies.

Conclusions should be developed and more information should be addressed.

Author Response

Please see attached response document. 

Author Response File: Author Response.docx