Identification of compound drought and heatwave events on a daily scale and across four seasons

Shan, Baoying; Verhoest, Niko E. C.; De Baets, Bernard

doi:https://doi.org/10.5194/egusphere-2023-147

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https://doi.org/10.5194/egusphere-2023-147

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08 Jun 2023

| 08 Jun 2023

Identification of compound drought and heatwave events on a daily scale and across four seasons

Baoying Shan, Niko E. C. Verhoest, and Bernard De Baets

Abstract. Compound drought and heatwave (CDHW) events can incur intensified damage to ecosystems, economies, and societies, especially on a warming planet. Although it has been reported that CDHW events in the winter season can also affect insects, birds, and wildfires, the literature generally focuses on the summer season exclusively. Moreover, the coarse resolution of droughts as determined on a monthly scale may hamper a precise identification of the start and/or end dates of CDHW events. Therefore, we propose a method to identify CDHW events on a daily scale that is applicable across the four seasons. More specifically, we use standardized indices calculated on a daily scale to identify four types of compound events in a systematic way. Based on the hypothesis that droughts or heatwaves should be statistically extreme and independent, we remove minor dry or warm spells and merge mutually dependent ones. To demonstrate our method, we make use of 120 years of daily precipitation and temperature observed at Uccle, Brussels-Capital Region, Belgium. Our method yields more precise start and end dates for droughts and heatwaves than what can be obtained with a classical approach acting on a monthly scale, thus also allowing for a better identification of CDHW events. Consistent with existing literature, we find an increase in the occurrence of the number of days in CDHW events at Uccle, mainly due to the increasing frequency of heatwaves. Our results also reveal a seasonality of CDHW events as droughts and heatwaves are negatively dependent in the winter season at Uccle, while they are positively dependent in the other seasons. Overall, the method proposed in this study is shown to be robust and shows potential for exploring how year-round CDHW events influence ecosystems.

Received: 01 Feb 2023 – Discussion started: 08 Jun 2023

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Journal article(s) based on this preprint

08 May 2024

Identification of compound drought and heatwave events on a daily scale and across four seasons

Baoying Shan, Niko E. C. Verhoest, and Bernard De Baets

Hydrol. Earth Syst. Sci., 28, 2065–2080, https://doi.org/10.5194/hess-28-2065-2024,https://doi.org/10.5194/hess-28-2065-2024, 2024

Short summary

Baoying Shan, Niko E. C. Verhoest, and Bernard De Baets

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-147', Anonymous Referee #1, 10 Jul 2023
Identification of compound drought and heatwave events on a daily scale and across four seasons

Authors: Baoying Shan et al.
Egusphere-2023-147

This paper discusses ways to determine heatwaves, droughts, and combination of them in a systematic way and at daily scale allowing to better capture the start and end date of the events. They use 120 years of data over Uccle (point) and suggested an objective method to remove minor spells and merge mutually dependent ones and discussed four ways to define a CDHW event. They eventually used the outcomes to study the seasonality of the drought, heatwaves, and combination of them. They found that seasonal patterns, with the occurrences of droughts and heatwaves being negatively dependent in winter but positively dependent in the other three seasons at Uccle.

I believe the paper is well structured and well written in almost all aspects: the science within the scope of the journal, relatively novel in the method and concepts, important conclusion, good literature review, and good discussion of the outcomes and associated interpretation. The conclusions are backed up with proper studies, the title and abstract are fine.
I only have few minor points:
The study is performed using a point measurement that has 120 years of daily temperature and precipitation data. Yet, the number of events, especially compound events, are not much. Is there any recommendation for the minimum length of the data record if someone wants to apply the method over other regions?

Line 26: remove “more” before regionally

Line 39: what is (7d)? Fig 7d in their paper?

Line 98: not sure how you come up with 91 days? Is the 91 day window for both 30yr and entire record analysis? I think it is better to further clarify this.

Line 224: shouldn’t it be the sum of “positive” SHI?

Line 254: Check if the orders is right. I think it should be 1.3, 1, and 0.5 instead.

Explain why no merging is carried out in some cases in Table 1.

For Nd=1 and SPI =-1.3, what does Md=51 mean? why all other Mds are much less? Are these results stable?
Citation: https://doi.org/10.5194/egusphere-2023-147-RC1
- AC1: 'Reply on RC1', Baoying Shan, 17 Sep 2023
  
  The response letter is attached as a pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-147-AC1
RC2:
'Comment on egusphere-2023-147', Anonymous Referee #2, 12 Jul 2023

This study presents a method to identify compound drought/heatwave daily events as well as develop a set of indices that can be used to characterize such events. The paper is well organized and the topic is appropriate for the journal. However, there are a couple of issues that should be addressed before publication (along with some other minor issues outlined below).
In particular, given the definition of drought and heatwave events I'm not sure that a "precise identification of the start and end dates of an event" is really realistic, I would probably focus more on things like duration etc. which are important and somewhat remove the aspect of a specific start/end date. Also, there is not much discussion on validating the identification of these compound events and although it is a difficult task I would have liked to see some additional text/analysis. A discussion section is also missing that would add some more details on how the results compare in the context of other studies; also, the implications of identifying the compound events can be expanded upon slightly (as the authors seem to attempt in the final sentence of the Conclusions section).
- l. 29: is the "coarse resolution" referring to spatial, temporal or both? From the next sentence it seems it's temporal but it'd be good to be explicit here.

- Introduction is otherwise well written.

- l. 76-80: I don't find this adds much value. Instead, an overview of the experimental design and its rationale would work better here.

- l. 82: why was only one meteorological station used in the study?

- l. 84: perhaps use "have been used in many studies" instead of "have been subjected to many studies"?

- l. 132-134: to what degree does the choice of distribution change the calculation of the indices? The use of AIC is an objective way of selecting the distribution, but what is the sensitivity of SPI/SHI?

- l. 148: the etymology of the term "deficiency" refers to quality, I'm wondering if there could be a better term to describe "how close two neighboring spells are" (unless I misunderstood the definition). Perhaps "proximity"?

- l. 168: I would remove the word "Concretely" here, perhaps "Particularly"?

- l. 230: not sure you need a separate subsection for this.

- l. 309: why specifically these values for Nd, Nh?

- l. 360-374: the Conclusions section is somewhat poorly written. For instance, the last sentence seems out of place while the statement "The results confirmed the effectiveness of the proposed method" needs a bit more context.

Citation: https://doi.org/10.5194/egusphere-2023-147-RC2
- AC2: 'Reply on RC2', Baoying Shan, 17 Sep 2023
  
  The response letter is attached as a pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-147-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-147', Anonymous Referee #1, 10 Jul 2023
Identification of compound drought and heatwave events on a daily scale and across four seasons

Authors: Baoying Shan et al.
Egusphere-2023-147

This paper discusses ways to determine heatwaves, droughts, and combination of them in a systematic way and at daily scale allowing to better capture the start and end date of the events. They use 120 years of data over Uccle (point) and suggested an objective method to remove minor spells and merge mutually dependent ones and discussed four ways to define a CDHW event. They eventually used the outcomes to study the seasonality of the drought, heatwaves, and combination of them. They found that seasonal patterns, with the occurrences of droughts and heatwaves being negatively dependent in winter but positively dependent in the other three seasons at Uccle.

I believe the paper is well structured and well written in almost all aspects: the science within the scope of the journal, relatively novel in the method and concepts, important conclusion, good literature review, and good discussion of the outcomes and associated interpretation. The conclusions are backed up with proper studies, the title and abstract are fine.
I only have few minor points:
The study is performed using a point measurement that has 120 years of daily temperature and precipitation data. Yet, the number of events, especially compound events, are not much. Is there any recommendation for the minimum length of the data record if someone wants to apply the method over other regions?

Line 26: remove “more” before regionally

Line 39: what is (7d)? Fig 7d in their paper?

Line 98: not sure how you come up with 91 days? Is the 91 day window for both 30yr and entire record analysis? I think it is better to further clarify this.

Line 224: shouldn’t it be the sum of “positive” SHI?

Line 254: Check if the orders is right. I think it should be 1.3, 1, and 0.5 instead.

Explain why no merging is carried out in some cases in Table 1.

For Nd=1 and SPI =-1.3, what does Md=51 mean? why all other Mds are much less? Are these results stable?
Citation: https://doi.org/10.5194/egusphere-2023-147-RC1
- AC1: 'Reply on RC1', Baoying Shan, 17 Sep 2023
  
  The response letter is attached as a pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-147-AC1
RC2:
'Comment on egusphere-2023-147', Anonymous Referee #2, 12 Jul 2023

This study presents a method to identify compound drought/heatwave daily events as well as develop a set of indices that can be used to characterize such events. The paper is well organized and the topic is appropriate for the journal. However, there are a couple of issues that should be addressed before publication (along with some other minor issues outlined below).
In particular, given the definition of drought and heatwave events I'm not sure that a "precise identification of the start and end dates of an event" is really realistic, I would probably focus more on things like duration etc. which are important and somewhat remove the aspect of a specific start/end date. Also, there is not much discussion on validating the identification of these compound events and although it is a difficult task I would have liked to see some additional text/analysis. A discussion section is also missing that would add some more details on how the results compare in the context of other studies; also, the implications of identifying the compound events can be expanded upon slightly (as the authors seem to attempt in the final sentence of the Conclusions section).
- l. 29: is the "coarse resolution" referring to spatial, temporal or both? From the next sentence it seems it's temporal but it'd be good to be explicit here.

- Introduction is otherwise well written.

- l. 76-80: I don't find this adds much value. Instead, an overview of the experimental design and its rationale would work better here.

- l. 82: why was only one meteorological station used in the study?

- l. 84: perhaps use "have been used in many studies" instead of "have been subjected to many studies"?

- l. 132-134: to what degree does the choice of distribution change the calculation of the indices? The use of AIC is an objective way of selecting the distribution, but what is the sensitivity of SPI/SHI?

- l. 148: the etymology of the term "deficiency" refers to quality, I'm wondering if there could be a better term to describe "how close two neighboring spells are" (unless I misunderstood the definition). Perhaps "proximity"?

- l. 168: I would remove the word "Concretely" here, perhaps "Particularly"?

- l. 230: not sure you need a separate subsection for this.

- l. 309: why specifically these values for Nd, Nh?

- l. 360-374: the Conclusions section is somewhat poorly written. For instance, the last sentence seems out of place while the statement "The results confirmed the effectiveness of the proposed method" needs a bit more context.

Citation: https://doi.org/10.5194/egusphere-2023-147-RC2
- AC2: 'Reply on RC2', Baoying Shan, 17 Sep 2023
  
  The response letter is attached as a pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-147-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to revisions (further review by editor and referees) (25 Sep 2023) by Narendra Das

AR by Baoying Shan on behalf of the Authors (11 Oct 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to revisions (further review by editor and referees) (05 Nov 2023) by Narendra Das

AR by Baoying Shan on behalf of the Authors (09 Nov 2023) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Dec 2023) by Narendra Das

RR by Anonymous Referee #3 (20 Jan 2024)

RR by Anonymous Referee #4 (29 Jan 2024)

Suggestions for revision or reasons for rejection

Identification of compound drought and heatwave events on a daily scale and across four seasons
Authors: Baoying Shan et al.

This paper introduces a method for identifying compound drought/heatwave events on a daily basis. In general, the study is well-written and the chosen topic is suitable for the journal. The authors have addressed most of the comments made by previous reviewers, but there are a few remaining issues that need to be resolved before publication, along with some minor concerns mentioned below.

In particular, the authors should clarify the specific term for drought. Droughts are generally categorized into two major types including conventional droughts and flash droughts. The conventional droughts are referred to those events that last longer and they are usually assessed on a monthly scale, while flash droughts are those events that develop and intensify rapidly, which have shorter durations (e.g. 20 days). In this study, while the authors proposed a method to detect droughts on a daily basis, they did not mention flash droughts or use specific method to detect droughts with short duration (e.g. 15, or 20 days). The major problem is that authors defined droughts on a daily basis using daily SPI, which I am afraid that it might not be a right approach. The SPI is used in various studies to define droughts on monthly scale, but it cannot detect rapid onset and development of droughts in finer temporal scales (e.g. 15 day in this study), as it might lead to false or unreliable drought signals, because flash droughts cause an intense reduction in soil moisture and a short reduction in precipitation without affecting soil or plants might not be a drought. Therefore, to capture these droughts, meteorological drought indices, including evaporation (potential or actual evaporation) should be used in the analysis, such as SSI (Standardized Soil moisture Index), SPEI or SNPI.

Specific Comments:
1- Page 2, line 30: what about flash droughts? Recent studies focused on the concurrent occurrence of flash droughts and heatwave. This type of droughts usually occur in finer time scales (e.g. 20 days). So, the statement ‘Generally, droughts are identified …’ might not be true. The authors could at least mention flash droughts in this paragraph. Besides, heatwaves can be the driver of flash droughts in some cases (Mo et al. 2015).
2- Page 4, line 89: please add ‘respectively’ to the end of this sentence: ‘as indices to identify droughts and heatwaves.’
3- In the caption of Figure S1, please correct 30-years. It should be ‘’in the past 30-year’’ or ‘’30 years’’.
4- Line 131-136: while the selection of probability function significantly influences the values of an indicator, why did you not use at least one non-parametric function?
The most attractive feature of the non-parametric drought frameworks is that it leads to statistically consistent drought indicators based on different variables (Farahmand and Aghakouchak, 2015). Most drought indicators rely on a representative parametric probability distribution function that fits the data. However, a parametric distribution function may not fit the data. Non-parametric drought indicators do not assume representative parametric distributions.
5- In page 6, line 155, please add description for m and i for both SPI and SHI.
6- While the authors explain the reason for using data from only one station (mainly because of availability of relatively long P and T records) in response to previous referees, still I’m not sure whether this approach would be practical in other regions or not. Several factors affect the occurrence, severity and characteristics of droughts, heatwaves, and compound extremes. By using just one station, the authors neglect important factors such as background aridity and weather patterns in their study and it still remains unclear how this methodology works in different climatic zones.
7- While the authors enhanced the conclusion as suggested by referee2, it still needs further improvement and lacks a comprehensive representation of results and assessments.
8- In line 419: please include references for this statement ‘the increasing temperatures contribute to the increase in CDHW events, which aligns with the current literature.’
9- In line 421: either further explain this statement ‘which could be explained by physical mechanisms.’ or add some references.
10- In line 422: I am not sure about this statement ‘our daily-scale identification captures variations that monthly or weekly scales often miss’, because I did not see any weekly analysis that confirms this assertion.

Mo, K. C., and D. P. Lettenmaier (2015), Heat wave flash droughts in decline, Geophys. Res. Lett., 42, 2823–2829, doi:10.1002/2015GL064018.
Farahmand, A., and A. AghaKouchak (2015), A generalized framework for deriving nonparametric standardized drought indicators, Advances in Water Resources, 76, 140-145.

Hide

ED: Publish subject to minor revisions (review by editor) (10 Mar 2024) by Narendra Das

AR by Baoying Shan on behalf of the Authors (17 Mar 2024) Author's response Author's tracked changes Manuscript

ED: Publish as is (24 Mar 2024) by Narendra Das

AR by Baoying Shan on behalf of the Authors (25 Mar 2024) Manuscript

Journal article(s) based on this preprint

08 May 2024

Identification of compound drought and heatwave events on a daily scale and across four seasons

Baoying Shan, Niko E. C. Verhoest, and Bernard De Baets

Hydrol. Earth Syst. Sci., 28, 2065–2080, https://doi.org/10.5194/hess-28-2065-2024,https://doi.org/10.5194/hess-28-2065-2024, 2024

Short summary

Baoying Shan, Niko E. C. Verhoest, and Bernard De Baets

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https://doi.org/10.5194/egusphere-2023-147-supplement

Baoying Shan, Niko E. C. Verhoest, and Bernard De Baets

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Short summary

This study developed a convenient and new method to identify the occurrence of droughts, heatwaves, and co-occurring droughts and heatwaves across four seasons. From that, we could know the drought (or heatwave) starts on which day and ends on which day. We found an increase in the frequency of heatwaves and the co-occurring droughts and heatwaves in Belgium caused by climate change. We also found that different months have different chances of co-occurring droughts and heatwaves.


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