the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Sep 2025
Compound droughts have become more widespread globally
Abstract. Compound droughts, in which meteorological, hydrological, and agricultural drought occur simultaneously, have a more substantial impact on ecological and socio-economic systems than any drought type alone. Yet, the global patterns – and particularly the multi-decadal trends – of these co-occurring droughts remain poorly understood. We analyse the global co-occurrence of meteorological, agricultural, and hydrological droughts from 1961 to 2020 using run theory and empirical drought indices from three global hydrological models (GHMs), each forced with three different meteorological forcing datasets. Our results indicate that compound droughts and their characteristics show distinct spatial patterns, varying across different hydrological regions. The findings suggest that compound droughts have become more widespread globally, particularly in the hydrological regions (i.e., hydrobelts) near the equator and in the southernmost regions, where the number of days under compound drought has increased rapidly over the past 60 years. Our results also show that compound drought behaviour in the boreal hydrobelt significantly differs from all other hydrobelts, showing a general wetting trend and no increase in compound droughts. We also, however, find a high uncertainty in the ensemble, highlighting a need to global hydrological modelling aimed toward droughts specifically. The results provide valuable global insights into complex phenomena of compound droughts, helping in drought preparedness actions and planning.
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Status: open (until 09 Nov 2025)
- RC1: 'Comment on egusphere-2025-3909', Anonymous Referee #1, 04 Nov 2025 reply
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RC2: 'Comment on egusphere-2025-3909', Anonymous Referee #2, 04 Nov 2025
reply
Review of the manuscript „Compound droughts have become more widespread globally“
This manuscript presents the analysis of simultaneous co-occurrence of droughts in different compartments (i.e., precipitation, soil moisture and streamflow) based on simulations of several global models. The study examines the likelihood of such co-occurrences around the globe and quantifies their trends.
Although investigating simultaneous co-occurrences of metrological, hydrological and agricultural droughts can indeed provide valuable insights for water resources management, the novelty of this study given a large body of drought propagation work is not clear. Moreover, lack of validation of model simulations combined with several methodological ambiguities in implementing standardized drought indexes likely result in high spatially-variable uncertainty of detected trends that is neither quantified nor reported in this study. Finally, very general conclusion that lacks any novel insights further indicates that more effort is needed to clarify the novelty of this manuscript before publishing. Please find below my detailed comments.
Major comments
Terminology and novelty: In the manuscript the terms of drought propagation and compounding are used in a very confusing and inconsistent manner. The authors define compounding drought as overlap of droughts in different terrestrial compartments: precipitation, soil moisture and streamflow. However, it is not clear how their study is different from a large body of studies on drought propagation and why it is justifiable and useful to use the compound event framework here. Although I find that studying the question whether or not drought occurs simultaneously in all three compartments and highlighting the regions where this is more likely – very interesting, I believe that the inconsistent terminology used in the manuscript makes its novelty rather unclear. While the introduction is more focused on compounding framework, the discussion refers to propagation. Given ambiguous terminology, the novelty of the study with regard to existing drought propagation studies is not clear, and on the other side it is not clear how the results of the study are comparable with the findings of studies that consider such events from compounding perspective.
Lack of model validation and incomplete model description: Several global models are used to demonstrate probabilities and trends in simultaneous cross-compartment droughts; however, no information is provided on the ability of these models in combinations with the different input datasets to actually simulate droughts. Moreover, very little information is provided about model structure and processes that they are able (or not able to simulate). Considerable uncertainty across models reported in this study, together with the reports on the poor functional performance of global models (Gnann et al., 2024 https://doi.org/10.1038/s44221-023-00160-y) indicates that reliability of these models has to be critically examined. Moreover, it is not clear to what extent human activities are considered in each of the models. All these information is necessary to judge the reliability of presented results.
Drought event definition: I think standardized drought indices can be a very useful tool for detecting drought anomalies. However, I am skeptical that drought events across different compartments can be effectively identified using the same aggregation period for precipitation, soil moisture and streamflow. The latter have clearly different temporal structure from precipitation and associated with much higher persistency. This point has to be clarified and the optimal aggregation period for each compartment has to be critically evaluated. Moreover, the implemented method seems to be suboptimal for dry regions with long consequent periods of no flow as the authors report themselves (Page 16 Line 8-9).
Inconsistent ENSO analysis: The analysis of ENSO suddenly appears in the Methods, only very roughly presented in the results and is neither discussed nor introduced properly. It is not clear why it was selected as a single climatic driver to be examined and it is not clear what was the rationale for this analysis. Consider either extending the scope to possible large-scale climatic drivers of drought co-occurrence across compartments or remove this part from the manuscript for clarity.
Conclusions: The conclusion section is very vague and very general. It barely touches on the actual results obtained in this study. The implications mentioned there are also very general and could have been resulted from almost any drought study again indicating that the novelty of the presented study is not quite clear. Please revise.
Detailed comments
Page 1 Line 29-31: From this sentence it sounds as precipitation deficit propagates into precipitation drought. Please revise.
Page 2 Line 5-10: At this point the authors should clearly clarify why is it worth to consider simultaneously co-occurring droughts across different compartments as compounding events and what is an added value of using such frameworks over drought propagation concept. I believe this can clarify the novelty of this study. Moreover, towards the Discussion section the authors converge back to discussing propagation rather than compoundness (e.g., Page 12 Line 9) making the added value of implementing compounding framework even more unclear.
Page 2 Line 18-20: It would be more instructive to summarize in this part what is known about co-occurrence of droughts across different compartments globally, instead of merely stating what was done without reflecting on their findings. Please revise.
Page 2 Line 25-40: I definitely agree that it is very related to the analysis of drought types. This also makes the novelty of this study rather unclear (see also my major comment). Introduction should identify existing gap more clearly and show how the analysis performed in this study aims to close it.
Page 3 Line 5-7: From this statement it is not clear what additional insights the global analysis will bring compared to the array of the regional studies listed above. Please clarify. See also my comment above on the need of clarifying the existing gap more clearly to highlight the novelty of this study.
Page 3 Line 7: It would be useful for the reader to already highlight in the Introduction that this is a global modeling study and reflect on the current accuracy of these tools and possible challenges associated with using them for global drought analysis.
Page 3 Line 21: This comes rather surprising. From the Introduction it was not clear that the connection to large-scale atmospheric drivers is in the scope of this manuscript. Without it the choice of this particular driver is not justified, i.e., it is not clear why only this one was used and no other alternative was considered.
Figure 1: Portions of panel B and C conserving events are identical. Please correct or clarify how these two analyses differ. Please also explain all acronyms in the caption.
Page 5 Line 1-12: The description of the models and the input is rather poor (see major comment). Moreover, it is not clear whether and how the models were calibrated/validated.
Page 5 Line 11-12: What about all other glaciers abundantly present in the European Alps, High Asia Mountains and in the mountainous ranges in North and South Americas? How is the glacier problem treated there? Please clarify.
Page 5 Line 22-27: Please clarify why two different periods had to be considered here.
Page 5 Line 23: Is the same accumulation period justifiable given the known difference in temporal persistence of precipitation, soil moisture and streamflow? Please provide rationale for your choice of particularly 3 moths as well.
Page 6 Line 1-2: Please provide reference for this statement.
Page 6 Line 5-10: Moreover, given this description it does not seem to me that these are events, these seem to be anomalies (see also my major comment).
Page 6 Line 13-15: References are needed to motivate the choice.
Page 6 Line 26: Run theory is mentioned on several occasions in the text, but is actually never properly introduced. Please add.
Page 7 Line 5-10: This is a very confusing description. Please clarify how the start and the end was selected and which compartment was chosen to define the overlapping periods.
Page 7 Line 20-25: This description is unclear. How droughts and phases was matched? Please clarify.
Page 8 Line 2: Why the word events is in brackets? Because it is actually not an event, but rather an anomaly?
Page 8 Line 5-10: It is not clear which count is meant here, seems that the definition differs, please revise.
Section 3: The explanation of the observed geographical patterns is missing here. Please provide it here or in the discussion section.
Page 9 Line 14: Are these patterns agree with observations? See Seo et al., 2025 doi: 10.1126/science.adq6529
Figure 3: Please explain all abbreviations.
Page 11 Line 11-15: These findings seem to be a direct result of the definitions used in this study. Please clarify.
Page 11 Line 25: Isn’t it very much expected that there are more precipitation droughts? I think it is well established that large portion of droughts never propagates (e.g., Brunner and Chartier-Rescan, 2025 https://doi.org/10.1029/2023GL107918).
Page 12 Line 11-12: I miss a discussion here on why this is observed in these regions.
Figure 4: Model uncertainty and regional variability has to be displayed in the aggregated bars of the panel G. Please revise
Page 14 Line 19: Please revise the sub-title, it is not the most eloquent choice.
Page 15 Line 24-29: I find this discussion not very helpful. Merely stating which threshold was chose in a different study is not very helpful for putting the results of this study into perspective. Please clarify here how the choice of the threshold in this study might have affected the outcomes.
Page 16 Line 8-9: If this is the known problem, why this method was not applied in this study? I would assume that intermittent flows is quite a common challenge across all arid regions. Are any of the observed trends possible artifacts of this limitation?
Page 16 Line 20-25: Which calibration is meant here? No calibration was mentioned in the Methods section. Please clarify and revise the Method section if needed.
Page 16 Line 25-30: Does it mean that the observed trends might be the artefacts of the dataset used? Please clarify.
Page 18 Line 24-26: I do agree that this is out of the scope, but reporting the results on drought trends without providing any statement on the reliability of the models for drought simulation is not very helpful (see my major comment).
Page 19 Line 2-3: I am not quite convinced that this is true. At least in Europe and Northen America soil moisture-based drought monitors are used for governance. Please reflect and revise.
Editorial comments
Page 1 Line 30 and elsewhere: please double-check the punctuations. On several occasions the commas are missing in the manuscript.
Page 5 Line 3: Why three? This is not yet clear at this point. Please revise the sentence.
Page 5 Line 11: Please use consistent acronyms in the text (ICE vs Ice)
Page 5 Line 13: This sentence makes no sense, please revise
Page 11 Line 20: Something is missing in this sentence.
Citation: https://doi.org/10.5194/egusphere-2025-3909-RC2 -
RC3: 'Comment on egusphere-2025-3909', Yannis Markonis, 07 Nov 2025
reply
Thank you for the opportunity to read this work. The manuscript addresses an important and timely question on how different drought types interact within the hydro-climate system, and it is clear that substantial thought and technical care have gone into the analysis and experimental design. My comments below are intended to help sharpen the framing, align terminology with the broader compound-events literature, and better showcase the strengths that are already present.
1. Terminology and definition
This is a valuable and timely analysis, and I appreciate that you already acknowledge neighboring concepts (propagation, two-type co-occurrence, multi-hazard compounding in Page 2, Lines 14–16). One place where a small editorial change could greatly improve readability is the term “compound drought.” In the extremes literature, “compound events” usually means multi-hazard combinations (e.g., heat and drought), whereas this paper focuses on simultaneous occurrence of multiple drought types within the hydro-climate system.
My suggestion to help readers land where you intend would be to consider adopting “concurrent multi-type drought” throughout (as you did in your previous work in Ahopelto et al., 2020). This small shift will better signal that the contribution is about the facets of drought rather than cross-hazard interactions, without changing your analysis or findings. It also helps to clarify what is being measured (so your strengths are obvious). As implemented, your core metric reads as the fraction of drought-affected days on which all three types co-occur at the grid-day scale, and “durations” are runs of such concurrent days nested within broader droughts, not separate “tri-type events.” Making this explicit will help readers interpret your trends correctly. For example, increases in concurrent days can reflect tighter synchronization (timing) even when overall drought frequency is flat or drops, while longer but less synchronized droughts could lower the concurrent share. Calling this out will showcase a key insight of your study.
If you prefer to retain the term “compound drought,” please consider these suggestions to improve the manuscript’s clarity:
- You could use a phase descriptor, e.g., “compound-drought phase/period”. In this manner you can help distinguish it from a parent drought event.
- You could add one-sentence definition (early in Introduction/Methods), similar to:
“We define a compound (concurrent multi-type) drought as the simultaneous co-occurrence of meteorological, agricultural (soil-moisture), and hydrological (streamflow) drought at the grid-day scale, evaluated with strict concurrence.” - You could add 2–3 sentences that clearly separate concurrent drought from drought propagation and from multi-hazard compound events, and most importantly explain why the concurrent framing is scientifically/operationally useful here (e.g., it pinpoints periods when multiple sectors face simultaneous stress).
- You should cite Wu (2022) when introducing/defining “compound drought,” and acknowledge the broader compound-events canon (e.g., Zscheischler et al., 2020) to demonstrate terminological awareness and to contrast with multi-hazard usage.
- You could add a small schematic illustrating a long meteorological drought with nested soil-moisture and streamflow deficits, highlighting the tri-type overlap segment analyzed (for example Fig.2 in AghaKouchak et al., 2023[1]).
These changes will sharpen the manuscript’s scope and ensure readers interpret your statistics and trends in the intended, operationally meaningful way.
2. Uncertainty in Methods/Results; ENSO out-of-scope
At present, uncertainty appears mainly in the Discussion, while the Methods and Results do not articulate a formal uncertainty framework. In contrast, ENSO receives dedicated methodological space that feels tangential to the central objective of characterizing concurrent multi-type drought climatology and trends. I recommend giving uncertainty a more visible and structured role and, if needed, streamlining or relocating the ENSO analysis.
In the Methods, you could introduce a short subsection titled “Ensemble and uncertainty framework” that explains how you use the multi-model ensemble, multiple forcings, or other choices to assess robustness. This may include, for example, how model spread is summarized, how sensitive results are to thresholds or definitions, and how agreement or disagreement across GHMs and forcings is interpreted. In the Results, it would be beneficial if each main finding is accompanied directly by an indication of uncertainty (for instance, ranges, confidence in sign of trend, or agreement levels), rather than having uncertainty emerge for the first time in the Discussion. This will strengthen readers’ trust in your conclusions and make the Discussion more focused and efficient.
Regarding ENSO, the stratification is interesting, but it is not essential for your core message on concurrent multi-type drought behavior and may distract from it or occupy space better used to formalize uncertainty. A reasonable solution would be either to remove the ENSO-focused analysis from the main text or move it to the Supplementary Material with a brief pointer in the main manuscript. This keeps the narrative tight while preserving the analysis for interested readers.
3. Discussion of Results
Your Results already contain rich and informative patterns; the Discussion will be more impactful if it stays tightly anchored to those findings and avoids opening threads that extend beyond what the analyses can support (Section 4.3 in particular currently feels somewhat beyond the demonstrated evidence).
A clear, evidence-led structure for each subsection of the Discussion could be as follows: first, restate a specific empirical result with explicit reference to the relevant figure or table; second, interpret this result mechanistically, drawing on known processes; third, connect it to existing literature that has ideally already been introduced in the Introduction, noting where your findings confirm, nuance, or challenge previous work; fourth, recognize key limitations or assumptions that affect confidence in this interpretation; and finally, where appropriate, outline practical or conceptual implications. Adopting this pattern will naturally transform the information from the Results into knowledge claims grounded in both data and process understanding.
Several questions arise directly from your reported patterns and could help structure the Discussion without expanding its scope. For example, why do concurrent multi-type drought days and durations increase in equatorial and southern hydrobelts while the boreal belt shows little change or a tendency toward wetting? Why do some regions show pronounced increases in concurrent days without commensurate increases in overall drought frequency, and to what extent might tighter synchronization among precipitation, soil moisture, and streamflow explain this? Why do trend magnitudes and even signs differ across models and forcings, and what does that say about structural or forcing-related uncertainties? Why are changes in duration sometimes stronger than changes in counts; does hydrological memory or catchment storage act to prolong concurrent phases once initiated? Why do coastal or monsoon-dominated regions behave differently from nearby continental interiors? And why do snow- or permafrost-affected regions deviate from global tendencies; is this due to genuinely distinct processes or to limitations in representing cryospheric processes and seasonality in the models? You do not need to address every one of these in depth, but framing selected subsections around such targeted questions will keep the Discussion tightly linked to figures, mechanisms, and literature, and ensure that interpretation remains firmly evidence-based.
In the same spirit, there are a few specific features in the figures that would benefit from direct comment. The step-like jumps visible in the Figure 3A time series (for example in NDR) should be discussed: please clarify whether they reflect genuine hydro-climatic regime shifts, arise from methodological or processing decisions, or could point to data inconsistencies. Applying or citing a suitable change-point or non-stationarity assessment would help to evaluate whether these shifts may bias trend estimates. Similarly, the apparent non-stationarities in Figure 5C and the striking similarity in statistical properties between H08 and WaterGAP warrant a short explanation. It would help readers if you could distinguish what aspects of cross-model similarity are likely linked to shared structural or parameterization choices and what aspects may be inherited from harmonized forcings or experimental setup. A concise clarification here will prevent over-interpretation of agreement as independent confirmation.
Finally, consider revisiting subsection 4.3 with a critical eye on whether every element is directly supported by your analyses. Content that is more speculative or forward-looking could be condensed into a final “Outlook” paragraph that clearly signals its role as perspective, leaving the core of the Discussion focused on results-backed insights.
4. Minor comments:
Page 2, Line 18: I could not locate the reference “Kallio, M., Heino, M., Kinnunen, P., Fallon, A., and Ahopelto, L.: Identifying Global Co-occurrence of Hydrological, Meteorological and Agricultural Droughts, 2019.” Please verify the citation details or provide additional information.
Page 5, Line 26: Please clarify whether “forcing” is used in the sense of “transforming” or whether you refer to external climate forcings; the current phrasing is ambiguous.
Page 6, Line 27: The phrase “probability of occurrence” suggests an annual probability that a compound drought will occur in a given year. Since your metric appears to describe the share of time steps with concurrence, a term such as “fraction of concurrency” or similar would likely be more transparent.
Page 10, Line 1: Figure 3A is difficult to read in its current composite form. You may wish to facet by hydrobelt with independent vertical axes so that regions with low variance are no longer compressed by those with higher variance. If time series by forcing and GHM remain crowded, consider separating them into distinct figures (for example, one by forcings and one by GHMs).
Page 10, Lines 2–9: Please add the full names of the hydrobelt regions in the caption here and similarly for Figures 4 and 5.
Page 15, Lines 8–9: The statement about “climate change” influencing increases in droughts would benefit from specifying which aspects (for example, shifts in precipitation regimes, increased evaporative demand) and briefly indicating the mechanism.
Page 16, Lines 5–7: When referring to dataset uncertainties, please make explicit that uncertainties differ across regions and environments and briefly indicate how this might affect interpretation [2].
Page 16, Lines 14–19: This section invites the question “why are we seeing this?” Adding one or two sentences that link the pattern to plausible processes or existing studies would strengthen it.
Page 16, Line 21: Please specify what kind of problems ERA5 is experiencing in this context (for instance, biases in precipitation, issues in particular regions or seasons).
Page 17, Line 1: It is not entirely clear from the legend whether Forcing Data are included in Figure 5B. Please clarify the labeling so that readers can distinguish all elements.
Page 18, Line 17: Clarify what you mean by “non-stationary” (for example, changes in mean, variance, or dependence over time), so readers understand which property of the series is being discussed.
Page 19, Lines 1–15: Please reconsider whether subsection 4.3 is fully aligned with the main findings and scope. If it is only loosely supported by your results, it may be more effective to shorten or reposition it as part of a brief Outlook rather than a core Discussion element.
References[1] AghaKouchak, A., Huning, L. S., Sadegh, M., Qin, Y., Markonis, Y., Vahedifard, F., ... & Kreibich, H. (2023). Toward impact-based monitoring of drought and its cascading hazards. Nature Reviews Earth & Environment, 4(8), 582-595.
[2] Markonis, Y., Vargas Godoy, M. R., Pradhan, R. K., Pratap, S., Thomson, J. R., Hanel, M., ... & Papalexiou, S. M. (2024). Spatial partitioning of terrestrial precipitation reveals varying dataset agreement across different environments. Communications Earth & Environment, 5(1), 217.Citation: https://doi.org/10.5194/egusphere-2025-3909-RC3
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First, I would like to question the authors about their motivation for conducting a study on droughts at the global scale. Studies of this nature commonly require methodological simplifications to make their execution feasible, and the present manuscript is no exception. Thus, what is the real advantage of sacrificing a higher level of detail and contextualization of a specific region in exchange for a global study, whose analyses are, by nature, more limited? Who, in fact, benefits from simplified and limited information or analyses about drought events? This is especially relevant considering that droughts are managed locally or regionally, not globally.
Additionally, the representativeness of grouping the planet into only nine regions is questionable. Beyond the presumed meteorological similarities, do these regions truly share comparable drought impact patterns? For example, the “SST” region encompasses both the Brazilian semiarid — with an average annual rainfall of approximately 600 mm, potential evapotranspiration close to 2000 mm, and intermittent rivers, making it highly susceptible to droughts — and the country’s South and Southeast regions, characterized by a markedly humid climate that includes some of the continent’s largest rivers, where droughts are not a recurring problem. In other words, these two regions do not share similarities in terms of drought occurrence patterns and drought impacts, so they should not be analyzed in the same group. Therefore, I suggest that the authors consider grouping regions not only based on hydrometeorological similarities but also on drought impact and occurrence patterns.
Given the profound environmental changes caused by human activities in all parts of the world, does it still make sense to consider droughts solely as a purely natural hazard, completely disregarding the anthropogenic influence on their onset and propagation? I suggest that the authors address this discussion or clearly state the rationale for considering it more useful to consider droughts exclusively as natural hazards.
Another relevant limitation concerns the temporal scale adopted for the calculation of drought indices. The study employs an empirical distribution methodology on a daily scale, later aggregated into three-month periods. What is the justification and advantage of performing a daily analysis for a type of disaster whose natural component is traditionally assessed using standardized indices at monthly scales (e.g. 3, 6, or 12 months)? What is the potential impact of this choice on identifying drought events in regions with highly seasonal rainfall regimes? I recommend converting all variables to a monthly basis and computing standardized indices using a 12-month scale, as this approach may be more appropriate for a global-scale study. Each time step would always consider one full year, which would always capture the rainy season and avoid the potential effect of the seasonality of the precipitation regime.
It is also important to discuss the effectiveness of the method adopted for identifying drought events. I suggest that the authors include a validation step based on historical drought records, in order to demonstrate the capability of the proposed methodology to adequately detect different types of drought events. The discussions and conclusions about the occurrence of compound droughts are only valid if it is shown, based on observed data, that the proposed approach can effectively identify the different drought types. Based on these observational data, more relevant regions of interest could then be defined for detailed analysis, replacing the nine generic regions currently considered.
The authors employed a combination of global models with a spatial resolution of 0.5°, using reanalysis data as input. In this context, one may ask: what is the accuracy and performance of this model? How was it calibrated, considering the distinct regional characteristics across the globe? The authors discuss model and index-related uncertainties, but it remains unclear how these could be improved and how the results should be evaluated in light of such uncertainties.