Response of Ecohydrological Variables to Meteorological Drought under Climate Change

Round 1

Reviewer 1 Report

Review of "Response of ecohydrological variables to meteorological drought under climate change"

By Zhang et al.

The study analyzed ecohydrological variables and drought episodes for a historical period of 1982-2015 and their future projections for three time slices upto year 2100. Specifically, the study looked at spatial patterns of evapotranspiration (ET), NDVI, vapor pressure deficit (VPD), and soil moisture (SM) during drought and non-drought periods and studied their behaviour over arid vs humid regions around the globe. The remote sensing component of the study lies in its use of land cover and NDVI datasets. The first part of the study provides important insights on how ecohydrogical variables vary in space during drought episodes, and how they are sensitive to temperature and radiation. It then quickly shifts to an analysis of how the drought characteristics conditioned on ecohydrological variables vary in space, and according to aridity and vegetation types. The "response of ecohydrological variables" aspect as mentioned in the title is therefore limited to the first couple of figures. 

Overall, I have a few  concerns, mainly related to the description of methods and results. Some places in the text require more clarity on methods used in order to better interpret the results. It is noted that the manuscript cites 15 supplementary figures and two supplementary tables to support various statements made in the text. However, my review is based only on the manuscript pdf file as the supplementary material is not available to me for review. My recommendation to the Journal is that the manuscript requires considerable revision (mainly of editorial nature) before it is accepted for publication. Please find below specific comments that support this recommendation.

Specific comments

1) Introduction: 

a) The references 11-16 are global scale studies on droughts and their impacts, yet the current study states (lines 61-62) that changes in ecohydrological variables at global scales is still uncertain. This statement needs stronger support. What aspects of changes in ecohydrological are uncertain? For example, what are the research gaps that were not addressed in studies 11-16? The incorrect sentence construction in lines 61-62 further adds to the lack of clarity.   

b) Lines 102-104: The time period of 34 years is not long enough to cover the atmospheric and oceanic decadal variability (e.g., Pacific Decadal Oscillation), which may influence the outcome of the study. What is the criterion for selecting this time period, when global data for longer periods are available (e.g., ERA-5 or other reanalysis data or the one in reference 32)? 

c) Line 120: Why only six models selected? More would be better given the uncertainty associated with projections.

2) Material and Methods

a) Sections 2.2 to 2.4: The study used SPI-12 and SPEI-12. I suggets that all occurrences of SPI after section 2.2 are changed to SPI-12. The same suggestion applies to SPEI. For example, in SPI and SPEI in line 207 should be changed to SPI-12 and SPEI-12 to be more specific.

b) Section 2.4: It is not clear where the trend analysis and Mann-Kendall test (line 217) are used in this study.

3) Results: 

a) In my opinion, the manuscript requires a figure showing spatial patterns of frequency, duration and severity of droughts. This should be the first figure of the results section. In particular, the spatial patterns of frequency (or number of episodes) is important as it gives some insight on any statistical sampling effects on averages and pdfs (of duration and severity).   

b) Fig 1d and the inset histogram show that SM anomaly is predominantly negative. However, it is not clear if the 93.47% mentioned in line 265 is simply the percentage of negative SM anomalies or if this value is based on a rigorous statistical test (Mann-Whitney-Wilcoxon test). If latter, the reader would benefit from a short description of how the test was applied (e.g. hypothesis, p-value etc.). To be consistent, please add the percentages for ET and NDVI as well. This would help better understand the rest of the figures (Fig 3-9) as all of them are conditioned on the significant changes to ecohydrological variables. A minor but related note: The ordering of variables in Fig 1 is not consistent with the rest of the manuscript (which follows ET, SM, VPD and NDVI order) and I wonder why Fig 1c-d were discussed first followed by Fig 1a-b. 

c) Line 270: Spatial consistency in Figure 1 is difficult to see. For example, the small positive anomaly patch in NDVI (Fig 1b) in Central Africa is less spatially coherent for ET anomaly (Fig 1a), i.e., there are quite a few negative ET anomalies when NDVI anomaly is positive. Similar behavior is seen for US Midwest. With this as a background, the statement in line 270 that "a decrease (or increase) in NDVI is often accompanied by a decrease (or increase) in ET during drought period" requires a stronger supporting quantification. What could be the reason for the lack of coherence in ET? One way to assess spatial consistency is via correlation between NDVI and ET but this needs to be done at regional level so that the global correlations do not mask regional differences.

d) Lines 294-299: This reasoning is not clear to me. Why are temperature, radiation lower during the drought episodes in NH, Amazon River Basin and Central Africa? And why and how is SM replenished during drought episode in these regions? Also, SM is treated as a climate factor in Fig 2 whereas it is also one of the ecohydrological variables as stated in line 106. 

e) I understand that the results in Fig 3 are spatial patterns of drought characteristics (duration, severity) conditioned on "significant change" (line 309) in ecohydrological variables. If so, are these "significant changes" identified from Mann-Whitney-Wilcoxon test and same as the ones presented in section 3.1? In that case, am I right in my interpretation that Fig 3b and 3f being conditioned on significant SM anomalies show drought characteristics for 93.47% regions of the world, whereas Fig 3c and 3g show drought characteristics for 85.41% regions of the world?  

f) Section 3.2 also requires considerable revision in the way the results are described. It is not clear how this section characterizes the "thresholds of ecohydrological variables in response to drought" as mentioned in the section heading. Firstly, what is the meaning of the "response degree" of a ecohydrological variable in line 313? Secondly, how did you arrive the observation in line 315 that "after a certain degree of drought occurs, different ecohydrological variables in the same area changed in different magnitude."?   

Minor comments

Line 89: Shouldn't this be "...are [related] closely to each other..."?

Lines 145-149: Incorrect sentence. Please check. I think it should be "The land cover classes used in this study includes...".

Lines 182-183: AI data should be included in Table 1. 

 

Author Response

We thank the reviewer for the comments. We have carefully considered all the reviewer’s comments as followed. Please refer to the attachment #Response to Reviewer 1 Comments.docx for the details of the revision

Author Response File: Author Response.docx

Reviewer 2 Report

Overall, It is an interesting paper. Relative behaviors of ET, SM, VPD and NDVI could be examined through the results in this study. However, there must be some points to be improved, and the manuscript can be more attractive.

1. Most of the results were derived simply by comparing and investigating the figures. This qualitative approach, even though it is important, may not be accepted, unless the difference is not so vivid. Some statistical analysis on the derived results must be tried in the revision of the manuscript.
2.  The introduction section is not well structured. I could not see why this study is really important in the introduction section. It seems like just a collection of related studies. 
3. Use of historic data is OK. However, more information should be given to the future data (temperature and precipitation) based on the climate change scenarios. We all know that the future drought is simply dependent upon the future climate data. As the future data is simply a scenario, the future drought should also be assumed as just one scenario. We cannot say that it is true and it will happen. 
4. It is not easy to understand how the threshold value was determined in Section 3.2. Please provide one example before providing the results and discussion.
5. Figures 4 and 5 compare the results on the derived thresholds. The result is good, but it is also necessary to explain why that kind of difference occurred. I could find some discussions in the following section, but was not enough.
6. In Section 3.5, it should be mentioned about the assumptions you adopted in the analysis, such as the vegetation and land use in the future.    
7. It is not easy to accept the role of human activities on this drought impact. If possible, some kind of statistical analysis can be helpful to prove these findings. It may be necessary to do the significance test on the difference among regions, climate zones, etc. 

Author Response

We thank the reviewer for the comments. We have carefully considered all the reviewer’s comments as followed. Please refer to the attachment #Response to Reviewer 2 Comments.docx for the details of the revision.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript investigates, at the global scale, the response of the ecohydrological variables to meteorological drought under climate change. The international literature needs this kind of research; however, the papers are partial contributions, such studies are very important to have more and systematic knowledge about the dynamics of our planet.

The proposed paper, using a multitude of free data sources, demonstrates how researchers can decrypt the spatial correlation between the duration and severity of drought and ecohydrological variables.    

The paper has a good structure and interesting contents of each section. The Title is an attractive one and the Abstract represents a relevant synthesis of the methodological analysis, findings, and comments. However, I suggest replacing “the extent and magnitude of drought” with the “duration and severity of drought”, just to use the terms met inside of the manuscript and to be more focused on the specificity of drought (the “extent” requires “spatial” extent, and “magnitude” is not the most appropriate term for drought).

Regarding the Introduction section, which is well conceived, covering a relevant literature and defining the research objectives, I suggest clearly summarize of the work hypothesis.  

I highly appreciate the detailed presentation of the Materials and Methods section, which is very useful for scholars who intend to continue such research, valorizing other data sources. The structure of this section reflects the main steps of methodological approach, providing enough elements on data collection, the analyzed period, processing of datasets, the main tests used to define the trends of drought duration and severity, other different quantitative methods, and indexes to measure the impact of drought on ecohydrological variables and their responses. An important contribution is the assessment of the occurrence probability of drought events. Using appropriate techniques and tools, the authors estimate the return period of such events with strong impact on ecohydrological variables changes.

Positive appreciations on the Results analysis made by authors. They demonstrate, by the planetary maps, which accompany this analysis, the practical issues of the research. It is underlined the spatial distribution of ecohydrological variables’ changes during the drought periods, the specific thresholds of these variables responding to drought, the effect of human activities and population density on the migration of drought thresholds at the global scale. Regarding the probability of drought events with impact on the ecohydrological variables dynamics, the authors’ results show that the consequences are different in humid and semi-humid regions vis-à-vis of arid and semi-arid ones. Starting from the relationships between the frequency of drought events and the changes of ecohydrological during the analyzed period and extrapolation for different future climate scenarios, the conclusion is that it could discuss about a significant downward trend. Specifying the main regions which can be strongly affected in the future, the study can be used in defining the regional and global policy to mitigate the effects of the increasing of the drought events frequency.

Discussions and Conclusion sections are well connected with the results, and I appreciate their contents. A short remark on the title of 4.3. Chapter, which directs the reader to find some elements of uncertainty and implications of the impact of the drought on ecohydrological changes. Obviously, it’s about the limits of the study! I suggest replacing the existing formulation with this one.

 Some minor elements:

• Row 327, Figure 3 – please replace Duation with Duration
• Row 445 – please eliminate “diamonds indicate outliers”!

Because there is a redundant remark between the last phrases from of the Discussion and Conclusion sections, I suggest keeping one of them. My option is for the Conclusion section, with few improvements.  

Author Response

We thank the reviewer for the comments. We have carefully considered all the reviewer’s comments as followed. Please refer to the attachment #Response to Reviewer 3 Comments.docx for the details of the revision.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The revised manuscript has adequately addressed the issues raised in my review of the previous version. I recommend that the manuscript be accepted for publication.   

Author Response

Thanks for your comments about the mansucript!