The dynamics of spatio-temporal droughts in northeast brazil&nbsp;

Pontes Filho, Joao Dehon; Souza Filho, Francisco Assis de; Studart, Ticiana Marinho de Carvalho; Estácio, Ályson Brayner Souza; Martins, Eduardo Sávio Passos Rodrigues

doi:10.5194/egusphere-2024-3166

Preprints

https://doi.org/10.5194/egusphere-2024-3166

Preprints

14 Nov 2024

| 14 Nov 2024

The dynamics of spatio-temporal droughts in northeast brazil

Joao Dehon Pontes Filho, Francisco Assis de Souza Filho, Ticiana Marinho de Carvalho Studart, Ályson Brayner Souza Estácio, and Eduardo Sávio Passos Rodrigues Martins

Abstract. This study presents a new framework for simplifying the spatio-temporal dynamics of droughts. The framework balances the complexity of drought analysis with the need for clear, actionable information. Using growth curves, growth rates, and acceleration of drought characteristics, the study provides a novel method to monitor drought severity, affected areas, and progression paths. By examining patterns and relationships between these characteristics, the framework identifies critical regions for targeted drought monitoring. The findings highlight that droughts originating in certain areas of Northeast Brazil pose greater risks, requiring closer attention. This approach offers decision-makers a more effective tool to predict and mitigate drought impacts, improving drought management strategies.

Received: 11 Oct 2024 – Discussion started: 14 Nov 2024

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Journal article(s) based on this preprint

29 Apr 2026

The dynamics of spatio-temporal droughts in Northeast Brazil

Joao Dehon Pontes Filho, Francisco Assis de Souza Filho, Ályson Brayner Sousa Estácio, Ticiana Marinho de Carvalho Studart, and Eduardo Sávio Passos Rodrigues Martins

Hydrol. Earth Syst. Sci., 30, 2473–2491, https://doi.org/10.5194/hess-30-2473-2026,https://doi.org/10.5194/hess-30-2473-2026, 2026

Short summary

Joao Dehon Pontes Filho, Francisco Assis de Souza Filho, Ticiana Marinho de Carvalho Studart, Ályson Brayner Souza Estácio, and Eduardo Sávio Passos Rodrigues Martins

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3166', Anonymous Referee #1, 03 Dec 2024

Dear authors, please find my comments on the supplement document.

Citation: https://doi.org/10.5194/egusphere-2024-3166-RC1
- AC2:
  'Reply on RC1', Joao Dehon Pontes Filho, 20 Mar 2025
  1) General Comments on the Review Process
  We sincerely appreciate the insightful feedback provided by the reviewers. Their detailed and constructive comments allowed us to recognize that the initial version of our manuscript presented some conceptual and structural ambiguities. In response, we have undertaken a major revision to refine and clarify our work.
  A key outcome of this revision was the recognition that the section focusing on the spatial distribution of drought occurrence was detracting from the core message of the study. Consequently, this section has been removed to enhance the manuscript’s clarity and focus. The revised version now concentrates on the methodological framework inspired by the dispersion analysis of COVID-19, as this aspect received greater attention and interest from the reviewers.
  Beyond this refinement, we have incorporated additional reflections and methodological advancements into the manuscript. Specifically, in addition to the original curves, that we had to reformulate their names, we have introduced a dynamic phase analysis of drought events, categorizing them into expansion, persistence, and contraction stages. To further evaluate the transitions between these phases, we applied a transition matrix approach, which provides valuable insights into the evolution of droughts and their implications for decision-making.
  Furthermore, based on observed drought events, we have developed a typology of drought evolution patterns. This classification system offers an additional contribution to decision-makers that can now better understand how events in their region are expected to evolve and the design of mitigation strategies tailored to the most probable drought evolution patterns.
  We believe that these revisions have significantly strengthened the manuscript, aligning it more closely with the journal’s scope and ensuring that the study provides clear and actionable insights for drought monitoring and management.
  2) Response to General Reviewer 1 Comments
  We are grateful for the thorough and constructive feedback provided by Reviewer 1. The comments allowed us to significantly refine and clarify the manuscript, leading to a substantial revision of both the methodological framework and the presentation of results. Below, we detail the major issues raised and the corresponding revisions made.
  Language:
  
  The reviewer noted grammatical errors and poorly structured sentences. Given that most of the text in the revised manuscript is new, we conducted a comprehensive English language review to ensure clarity, coherence, and grammatical accuracy.
  
  Materials and Methods:
  
  The reviewer pointed out that the study area was not sufficiently presented and that the methodology lacked clarity. In response, we have restructured the methodology section to provide a clearer description of the study area and methods. Additionally, we incorporated a flowchart to enhance the comprehension of the methodological steps.
  
  Results:
  
  Several figures had critical flaws, including missing labels and incorrect values. Additionally, some analyses, such as the 4D analysis, were not adequately introduced in the methods, and the concept of correlation was misapplied. We have carefully revised all figures, replaced some analyses, and removed figures that were no longer relevant to the refined scope of the study.
  
  Discussion:
  
  The reviewer observed that the results lacked proper discussion and identified a duplicated paragraph. To address this, we have integrated the discussion of results alongside their presentation, ensuring a clear narrative that highlights the practical implications and informational gains provided by the proposed methodology.
  
  Novelty:
  
  The reviewer questioned the novelty of using growth curves and growth rates, arguing that they do not provide a significant technical gain over classical approaches such as accumulated severity and total affected area. In response, we made a significant effort to better articulate the technical advantages of our approach. The newly proposed typology improves the characterization of drought behavior in a given region, enabling better preparedness for expected drought dynamics. Additionally, the transition matrix analysis allows decision-makers to gain confidence in the reliability of the information derived from the dynamic evolution of droughts.
  
  Applicability for Decision-Makers:
  
  The reviewer requested further demonstration of how the proposed tool enhances drought prediction and mitigation. To address this, we incorporated an in-depth discussion on how the tool supports proactive drought management by facilitating region-specific action planning. For instance, in the study area, where droughts tend to expand and contract rapidly while maintaining prolonged persistence, a swift response in both the initiation and termination of mitigation measures is crucial. The revised manuscript now explicitly connects these insights to actionable decision-making strategies.
  
  3) Response to Specific Reviewer 1 Comments
  We sincerely appreciate reviewer 1 (RC1) detailed and constructive feedback. Below, we provide a point-by-point response to the specific comments raised, outlining the revisions made to the manuscript.
  "The CRU TS v4.05 time series has been available since 1901, but we chose only to use data from the mid-20th century onwards as the region did not have many rain gauges at the beginning of the century and the dataset interpolation may add noise for the grid analysis."
  
  Response: The reviewer correctly pointed out that the phrase "has been available since 1901" could be more accurately stated as "provides data for the period starting in 1901." We have made this change, and the revised paragraph now reads:
  
  "Monthly precipitation data were sourced from the CRU TS v4.05 dataset, which has a spatial resolution of 0.5° × 0.5° and covers the period from 1950 to 2018 (Harris et al., 2020). Although the CRU TS v4.05 time series extends back to 1901, we opted to use data from 1950 onward due to the limited availability of rain gauge records in the study region during the early 20th century, which could introduce uncertainties in data interpolation."
  
  "How many stations are sufficient to avoid noise in dataset interpolation?"
  
  Response: It is difficult to precisely quantify the number of stations required to eliminate interpolation noise. However, the fewer the stations, the greater the uncertainty, as precipitation patterns from distant locations may be imposed on regions with insufficient data. Given our understanding of the study area and the historical distribution of meteorological stations, we believe that by the 1950s, the density of stations was adequate to provide reliable information for grid-based analysis.
  
  "The first step for drought spatio-temporal definition is data processing is the first step."
  
  Response: Corrected for clarity.
  
  "(e.g. hydrological" (incomplete sentence).
  
  Response: Corrected.
  
  "Run theory (reference)?"
  
  Response: The missing reference was added: Run theory (Yevjevich, 1967).
  
  "Drought duration was not chosen to be analyzed since it is present in all the other three drought characteristics."
  
  Response: This sentence was removed from the text, as we no longer consider it essential for conveying our message. The original intention was to highlight that drought duration could not be treated as a standalone characteristic because our proposed methodology already incorporates temporal evolution in the analysis of severity and affected area.
  
  "The growth rate is the first derivative (of what)?"
  
  Response: To clarify, we revised the terminology of the curves. What was previously referred to as the growth rate is now termed the state curve. The revised section now reads:
  
  "Each drought characteristic, in this study the affected area and severity, was used to compute the three-curve model:
  
  Growth Curve: Cumulative drought characteristic (integral of the State Curve).
  
  State Curve: Drought characteristic.
  
  Dynamic Curve: Change in the State Curve (first derivative of the State Curve)."*
- "What is the utility of the centroid curve/rate/acceleration? The drought-affected area might grow while the centroid remains stationary. Also, a mild drought signal might move around within the study area."
  
  Response: This comment prompted us to reassess the necessity of including centroid-related analyses. We concluded that the centroid did not provide significant added value in explaining drought dynamics. Consequently, we removed this variable from the analysis, focusing instead on the most informative indicators—affected area and severity.
- "By 'dangerous,' do you mean 'hazardous'?"
  
  Response: Yes, we have replaced dangerous with hazardous to align with appropriate scientific terminology.
- "Consider showing Figure 2A as a separate figure."
  
  Response: We created a separate figure specifically to illustrate the spatio-temporal evolution obtained from the 3D analysis.
- "The study area (this is the first mention of a 'study area.' By convention, it should be presented in the methods section)."
  
  Response: A dedicated Study Area subsection has been added to the methodology section, providing a clear description of the location where the proposed framework was applied.
- "A 4D analysis (of what)?"
  
  Response: This section of the study was removed.
- "Correlation (no correlation is presented; this is a pairplot)."
  
  Response: This section was removed.
- "The area overlap criteria did not result in any changes in the definition of drought events. (That is a rather surprising result. Do you have any justification or discussion to contribute?)"
  
  Response: This section was removed.
  
  The new version of the paper is attached as supplement to this comment. We greatly appreciate the opportunity to refine our study based on the reviewers', and we hope that the revised manuscript meets the expectations of the journal.

Citation: https://doi.org/10.5194/egusphere-2024-3166-AC2

RC2: 'Comment on egusphere-2024-3166', Anonymous Referee #2, 14 Feb 2025

This paper presents a methodology for drought assessment based on methods that have been used to monitor the COVID-19 pandemic. Although it is a highly relevant subject and of interest to the general public at HESS, fundamental problems need to be corrected in order for this article to be in a publishable format.

General Comments:

The fundamental issue of this study lies in how the authors chose to identify and track drought events. Droughts result from complex interactions between human actions and the environment, extending beyond a purely meteorological phenomenon. Evaluating drought solely through standardized indices such as SPI has already been extensively explored in this region and presents a simplistic analysis that ignores various local aspects, especially those related to water availability. The authors themselves state that drought perception in the region is directly influenced by this factor, yet they practically ignore it by relying only on SPI. If the presented method had at least combined multiple indices capable of capturing the forcings associated with drought emergence in the region, it would offer a more robust approach. However, this was not done, nor was it mentioned as a study limitation. This choice undermines the paper's innovation, as the technical relevance of the proposed approach is not clearly demonstrated.

Another supposed innovation is the drought assessment in four dimensions: duration, severity, area, and onset centroid. Except for the latter variable, the others have already been extensively analyzed in previous studies. Regarding the centroid, its real relevance is unclear, as is whether it makes sense to consider a multifactorial disaster like a drought event based on the analysis of a single variable. I believe this concept would be more useful if it actually demonstrated spatial causality in the propagation of the precipitation deficit identified by SPI. In other words, how a lack of rainfall in one region of the study area may the lack of rainfall in another. This was implicitly considered by evaluating only SPI and the supposed spatial propagation of its deficit.

Specific Comments:

The text presents several writing errors, and the structure, with overly short sentences and low textual cohesion, does not meet the technical quality expected for a high-impact scientific journal such as HESS. Moreover, there are phrasing errors throughout the text, which I believe result from poor translation. Here are some examples:

"The first step for drought spatio-temporal definition is data processing is the first step." (Line 85).
"However, the methodology proposed in this study for the drought spatio-temporal analysis fits any gridded time-series of drought index and any drought definition (e.g., hydrological." (Line 90).

A significant portion of the results discussion section is based on presenting characteristics of the region’s meteorological dynamics, which have already been extensively discussed in the scientific literature, without establishing a direct connection to the study’s findings. This, once again, raises questions about the study's innovation.

Even though the authors claim that Brazil’s Northeast region is well known—which may be true—I suggest providing more details and characteristics of this region.

Citation: https://doi.org/10.5194/egusphere-2024-3166-RC2

AC1:
'Reply on RC2', Joao Dehon Pontes Filho, 20 Mar 2025
1) General Comments on the Review Process
We sincerely appreciate the insightful feedback provided by the reviewers. Their detailed and constructive comments allowed us to recognize that the initial version of our manuscript presented some conceptual and structural ambiguities. In response, we have undertaken a major revision to refine and clarify our work.
A key outcome of this revision was the recognition that the section focusing on the spatial distribution of drought occurrence was detracting from the core message of the study. Consequently, this section has been removed to enhance the manuscript’s clarity and focus. The revised version now concentrates on the methodological framework inspired by the dispersion analysis of COVID-19, as this aspect received greater attention and interest from the reviewers.
Beyond this refinement, we have incorporated additional reflections and methodological advancements into the manuscript. Specifically, in addition to the original curves, that we had to reformulate their names, we have introduced a dynamic phase analysis of drought events, categorizing them into expansion, persistence, and contraction stages. To further evaluate the transitions between these phases, we applied a transition matrix approach, which provides valuable insights into the evolution of droughts and their implications for decision-making.
Furthermore, based on observed drought events, we have developed a typology of drought evolution patterns. This classification system offers an additional contribution to decision-makers that can now better understand how events in their region are expected to evolve and the design of mitigation strategies tailored to the most probable drought evolution patterns.
We believe that these revisions have significantly strengthened the manuscript, aligning it more closely with the journal’s scope and ensuring that the study provides clear and actionable insights for drought monitoring and management.

2) Response to General Reviewer 2 Comments
We sincerely appreciate the thoughtful and detailed feedback provided by Reviewer 2. The comments prompted substantial revisions, leading to a more refined and focused manuscript. Below, we outline the key concerns raised and the corresponding modifications implemented in response.
Methodological Limitations – Use of a Single Drought Index (SPI):

The reviewer highlighted that evaluating drought events solely through the Standardized Precipitation Index (SPI) oversimplifies the phenomenon by neglecting key local aspects, particularly water availability. We acknowledge this limitation and agree that a multi-index approach could provide a more comprehensive representation of drought conditions. However, the primary goal of our study is to propose a methodological framework that enhances the information available to decision-makers rather than to provide an exhaustive drought characterization. The methodology we present is inherently flexible and can be adapted to incorporate other drought indices, such as the Standardized Precipitation Evapotranspiration Index (SPEI) or hydrological drought indicators. To reflect this, we have explicitly mentioned this methodological limitation in the revised manuscript and suggested future research avenues that could explore the integration of additional indices.

Use of the Drought Centroid:

The reviewer questioned the relevance of tracking the drought centroid, suggesting that this metric would be more valuable if it demonstrated spatial causality in precipitation deficits. Upon further reflection, we recognize that the initial version of the manuscript did not fully establish a causal relationship between centroid movement and drought propagation. Given this, and in an effort to streamline the manuscript’s focus, we have decided to remove this analysis from the revised version. This change allows us to concentrate on the methodological aspects that provide decision-makers with clearer and more actionable insights.

Clarity and Textual Coherence:

The reviewer pointed out issues related to writing quality, including weak structure, short sentences, and limited textual cohesion. As mentioned in our response to Reviewer 1, the majority of the manuscript has been rewritten, and we have conducted a thorough English language revision to improve clarity, coherence, and readability. We hope that these improvements effectively address the linguistic concerns raised in the initial review.

Discussion – Connection to Study Findings:

The reviewer noted that a substantial portion of the discussion repeated well-known meteorological dynamics of the study region without sufficiently linking them to the study’s findings. To address this, we have integrated the discussion directly into the results section. This restructuring allows us to explain the findings in a way that immediately clarifies their practical implications, ensuring that each result is contextualized within the broader objective of enhancing drought monitoring and decision-making.

These revisions have strengthened the manuscript by enhancing methodological transparency, focusing on the most informative analyses, and improving overall readability. We greatly appreciate the reviewer’s insights, which have significantly contributed to refining our study’s impact and clarity.
The new version of the paper is attached as supplement to this comment.
Citation: https://doi.org/10.5194/egusphere-2024-3166-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-3166', Anonymous Referee #1, 03 Dec 2024

Dear authors, please find my comments on the supplement document.

Citation: https://doi.org/10.5194/egusphere-2024-3166-RC1
- AC2:
  'Reply on RC1', Joao Dehon Pontes Filho, 20 Mar 2025
  1) General Comments on the Review Process
  We sincerely appreciate the insightful feedback provided by the reviewers. Their detailed and constructive comments allowed us to recognize that the initial version of our manuscript presented some conceptual and structural ambiguities. In response, we have undertaken a major revision to refine and clarify our work.
  A key outcome of this revision was the recognition that the section focusing on the spatial distribution of drought occurrence was detracting from the core message of the study. Consequently, this section has been removed to enhance the manuscript’s clarity and focus. The revised version now concentrates on the methodological framework inspired by the dispersion analysis of COVID-19, as this aspect received greater attention and interest from the reviewers.
  Beyond this refinement, we have incorporated additional reflections and methodological advancements into the manuscript. Specifically, in addition to the original curves, that we had to reformulate their names, we have introduced a dynamic phase analysis of drought events, categorizing them into expansion, persistence, and contraction stages. To further evaluate the transitions between these phases, we applied a transition matrix approach, which provides valuable insights into the evolution of droughts and their implications for decision-making.
  Furthermore, based on observed drought events, we have developed a typology of drought evolution patterns. This classification system offers an additional contribution to decision-makers that can now better understand how events in their region are expected to evolve and the design of mitigation strategies tailored to the most probable drought evolution patterns.
  We believe that these revisions have significantly strengthened the manuscript, aligning it more closely with the journal’s scope and ensuring that the study provides clear and actionable insights for drought monitoring and management.
  2) Response to General Reviewer 1 Comments
  We are grateful for the thorough and constructive feedback provided by Reviewer 1. The comments allowed us to significantly refine and clarify the manuscript, leading to a substantial revision of both the methodological framework and the presentation of results. Below, we detail the major issues raised and the corresponding revisions made.
  Language:
  
  The reviewer noted grammatical errors and poorly structured sentences. Given that most of the text in the revised manuscript is new, we conducted a comprehensive English language review to ensure clarity, coherence, and grammatical accuracy.
  
  Materials and Methods:
  
  The reviewer pointed out that the study area was not sufficiently presented and that the methodology lacked clarity. In response, we have restructured the methodology section to provide a clearer description of the study area and methods. Additionally, we incorporated a flowchart to enhance the comprehension of the methodological steps.
  
  Results:
  
  Several figures had critical flaws, including missing labels and incorrect values. Additionally, some analyses, such as the 4D analysis, were not adequately introduced in the methods, and the concept of correlation was misapplied. We have carefully revised all figures, replaced some analyses, and removed figures that were no longer relevant to the refined scope of the study.
  
  Discussion:
  
  The reviewer observed that the results lacked proper discussion and identified a duplicated paragraph. To address this, we have integrated the discussion of results alongside their presentation, ensuring a clear narrative that highlights the practical implications and informational gains provided by the proposed methodology.
  
  Novelty:
  
  The reviewer questioned the novelty of using growth curves and growth rates, arguing that they do not provide a significant technical gain over classical approaches such as accumulated severity and total affected area. In response, we made a significant effort to better articulate the technical advantages of our approach. The newly proposed typology improves the characterization of drought behavior in a given region, enabling better preparedness for expected drought dynamics. Additionally, the transition matrix analysis allows decision-makers to gain confidence in the reliability of the information derived from the dynamic evolution of droughts.
  
  Applicability for Decision-Makers:
  
  The reviewer requested further demonstration of how the proposed tool enhances drought prediction and mitigation. To address this, we incorporated an in-depth discussion on how the tool supports proactive drought management by facilitating region-specific action planning. For instance, in the study area, where droughts tend to expand and contract rapidly while maintaining prolonged persistence, a swift response in both the initiation and termination of mitigation measures is crucial. The revised manuscript now explicitly connects these insights to actionable decision-making strategies.
  
  3) Response to Specific Reviewer 1 Comments
  We sincerely appreciate reviewer 1 (RC1) detailed and constructive feedback. Below, we provide a point-by-point response to the specific comments raised, outlining the revisions made to the manuscript.
  "The CRU TS v4.05 time series has been available since 1901, but we chose only to use data from the mid-20th century onwards as the region did not have many rain gauges at the beginning of the century and the dataset interpolation may add noise for the grid analysis."
  
  Response: The reviewer correctly pointed out that the phrase "has been available since 1901" could be more accurately stated as "provides data for the period starting in 1901." We have made this change, and the revised paragraph now reads:
  
  "Monthly precipitation data were sourced from the CRU TS v4.05 dataset, which has a spatial resolution of 0.5° × 0.5° and covers the period from 1950 to 2018 (Harris et al., 2020). Although the CRU TS v4.05 time series extends back to 1901, we opted to use data from 1950 onward due to the limited availability of rain gauge records in the study region during the early 20th century, which could introduce uncertainties in data interpolation."
  
  "How many stations are sufficient to avoid noise in dataset interpolation?"
  
  Response: It is difficult to precisely quantify the number of stations required to eliminate interpolation noise. However, the fewer the stations, the greater the uncertainty, as precipitation patterns from distant locations may be imposed on regions with insufficient data. Given our understanding of the study area and the historical distribution of meteorological stations, we believe that by the 1950s, the density of stations was adequate to provide reliable information for grid-based analysis.
  
  "The first step for drought spatio-temporal definition is data processing is the first step."
  
  Response: Corrected for clarity.
  
  "(e.g. hydrological" (incomplete sentence).
  
  Response: Corrected.
  
  "Run theory (reference)?"
  
  Response: The missing reference was added: Run theory (Yevjevich, 1967).
  
  "Drought duration was not chosen to be analyzed since it is present in all the other three drought characteristics."
  
  Response: This sentence was removed from the text, as we no longer consider it essential for conveying our message. The original intention was to highlight that drought duration could not be treated as a standalone characteristic because our proposed methodology already incorporates temporal evolution in the analysis of severity and affected area.
  
  "The growth rate is the first derivative (of what)?"
  
  Response: To clarify, we revised the terminology of the curves. What was previously referred to as the growth rate is now termed the state curve. The revised section now reads:
  
  "Each drought characteristic, in this study the affected area and severity, was used to compute the three-curve model:
  
  Growth Curve: Cumulative drought characteristic (integral of the State Curve).
  
  State Curve: Drought characteristic.
  
  Dynamic Curve: Change in the State Curve (first derivative of the State Curve)."*
- "What is the utility of the centroid curve/rate/acceleration? The drought-affected area might grow while the centroid remains stationary. Also, a mild drought signal might move around within the study area."
  
  Response: This comment prompted us to reassess the necessity of including centroid-related analyses. We concluded that the centroid did not provide significant added value in explaining drought dynamics. Consequently, we removed this variable from the analysis, focusing instead on the most informative indicators—affected area and severity.
- "By 'dangerous,' do you mean 'hazardous'?"
  
  Response: Yes, we have replaced dangerous with hazardous to align with appropriate scientific terminology.
- "Consider showing Figure 2A as a separate figure."
  
  Response: We created a separate figure specifically to illustrate the spatio-temporal evolution obtained from the 3D analysis.
- "The study area (this is the first mention of a 'study area.' By convention, it should be presented in the methods section)."
  
  Response: A dedicated Study Area subsection has been added to the methodology section, providing a clear description of the location where the proposed framework was applied.
- "A 4D analysis (of what)?"
  
  Response: This section of the study was removed.
- "Correlation (no correlation is presented; this is a pairplot)."
  
  Response: This section was removed.
- "The area overlap criteria did not result in any changes in the definition of drought events. (That is a rather surprising result. Do you have any justification or discussion to contribute?)"
  
  Response: This section was removed.
  
  The new version of the paper is attached as supplement to this comment. We greatly appreciate the opportunity to refine our study based on the reviewers', and we hope that the revised manuscript meets the expectations of the journal.

Citation: https://doi.org/10.5194/egusphere-2024-3166-AC2

RC2: 'Comment on egusphere-2024-3166', Anonymous Referee #2, 14 Feb 2025

General Comments:

Specific Comments:

"The first step for drought spatio-temporal definition is data processing is the first step." (Line 85).
"However, the methodology proposed in this study for the drought spatio-temporal analysis fits any gridded time-series of drought index and any drought definition (e.g., hydrological." (Line 90).

Even though the authors claim that Brazil’s Northeast region is well known—which may be true—I suggest providing more details and characteristics of this region.

Citation: https://doi.org/10.5194/egusphere-2024-3166-RC2

AC1:
'Reply on RC2', Joao Dehon Pontes Filho, 20 Mar 2025
1) General Comments on the Review Process
We sincerely appreciate the insightful feedback provided by the reviewers. Their detailed and constructive comments allowed us to recognize that the initial version of our manuscript presented some conceptual and structural ambiguities. In response, we have undertaken a major revision to refine and clarify our work.
A key outcome of this revision was the recognition that the section focusing on the spatial distribution of drought occurrence was detracting from the core message of the study. Consequently, this section has been removed to enhance the manuscript’s clarity and focus. The revised version now concentrates on the methodological framework inspired by the dispersion analysis of COVID-19, as this aspect received greater attention and interest from the reviewers.
Beyond this refinement, we have incorporated additional reflections and methodological advancements into the manuscript. Specifically, in addition to the original curves, that we had to reformulate their names, we have introduced a dynamic phase analysis of drought events, categorizing them into expansion, persistence, and contraction stages. To further evaluate the transitions between these phases, we applied a transition matrix approach, which provides valuable insights into the evolution of droughts and their implications for decision-making.
Furthermore, based on observed drought events, we have developed a typology of drought evolution patterns. This classification system offers an additional contribution to decision-makers that can now better understand how events in their region are expected to evolve and the design of mitigation strategies tailored to the most probable drought evolution patterns.
We believe that these revisions have significantly strengthened the manuscript, aligning it more closely with the journal’s scope and ensuring that the study provides clear and actionable insights for drought monitoring and management.

2) Response to General Reviewer 2 Comments
We sincerely appreciate the thoughtful and detailed feedback provided by Reviewer 2. The comments prompted substantial revisions, leading to a more refined and focused manuscript. Below, we outline the key concerns raised and the corresponding modifications implemented in response.
Methodological Limitations – Use of a Single Drought Index (SPI):

The reviewer highlighted that evaluating drought events solely through the Standardized Precipitation Index (SPI) oversimplifies the phenomenon by neglecting key local aspects, particularly water availability. We acknowledge this limitation and agree that a multi-index approach could provide a more comprehensive representation of drought conditions. However, the primary goal of our study is to propose a methodological framework that enhances the information available to decision-makers rather than to provide an exhaustive drought characterization. The methodology we present is inherently flexible and can be adapted to incorporate other drought indices, such as the Standardized Precipitation Evapotranspiration Index (SPEI) or hydrological drought indicators. To reflect this, we have explicitly mentioned this methodological limitation in the revised manuscript and suggested future research avenues that could explore the integration of additional indices.

Use of the Drought Centroid:

The reviewer questioned the relevance of tracking the drought centroid, suggesting that this metric would be more valuable if it demonstrated spatial causality in precipitation deficits. Upon further reflection, we recognize that the initial version of the manuscript did not fully establish a causal relationship between centroid movement and drought propagation. Given this, and in an effort to streamline the manuscript’s focus, we have decided to remove this analysis from the revised version. This change allows us to concentrate on the methodological aspects that provide decision-makers with clearer and more actionable insights.

Clarity and Textual Coherence:

The reviewer pointed out issues related to writing quality, including weak structure, short sentences, and limited textual cohesion. As mentioned in our response to Reviewer 1, the majority of the manuscript has been rewritten, and we have conducted a thorough English language revision to improve clarity, coherence, and readability. We hope that these improvements effectively address the linguistic concerns raised in the initial review.

Discussion – Connection to Study Findings:

The reviewer noted that a substantial portion of the discussion repeated well-known meteorological dynamics of the study region without sufficiently linking them to the study’s findings. To address this, we have integrated the discussion directly into the results section. This restructuring allows us to explain the findings in a way that immediately clarifies their practical implications, ensuring that each result is contextualized within the broader objective of enhancing drought monitoring and decision-making.

These revisions have strengthened the manuscript by enhancing methodological transparency, focusing on the most informative analyses, and improving overall readability. We greatly appreciate the reviewer’s insights, which have significantly contributed to refining our study’s impact and clarity.
The new version of the paper is attached as supplement to this comment.
Citation: https://doi.org/10.5194/egusphere-2024-3166-AC1

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 Apr 2025) by Marleen de Ruiter

AR by Joao Dehon Pontes Filho on behalf of the Authors (29 Apr 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (12 May 2025) by Marleen de Ruiter

RR by Anonymous Referee #2 (26 May 2025)

Suggestions for revision or reasons for rejection

The authors have submitted a manuscript that aims to simplify the so-called 3D drought analysis by using a methodology based on growth curves originally developed for studying the evolution of COVID-19 infections and drought typologies. This goal is proposed based on the premise that the methodologies currently available in the literature for this type of analysis are overly complex, limiting their practical application by decision-makers in the field of water resources and drought management. The current version of the text shows substantial improvements compared to the original, especially in terms of readability and grammatical coherence. However, it still fails to clearly demonstrate the scientific relevance and innovation of the proposed study. For the reasons detailed below, I recommend the rejection of this manuscript.
Although the manuscript has improved considerably, it still lacks in-depth analysis or a revision of the methodology that would be necessary to demonstrate its originality and scientific relevance. Firstly, the entire methodology is based solely on the SPI, meaning that, for the authors, a drought is merely a period of low precipitation. This choice is justified by the claim that the WMO recommends the use of this index for drought assessment studies. Furthermore, the authors downplay the limitation of using SPI alone by stating that any other index could be applied in its place (lines 115–125).
It is important to highlight that this WMO declaration was made in the context of meteorological drought analysis and that even within this context, it was stated that SPI should not replace other indices already used locally. Moreover, the WMO recommendation dates back to 2009, a time when the use of remote sensing products was not yet widespread in drought assessment studies. Back then, in many regions, precipitation was the only available data for studying drought hazards (Hayes et al., 2011). It is necessary to acknowledge that the state of the art in drought assessment has advanced. I believe that, considering HESS’s recent special issues on compound and multi-risk analysis and management, it would be more appropriate to publish studies that align with the growing understanding that drought events are highly complex disasters that cannot be reduced to a single meteorological variable, ignoring the broader socioeconomic dimensions of such events (Van Loon et al., 2024; Walker et al., 2024).
The study area addressed by the authors (Northeast Brazil, NEB) is not lacking in hydro-socio-agro-meteorological data to the extent that would justify reducing the analysis solely to precipitation data via SPI. Nor can one ignore the fact that droughts in this region—like in much of the world—are highly complex, intrinsically linked to water availability and local agricultural production methods. If such features are so essential for evaluating droughts in NEB, how can their complete exclusion be justified? This becomes even more evident when considering that NEB has an operational drought monitor, updated monthly, which uses indices based on various variables (not limited to precipitation) and includes field validation (monitordesecas.ana.gov.br). If the proposed methodology were at least applied to the information from this monitor, perhaps some relevant innovation could be demonstrated. However, the monitor is not mentioned, nor is there any discussion on how the authors’ methodology could enhance or modify the existing monitoring activities in the region.The authors make general claims that their methodology could be useful for large-scale monitoring systems, but they fail to demonstrate this with concrete examples — they do not even mention the main drought monitoring system in the study area, which would be the minimum expected.
Even disregarding the fact that the authors adopt a simplistic and overly conservative view of what constitutes a drought—seeing it only as a period of low precipitation—the manuscript still does not demonstrate scientific relevance or practical applicability of the presented methodology when compared to what is already available in the literature. The problem statement in the introduction is built on the idea that existing methodologies are overly complex, hindering their application. Once this criticism is made, it would be expected that the manuscript would show how its methodology can be implemented in practice and what its benefits would be for drought management. However, this is done in a generic manner, with discussions that could have been developed without applying the proposed method—or even without presenting any results—highlighting the lack of innovation in the manuscript.
For example, the authors propose a 3D approach to drought analysis, primarily based on two characteristics: affected area and severity. In most cases, this can be done directly through conventional SPI analysis, without the need for the proposed methodology, as extensively documented in the literature. One of the conclusions of the manuscript is that severity—considered as the mean SPI value during the identified drought events—is less suitable for evaluation and decision-making than the affected area because it exhibits so-called “erratic behavior” (lines 400–405). This is presented as a novel insight, but it is actually a direct consequence of the methodology itself. SPI is based solely on precipitation, accumulated over a moving window (12 months in the manuscript). Given the natural variability of precipitation—which, according to the authors themselves (lines 83–95), is even more pronounced in the study area—reductions in this variable will cause longer-term changes in the SPI time series. Therefore, severity will necessarily exhibit greater variability than the affected area, which is defined by a fixed threshold of the SPI. In other words, for any region, one does not expect SPI12 to show a noisy variation where the index switches in each timestep between extreme negative and positive values, resulting in an erratic affected area. On the other hand, it is natural for the severity's average over time to show greater variability.
Another central aspect of the study is the definition of drought typologies, which are essentially based on the intensification of severity or the growth/contraction of the affected area. These typologies are defined a posteriori—that is, it is only possible to determine the typology of a drought event once it has ended. The authors argue that classifying events into one of the four proposed typologies is useful for decision-makers, but this is not convincingly demonstrated. The usefulness of such classification becomes even more questionable when the identified events do not correspond exclusively to a single typology. The authors also suggest measures to be taken based on each typology (lines 450–460), but this discussion is generic and essentially describes a disaster risk reduction cycle, which is recommended for almost any situation (e.g., Wilhite et al., 2014).
The typologies essentially describe how the precipitation deficit evolves over time — whether the deficit starts intense or not, whether it later increases or decreases, and whether it ends abruptly or gradually. Depending on these aspects, combinations are created to define the typologies and their possible impacts. However, as the analysis is based exclusively on precipitation data, it does not consider, for instance, how the impacts may change or intensify in the case of independent but consecutive events. The manuscript suggests that droughts with larger affected areas but lower severity impact hydrology more, while those with smaller affected areas and higher severity affect agriculture more. However, they do not explain how this occurs or what local characteristics are related to this dynamic. This lack of detail raises further questions: Is there homogeneity in the water system of the study area? Is agriculture uniform? What levels of severity cause hydrological and agricultural impacts? Will the same severity level lead to the same impacts in different contexts?
It would have been interesting if the authors had further detailed their case study, fully analyzing one of the many multi-year drought events that occurred in the region using the proposed methodology. For instance, they could have conducted an analysis based on alternative drought narratives: one describing what happened using conventional monitoring methodologies, detailing the evolution of drought threats and the timeline of impacts; and another exploring what could have occurred if the proposed methodology had been applied. This could effectively highlight the method’s advantages and innovations.
In summary, although the idea of evaluating droughts through a methodological framework inspired by approaches used to monitor COVID-19 infections during the pandemic may seem appealing — even emotionally resonant — and the manuscript presents a reasonable effort to simplify drought analysis, it fails to demonstrate scientific innovation, practical applicability, and alignment with the current state of the art in drought assessment. Without addressing methodological limitations, contextual relevance, and integration with existing systems, the proposed approach remains of limited value to both the scientific community and decision-makers.
References
Hayes, M., Svoboda, M., Wall, N., & Widhalm, M. (2011). The Lincoln declaration on drought indices: universal meteorological drought index recommended. Bulletin of the American Meteorological Society, 92(4), 485-488.
Wilhite, D. A., Sivakumar, M. V., & Pulwarty, R. (2014). Managing drought risk in a changing climate: The role of national drought policy. Weather and climate extremes, 3, 4-13.
Van Loon, A. F., Kchouk, S., Matanó, A., Tootoonchi, F., Alvarez-Garreton, C., Hassaballah, K. E., ... & Werner, M. (2024). Drought as a continuum–memory effects in interlinked hydrological, ecological, and social systems. Natural Hazards and Earth System Sciences, 24(9), 3173-3205.
Walker, D. W., Oliveira, J. L., Cavalcante, L., Kchouk, S., Neto, G. R., Melsen, L. A., ... & Van Oel, P. R. (2024). It's not all about drought: What “drought impacts” monitoring can reveal. International Journal of Disaster Risk Reduction, 103, 104338.

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RR by Anonymous Referee #3 (07 Aug 2025)

Suggestions for revision or reasons for rejection

The manuscript 'The dynamics of spatio-temporal droughts in Northeast Brazil' introduces a growth-curve-based approach for characterising drought evolution using SPI data. The study aims to contribute to drought characterisation analysis. The methodological and scientific contribution of the manuscript requires further elaboration on core technical aspects. The manuscript is well-structured and has improved from earlier stages, but there are several points that require significant attention before publication; therefore, I recommend a major revision. Below are my major and minor comments to improve the document.

Best regards

Major comments
1) The application of growth curves, as adopted from epidemiological modelling (Utsunomiya et al., 2020), is not a novel concept. Similar approaches have been used in recent drought studies, e.g. Banfi et al. (2024), who also calculate and characterise drought development across Europe. Generally, such methods are also utilised in other areas of Natural sciences. The manuscript needs to explicitly position the methodological contribution.
Is the aim of the growth curve-based approach to help characterise how drought changes? Is the aim to provide more information to enhance drought monitoring? If yes, how, why?

2) The methodology for calculating 3D drought clusters lacks clarity. The authors refer to area-overlap thresholds, but the rationale for using a fixed 1.6% threshold needs stronger justification. A sensitivity analysis of the overlap threshold is strongly recommended to evaluate its impact on calculated droughts and their characterization.
How does the method for joining 2d drought clusters differ in principle and execution from previous works such as Diaz et al. (2020) or Herrera-Estrada et al. (2017), who also relied on 2d clustering using similar approaches to join drought areas?
Readers will benefit from a diagram that helps to better understand how 2d clusters are joined.
Why not adopt direct 3D clustering methods as done in Corzo et al. (2011), Cammalleri and Toreti (2023), or Diaz et al. (2023), which avoid arbitrary thresholds and are arguably more robust?

3) Although the proposed method tracks spatial and temporal drought evolution, the paper falls short in presenting detailed descriptive statistics or maps on how droughts evolve in the region. For example, how do drought areas or durations vary across events? Are there spatial patterns across the sub-regions of Northeast Brazil? These are critical to understanding the utility of the typology and transition matrix analysis. The authors could include summary statistics, spatial footprint evolution, and clustering maps of representative events to better demonstrate the tool's utility.

4) The manuscript presents a framework for characterising drought dynamics but does not provide any comparison to classical drought characterisation techniques (e.g., SAD curves, drought area-duration maps). Nor does it benchmark its added value against similar 3D clustering tools. A brief comparison or example case where the proposed method offers clearer insight would significantly strengthen the case for its adoption.

5) The CRU dataset’s 0.5° resolution is quite coarse, particularly for a region with high spatial rainfall variability. The authors should comment on the representativeness and limitations of using such a dataset for event-scale drought calculation in Northeast Brazil.

6) The manuscript lacks a stand-alone discussion section. The integration of results and discussion blurs critical reflections on limitations, transferability, and the comparison with prior studies/methods.

Minor comments
1) While the authors note the flexibility of their method to accommodate other indices, this is not demonstrated nor discussed. Will the area threshold still be valid with the use of other drought indicators, such as soil moisture-based ones, or will it be necessary to switch to 3d clustering?

2) It is not clear whether the typology assignment is automated or based on visual/manual interpretation. This needs clarification. Who labels the drought development? The decision-makers, the modellers?

3) Terms like “growth curve” and “dynamic curve” may be confusing without more precise definitions or graphical support. Consider clarifying with a single schematic diagram showing curve definitions as done by Utsunomiya et al. (2020).

4) Figures could be improved with clearer labels and larger text. (I'm not sure if this applies, but if you are working in Office Word, check that the resolution of figures is not compressed or set automatically to 220 ppi. You can verify this in Word Options > Advanced).

5) The reference of Utsunomiya et al. 2020 is missing in the Reference list. Diaz et al. 2019 is Diaz et al. 2020?

References
Banfi, F., Cammalleri, C., & De Michele, C. (2024). A joint spatio-temporal characterization of the major meteorological droughts in Europe. Environmental Research Letters, 19(9), 094041. https://doi.org/10.1088/1748-9326/ad6ba9

Cammalleri, C., & Toreti, A. (2023). A generalized density-based algorithm for the spatiotemporal tracking of drought events. Journal of Hydrometeorology, 24(3), 537–548. https://doi.org/10.1175/JHM-D-22-0115.1

Corzo Perez, G. A., van Huijgevoort, M. H. J., Voß, F., & van Lanen, H. A. J. (2011). On the spatio-temporal analysis of hydrological droughts from global hydrological models. Hydrology and Earth System Sciences, 15(9), 2963–2974. https://doi.org/10.5194/hess-15-2963-2011

Diaz, V., Corzo Perez, G. A., Van Lanen, H. A. J., Solomatine, D., & Varouchakis, E. A. (2020). An approach to characterise spatio-temporal drought dynamics. Advances in Water Resources, 137, 103512. https://doi.org/10.1016/j.advwatres.2020.103512

Diaz, V., Corzo Perez, G. A., Van Lanen, H. A. J., Solomatine, D., & Varouchakis, E. A. (2020). Characterisation of the dynamics of past droughts. Science of The Total Environment, 718, 134588. https://doi.org/10.1016/j.scitotenv.2019.134588

Diaz, V., Corzo Perez, G. A., Van Lanen, H. A. J., & Solomatine, D. P. (2023). Three-dimensional clustering in the characterization of spatiotemporal drought dynamics: Cluster size filter and drought indicator threshold optimization. In G. A. Corzo Perez & D. P. Solomatine (Eds.), Drought Science and Policy (Chapter 11). Wiley. https://doi.org/10.1002/9781119639268.ch11

Herrera-Estrada, J. E., Satoh, Y., & Sheffield, J. (2017). Spatiotemporal dynamics of global drought. Geophysical Research Letters, 44(5), 2254–2263. https://doi.org/10.1002/2016GL071768

Utsunomiya, Y. T., Utsunomiya, A. T. H., Torrecilha, R. B. P., Paulan, S. C., Milanesi, M., & Garcia, J. F. (2020). Growth rate and acceleration analysis of the COVID-19 pandemic reveals the effect of public health measures in real time. Frontiers in Medicine, 7, 247. https://doi.org/10.3389/fmed.2020.00247

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ED: Reconsider after major revisions (further review by editor and referees) (11 Aug 2025) by Marleen de Ruiter

AR by Joao Dehon Pontes Filho on behalf of the Authors (03 Nov 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (04 Nov 2025) by Marleen de Ruiter

RR by Vitali Diaz (04 Dec 2025)

Suggestions for revision or reasons for rejection

The revised version of the manuscript ‘The dynamics of spatio-temporal droughts in Northeast Brazil’ shows that the authors have incorporated most of the observations made by the other reviewers, and the manuscript now shows clear improvements in clarity and methodological grounding.

After reviewing the new version, I have only a few minor comments aimed at strengthening methodological clarity.

- It would be helpful to clarify whether the classification of drought typologies is performed automatically or through visual assessment. I couldn’t find it.

- Authors could briefly expand the conceptual comparison with recent studies that use cumulative curve approaches (for example, Banfi et al. 2024) to better highlight the distinct contribution of your proposal.

- Although the use of the 1.6 percent threshold for spatial overlap is now better justified, adding one sentence on how this parameter may influence event identification, and most importantly, events’ characteristics such as duration, a characteristic that potentially changes the typological classification, because a longer duration produces a longer total drought area.

-Include a short conceptual comparison of the construction of drought events from the 2D approach following the overlapping areas threshold approach with the fully 3D approach, for instance Diaz et al. (2023).

These comments do not require further analyses or extensive changes; they only aim to increase the precision with which the methodology is presented.

Overall, I believe the manuscript has advanced considerably and that these remaining minor improvements will contribute to a clearer and stronger final version.

Sincerely,
Vitali

References
Banfi, F., Cammalleri, C., & De Michele, C. (2024). A joint spatio-temporal characterization of the major meteorological droughts in Europe. Environmental Research Letters, 19(9), 094041. https://doi.org/10.1088/1748-9326/ad6ba9

Diaz, V., Corzo Perez, G. A., Van Lanen, H. A. J., & Solomatine, D. P. (2023). Three-dimensional clustering in the characterization of spatiotemporal drought dynamics: Cluster size filter and drought indicator threshold optimization. In G. A. Corzo Perez & D. P. Solomatine (Eds.), Drought Science and Policy (Chapter 11). Wiley. https://doi.org/10.1002/9781119639268.ch11

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RR by Anonymous Referee #4 (07 Dec 2025)

ED: Reconsider after major revisions (further review by editor and referees) (08 Dec 2025) by Marleen de Ruiter

AR by Joao Dehon Pontes Filho on behalf of the Authors (19 Jan 2026) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (20 Jan 2026) by Marleen de Ruiter

RR by Anonymous Referee #4 (17 Feb 2026)

ED: Publish subject to minor revisions (review by editor) (17 Feb 2026) by Marleen de Ruiter

AR by Joao Dehon Pontes Filho on behalf of the Authors (27 Feb 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (09 Mar 2026) by Marleen de Ruiter

ED: Publish as is (09 Mar 2026) by Thom Bogaard (Executive editor)

AR by Joao Dehon Pontes Filho on behalf of the Authors (12 Mar 2026)

Journal article(s) based on this preprint

29 Apr 2026

The dynamics of spatio-temporal droughts in Northeast Brazil

Joao Dehon Pontes Filho, Francisco Assis de Souza Filho, Ályson Brayner Sousa Estácio, Ticiana Marinho de Carvalho Studart, and Eduardo Sávio Passos Rodrigues Martins

Hydrol. Earth Syst. Sci., 30, 2473–2491, https://doi.org/10.5194/hess-30-2473-2026,https://doi.org/10.5194/hess-30-2473-2026, 2026

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Joao Dehon Pontes Filho, Francisco Assis de Souza Filho, Ticiana Marinho de Carvalho Studart, Ályson Brayner Souza Estácio, and Eduardo Sávio Passos Rodrigues Martins

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This study introduces a new way to analyze droughts over time and space, making it easier to track and understand their progression. By focusing on the severity, affected areas, and the movement of droughts, the research identifies regions in Northeast Brazil that require special attention. The findings help decision-makers better manage drought impacts, improving planning and response strategies to reduce risks. This work was driven by the need to enhance drought management tools.


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