Early Warning Signals of Dry-Wet Transition Based on the Critical Slowing Down Theory: An Application in the Two-Lake Region of China

Round 1

Reviewer 1 Report

The main purpose of this paper is to investigate the early-warning signals for the dry-wet transition events over the Poyang Lake basin and the Dongting Lake basin. To this aim, the authors use the Critical Slowing Down Theory and other statistical methods.

General Comments

The topic of the study is interesting, but in my opinion the manuscript is not suitable for publication in its current version since it presents many unclear methodological aspects that need to be deeply revised and integrated with new considerations and clarifications. Furthermore, the paper requires a thorough linguistic revision.

1) The Introduction is very difficult to understand and incomprehensible in some parts (see following specific comments).

2) The methodology needs to be clarified and explained better. All the statistical methods used in the work are only poorly described and references are missing in most cases.

3) Some results of the paper are just described and not discussed. Moreover, the results should be reviewed in the light of a clear description of the methodology which is currently not clear to me.

Specific Comments

Keywords. Page 1. Line 32.

Only acronyms are indicated here, they do not facilitate the readers.

Introduction. Page 1. Lines 36-37.

The geographical description of the analysis domain is not clear, a map would help the reader in understanding.

Introduction. Page 2. Lines 53-54: “… the prediction studies are mainly via models where the external forcings (sea temperature, sea ice, etc.) are taken as a bridge.”

I don't understand the meaning of this sentence. Furthermore, it is not clear what kind of model the authors refer to. I supposed numerical weather prediction models, but in the following sentence (line 55), linear methods are mentioned, so I do not understand.

Introduction. Page 2. Line 61. “…the prediction theories and technologies proposed before.”

What theories and methods are the authors referring to?

Introduction. Page 2. Lines 64-67.

This sentence is not clear.

Introduction. Page 2. Lines 73-75.

This sentence is not clear.

Data and Methods. Page 3. Line 120.

The variable ML is not represented on Fig.1.

Data and Methods. Page 3. Line 128. “…backward…”

Backward or forward?

Data and Methods. Page 4. Line 142. “The detailed description can be found in the reference [25].”

Reference [25] is in Chinese and it is not available for consultation for readers who do not know Chinese. A description should be reported in the manuscript.

Data and Methods. Page 4. Subsection 2.2.3.

MTT method is not described and references are missing.

Results and analysis. Page 4. Subsection 3.1.

REOF method is not described and references are missing.

Results and analysis. Page 4. Subsection 3.1. Figure 2.

How are the maps obtained? is it an interpolation of station points? No explanations about it are present in the paper.

Figures 2-3

In these figures, the letters associated to each panel ((a), (b), etc…) are missing.

Concerning the results, they should be evaluated after a deep revision of the manuscript and a clear description of the methodology that is missing in this version of the paper. Same considerations are valid for the conclusions.

Author Response

Thank you very much for your useful suggestions. Over the past weeks, we revised the manuscript according to the comments point by point in the appendex. We hope this edition meets the requirements.

Author Response File: Author Response.docx

Reviewer 2 Report

In the reviewed manuscript there are several points that should be complemented/corrected, including:

 - Page 1 line 32 Keywords: I suggest changing symbols to full names.

 - Page 1  Introduction (line 35 – 41) – I suggest instead of describing the research area at the point Introduction, adding in the manuscript the point: 'study area' and including in it a short description of the study area. Moreover, I suggest adding figure with research area and on it marked measuring stations

 - Point "Introduction" needs to be complemented with information about the results of research of scientists from other regions of the world in the scope of the studied topic

 - At the manuscript, the purpose is not clearly enough.

 -Page 2 line 61 ‘It is suggested that the critical thresholds or the so-called tipping points…’ – the notation is not clear.

 - Monthly precipitation totals from 1961-2020 from many measuring stations (page 3 line 102-103) had been taken into account, but their homogeneity had not been mentioned. The homogenity of the data is a key issue in the analysis, but it is not confirmed in any way. I suggest enriching the manuscript with a short description of the pluvial conditions of the research area

 -Page 3 line 104-105 ‘By using the  monthly data of precipitation at each station, the standardized precipitation index (SPI) is  calculated’ – had SPI been appointed for 1-month, 3-months or 6-months? I suggest enriching the manuscript with this information.

 -Page 3 line 109 ‘This study adopts the annual SPI…’ – the notation is not clear. I suggest paying attention on the sentence: page 3 line 104 –‘ By using the  monthly data of precipitation at each station, the standardized precipitation index (SPI) is  calculated’

 -Page 3 line 111

It is necessary to provide a formula for determining the SPI value and the criteria for assessing the period

 -Page 3 line 112

The title of the point 2.2.1 – I suggest giving full name.

-Page 4 line 142 point 2.2.2 and point 2.2.3

There are only 3 sentences in each of the points – instead of 'The detailed description can be found in the reference [25]' – more information needs to be added

 -Page 4 line 154

‘Due to the 154 complex topography of the two-lake region…’ – a short characteristic of the research area is needed.

 -Page 4/5 line 169-170 ‘Then, the grid area-weighted average method [34] is used to calculate the total SPI over each sub-region  based on the SPI at each selected station’ – I suggest enriching the manuscript with a description of the method of designating the SPI for the highlighted area

 -Page 7 Figure 3

Figure 3b ‘t1=t2=10a’, figure 3c ‘t1=t2=15a‘ – the notation is not clear.

 The conclusion is too general – I suggest adding more details.

Author Response

Thanks for the valuable suggestions. Over the past weeks, we modified the manuscript according to the comments point by point in the appendex. We hope this edition meets the requirements.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors analyze monthly precipitation data over a region in the middle Yangtze valley to search for predictors of dry-wet transitions in the region. They divide the region into four sections by using REOF analysis. The four regions chosen are locations of associated with the first four modes of variability in the study area. The precipitation transitions in these regions are then analyzed to search for an early warning signal using the critical slowing down (CSD) phenomenon. By looking at the autocorrelation, early signs of the events can be seen with an abrupt increase in AC from a minimum at approximately 10 years before the event. 

This study is an early investigation of this method. I think that the authors do a good job showing that this technique is feasible. I only have a couple of general comments. My recommendation is that this manuscript is acceptable after minor revision.

General comments:

1. In Figure 3, the authors identify the main dry-wet transitions on the decadal scale. Is there a certain magnitude of transition that this method can pick up? Does this technique work for any transition events before 1990?

2. Are the modes in the REOF analysis robust? Do similar patterns appear if using a slightly larger area and a subset of the 60-year period analyzed?

Author Response

Thank you so much for the valuable comments. Over the past weeks, we modified the manuscript according to your useful comments. More information is in the appendex. We hope this edition meets the requirements.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have improved the paper according to the reviewer’s comments and in my opinion the current version of the manuscript could be considered for publication, but an extensive editing of English language and style is still required.

Author Response

Dear Reviewer,

We thank you very much for the kindness suggestion. With the professional help of MDPI, the manuscript has been improved. We hope that the manuscript meets the publishing standard now.

Best wishes,

The authors.

Reviewer 2 Report

Thank the authors for their answers

 However:

- In the manuscript, the purpose is not clearly enough

 -page 3 line 112

‘The data has been quality controlled and is reliable’– information is too general, I suggest adding more details.

 -Page 3 line 123

‘The climate is mild with four distinct seasons. There is sufficient heat and concentrated precipitation. The spring temperature is changeable, and the summer and autumn are  dry. The cold period is short and the hot period is long…’ – such characteristic can be a description for many area of the globe, I suggest adding more details.

 In first review of the manuscript (Page 3 line 111) reviewer had suggested ‘It is necessary to provide a formula for determining the SPI value and the criteria for assessing the period’

‘REPLY: The drought index (SPI) we used is developed by the National Meteorological Information Center, China Meteorological Administration according to the National Standard of China (No. GB/T 20481-2017). Thus, the formula for determining the SPI value is as follows:…’ – I suggest those information add in manuscript and link it with the text from manuscript (page 3 line 116): ‘The SPI established by McKee et  al. [35] is used to describe the intensity of moisture, which can be calculated by referring to Belayneh et al. [36]. The calculation program can be downloaded from the following website (https://drought.unl.edu/monitoring/SPI/SPIProgram.aspx)’

 -page 9 line 275

Table 2 – I suggest starting title with a capital letter

Author Response

Dear Reviewer,

Thanks for your considerate comment. We modify the manuscript carefully, and reply the question one by one as the appendex.

Author Response File: Author Response.docx