Why Flash Floods Occur Differently across Regions? A Spatial Analysis of China

Round 1

Reviewer 1 Report

Manuscript title: "Why flash floods occur differently across regions? A spatial analysis of China"
Authors: Yesen, Liu and Yaohuan, Huang

This study conducted a geospatial overlay analysis of different historical spatial datasets from China to determine their association with flash floods and the regional heterogeneity of the associations. The study fits the scope of the journal Water. The results provide interesting insights on relationships between flash floods, precipitation, land use and population. The manuscript is logically organised, but the description of some sections needs to be improved. Especially, the first three sections require considerable revision. My recommendation to the Journal is that the manuscript be sent back for a major revision. Please find below more specific comments that support my recommendation.

1) Introduction: In its current form, the introduction is a bit incoherent and lacks clear problem statement. For example, the first paragraph ends with a statement "except for the common explanations that floods are due to rainfall frequency and intensity [5, 6] and land use change [7], there is little inquiry into other reasons and their spatial heterogeneity". But later in page 2, the authors state "Significant progress has been made in analyzing influence factors, including the underlying surfaces, residential distribution, and precipitation". I understand that these statements are about identification of knowledge gaps, but they are somewhat contradictory. Also, statements about interactions between humans and natural environment were repeated many times (lines 35, 42, 48). Further, what is the basis for the statement in line 52 that hydrologic models largely focus on precipitation and underestimate complexities of human behaviour? I would like to see such statements supported by references.

2) Data: Of all the data listed in Table 1, only flash floods and precipitation data are muti-year datasets. Other variables such as vegetation, population and land use are static datasets and also belong to different years. Please add a few sentences on why multi-year data on population and land use were not considered, when they are easily available.

3) Precipitation data processing: The first sentence (lines 105-106) of the paragraph on precipitation data processing "daily precipitation has been confirmed to be strongly correlated to precipitation in various time-steps (e.g., daily)" is not clear. This is the only sentence in the manuscript on why daily resolution data has been used in this study. Please clarify and revise accordingly. Further, the precipitation factor analysis and ANUSPLIN based interpolation requires a more detailed description than what is provided in section 2.2 using equation 2. Is the inner summation in equation 2 a sum of indicator variables (i.e. 1-0 values depending on whether Pk is in a specific class)? What are the units of S in equation 2? On the same note, what does "proved to be available in China" in line 110 mean?

4) Methods: Line 140 states all data were classified using Jenks natural breaks. It is not clear why it is used and how the ouput fields are used later. In equation 3 and line 146, is the "region" same as the ecoregion e? It is also not clear what the authors mean by a "zone". Is it a watershed?

5) There are many places in the manuscript where there are grammatical errors. I suggest that the authors conduct a thorough grammar check before submitting the revision.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

  The problem analyzed in the manuscript “Why flash floods occur differently across regions? A spatial analysis of China” by Yesen Liu and Yaohuan Huang is the spatial distribution of dangerous flash floods (FF) in China. As declared by the Authors “This paper aims to determine the spatial heterogeneity and associated factors causing FFs […]” (lines 38-39). The research is done based on the data collected in the period 1950 – 2015. The approach applied is quite original, in my opinion. The Authors used sophisticated statistical methods and GIS to find factors responsible for the occurrence and severity of the flooding. The purpose is ambitious and the problem is important. These are the advantages of the presented work. However, there are also several drawbacks, which decrease the entire evaluation of the work. The main noticed problems are discussed below.

1. The main and the most painful problem is the description of the available data and applied methods (subsections 2.2 and 2.3). The manuscript is relatively short, which should be evaluated positively. However, the description of the data and methods shows that this is too short. This description is unclear and chaotic. The examples are given below:
   1. The method mentioned in line 96, namely the Getis-Ord Gi statistics is crucial for a proper explanation of the data processing, but it’s not described properly.
   2. The meaning of the sentence written in lines 105-106, “Based on the available dataset …” is such that the daily precipitation is correlated to precipitation. Total misunderstanding.
   3. It’s not explained before what are symbols P(<10), P(10-25), … in lines 116-117 as well as the ranges 0-10, 10-25, … in line 118.
   4. It’s not known what is the “precipitation factor” in line 116. This is explained in the caption of Figure 2, in line 184.
   5. In my opinion, it should be explained what is the “physical” or “real” meaning of such parameters as “Factor detector” (line 142) and “Interaction detector” (line 150).
   6. It’s also not known what are the “zones for one factor” in line 146.
2. An additional problem is the “Geodetector method” (line 194) or “Geodetector tool” (line 134). These two expressions are used concerning the main tool/method used in this research. However, their meaning is different. If the applied “Geodetector” is a tool in the meaning “piece of software”, this tool should be carefully described and the authors of this tool should be mentioned. If “Geodetector” is a set of implemented methods, the methods should be discussed and the implementation should be described. I’m not sure how it was done, but I suppose that something as R package is implemented in this research.
3. In my opinion, the part of the Discussion, subsection 4.2 entitled “Different regional strategies to cope with flash floods” is beyond the defined purpose and the scope of the paper. So, this part should be removed or the purpose should be changed.
4. In my opinion, there is also a problem in the content of the section Conclusions. This text is written as a summary of the presented research. It should be the discussion of the research importance for other fields of scientific development, about further steps and possible limitations.

I also suggest the removal of passage in lines 70-75. Such a list of the paper content is good for technical reports, but it’s not necessary for the scientific paper.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Manuscript title: "Why flash floods occur differently across regions? A spatial analysis of China"
Authors: Yesen, Liu and Yaohuan, Huang

 

The revised manuscript is an improvement over the previous version, and reads much better. Nonetheless there are a few points (listed below) to be further addressed. My recommendation to the Journal is that the manuscript be sent back for a minor revision.

 

1) Line 48: The use of "except for precipitation" in this statement is not clear. Why precipitation is an exception here? Why it does not alter natural characteristics of extreme floods? Please clarify.

 

2) Line 80: I suggest to remove "elevation".

 

3) Lines 82-83: The sentence "For the acquisition difficulty of some data, we applied single-year of some data such as population and land use in this study" is not well constructed. Perhaps, rephrase it as "Because of difficulties in acquiring these datasets for multiple years, we employed single-year datasets for this study." But I have a larger issue with this reason mentioned by authors. The sentence in lines 84-87 "We have checked the population and land use distribution in different periods and found that although there are changes in some areas, the spatial distribution pattern of the whole country changed little, especially in the mountainous areas where mountain FFs occur, the change of population distribution is smaller." is not consistent with the preceding statement that there were acquisition difficulties related to multi-year datasets of population. If the authors were able to access population and land use datasets, acquisition issues cannot be the reason for not including them in study.

 

4) Line 107: Consider replacing short-term with "short-duration (e.g. hourly)"

 

5) Lines 123-125: "...ANUSPLIN software [33] [34],which is applied by RESDC to spatialize precipitation data of china". Does it mean the interpolation was carried out by RESDC and authors obtained the interpolated data? If the authors obtained point rainfall data and perform interpolation for this study using ANUSPLIN, then the sentence needs to be rephrased accordingly, and details on the core algorithm (e.g. kriging, inverse-distance weighting) used for interpolation needs to be provided.

 

6) Line 144: "This tool is widely used in spatial analysis [39,40]". For this statement on the wide usage of the Geodetector tool (published in 2017), the authors cite a 1967 paper on Jenks algorithm. Please correct.

 

7) Line 149: The scale of 1 to 5 (high to low) refers to Jenks algorithm?

 

8) The novelty of the study is in the usage of Geodetector and its component factor and influence detectors for flash flood spatial heterogeneity analysis. While the description of the methodology (section 2.3) has improved from previous version, the contribution of factor X in equation 3 is still not clear. The variables N, N_h, sigma, sigma_h on the RHS of equation 3 are related to flash floods. So, my guess is that the information about factor X comes via Jenks spatial zoning h. If so, please state this clearly. Further the factor detector results (Table 3) are not adequately described in section 3.2. Please add details on each variable in equation 3 (number of zones, N, N_h etc.) for an example ecoregion, followed by the range of values for all ecoregions. This will help the reader with physical interpretation of results.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

I appreciate the Author's effort in improving their manuscript. I see the text is corrected according to the suggestions. In my opinion, it's ready for publication. Congratulations.

Author Response

We greatly appreciate the reviewer's valuable comments, which is helpful for our improving this paper to publised in water.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.