Accelerated Hydrological Wet-to-Dry Transitions and Their Driving Mechanisms over Africa

Gurara, Megersa Adugna; Zhao, Gang; Liu, Xingcai; Xu, Ximeng; Gaffney, Paul P. J.; Tang, Qiuhong

doi:10.5194/egusphere-2026-1969

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https://doi.org/10.5194/egusphere-2026-1969

Preprints

27 Apr 2026

| 27 Apr 2026

Accelerated Hydrological Wet-to-Dry Transitions and Their Driving Mechanisms over Africa

Megersa Adugna Gurara, Gang Zhao, Xingcai Liu, Ximeng Xu, Paul P. J. Gaffney, and Qiuhong Tang

Abstract. Abrupt wet-to-dry (W2D) events represent a damaging natural disaster, exerting more far-reaching impacts on the environment and society than single extreme events. While sub-seasonal and interannual precipitation whiplash have recently been analyzed, hydrological W2D transitions, especially at smaller time scales such as sub-monthly, have yet to be examined. In this study, we quantify changes in the characteristics of hydrological W2D transitions based on soil moisture in Africa and identify the drivers behind these changes. The results show that the total W2D transition has accelerated markedly: transition speed increased by 19 % and duration shortened by 10 % from 1981 to 2024. The area averaged proportion of rapid W2D events to total W2D events has increased from 52 % during 1981–2000 to 58 % during 2001–2024. The spatial extent of rapid W2D transitions has increased significantly. On average, 13 % of the continent has experienced rapid W2D transition in the 1980s, increasing to 17 % after 2010. These findings suggest a general shift from slow to rapid hydrological W2D transitions on a sub-monthly timescale. We further find that the speeding up of W2D transition onset is driven by greater precipitation deficits, higher temperature, and higher evaporative demand during the transition onset period. Overall, the shift from slow to rapid W2D transitions reduces the predictability of hydrological volatility regimes, which has adverse impacts on agriculture, ecological stability, and water resources management.

Received: 08 Apr 2026 – Discussion started: 27 Apr 2026

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Megersa Adugna Gurara, Gang Zhao, Xingcai Liu, Ximeng Xu, Paul P. J. Gaffney, and Qiuhong Tang

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-1969', Shuo Wang, 24 Jun 2026
This manuscript investigates the acceleration of wet‑to‑dry (W2D) events across Africa and explores the potential mechanisms driving this trend. Utilizing ERA5‑Land soil moisture data and CHIRPS precipitation data, the authors systematically examine the spatiotemporal patterns and trends of W2D events across the continent. They further compare precipitation, evaporative demand, and temperature between rapid and slow W2D transitions. This topic is of particular relevance given Africa’s socioeconomic vulnerability to hydroclimatic extremes. Nevertheless, prior to publication, the manuscript would benefit from refinements in its conceptual framework, a more in‑depth mechanistic analysis, and clearer presentation of the key findings.
Major comments:
In the Abstract (Lines 15–17) and Introduction (Lines 56–57), the authors identify sub-monthly W2D events as an important research gap. However, the manuscript does not sufficiently justify why the sub-monthly timescale is scientifically or practically important, nor does it clarify how this timescale differs from monthly or longer ones. Before presenting this research gap, the authors should provide a stronger rationale for focusing on sub-monthly W2D transitions. Figure C2c is relevant to this point, but its current classification is too coarse. Rather than using broad categories such as "<30 days" and "30–60 days," the authors should consider subdividing transition durations into finer classes (e.g., 10, 15, or 20 days), as the present grouping does not directly address the sub-monthly issue.

The title emphasizes the driving mechanisms of W2D events, and in the Introduction (Lines 64–70), the authors summarize previous studies on the possible roles of precipitation, temperature, and evaporative demand. However, the results presented in Lines 25–26 and Section 2.3.5 do not extend beyond these three variables. The mechanistic analysis largely consists of comparing mean values of precipitation, evaporative demand, and temperature between rapid and slow W2D transitions. This approach appears too simplistic to support a strong claim about driving mechanisms and does not fully respond to the importance of "both precipitation and terrestrial hydrological indices" noted in Lines 73–74. If the authors wish to retain and emphasize the mechanistic contribution, Sections 2.3.5 and 3.3 should be strengthened. Beyond mean differences, the analysis should incorporate land–atmosphere interactions, variability, and extreme values.

The manuscript adopts the flash drought definition from Yuan et al. (2023) to define sub-monthly W2D events. According to Figure C3 and the sensitivity analysis in Section 2.3.4, the dry threshold is fixed at the 20th percentile, while wet thresholds of the 80th, 85th, and 90th percentiles are tested. In comparison, Yuan et al. (2023) identified sub-seasonal drought events as pentad-mean soil moisture declines from above the 40th percentile to below the 20th percentile, followed by recovery above the 20th percentile. Under the present method, events starting from above the 80th percentile represent a subset of those defined by Yuan et al.; thus, consistent conclusions are expected. Nevertheless, the Introduction and Discussion do not clearly articulate how W2D events differ from flash droughts or why this distinction is particularly relevant for Africa. The authors should clarify the added value of their W2D definition relative to existing flash drought frameworks.

The authors should add a sensitivity analysis concerning transition duration (Lines 137–140). Specifically, the manuscript should explain how many days between wet and dry conditions can still be considered part of the same W2D event. If the transition exceeds 60 days, it remains unclear whether the event can still be regarded as meaningful or impactful.

The authors should incorporate additional soil moisture datasets, such as the GLEAM root-zone soil moisture product. The results in Lines 177–185 could be revised to compare ERA5-Land with GLEAM. More importantly, the manuscript should demonstrate whether the key characteristics of rapid W2D events, including their acceleration and behavior across different timescales, remain consistent across multiple datasets.

Minor comments:
Figure 1 should be revised. The aridity index is not substantially discussed in the main text; the authors should either integrate it more fully into the manuscript or remove it from the figure. Additionally, Lines 155–156 refer to "agricultural and ecological implications," but Figure 1 does not directly support this claim. The figure would be more useful if it displayed the spatial distribution of soil moisture, precipitation, and heavy rainfall, and if it included a schematic definition of rapid and slow W2D events.

The statement in Lines 195–200 is not directly quantified in Figure 2c. These important results should be reorganized and presented more clearly in Figure 2.

For the regions discussed in Lines 221–228, the authors could add regional boundaries or color-coded boxes in Figure 3 to help readers connect the text with the spatial patterns shown. The argument also requires stronger evidence; for example, statements regarding "high climate variability" and "brief and intense rainfall, and rapid soil moisture depletion" should be supported by either additional data analysis or appropriate references.

The definition of rapid and slow W2D events in Section 2.3.3 should be moved before Table 1. Otherwise, readers encounter Table 1 without having been introduced to these definitions, which may cause confusion.

In Line 245, the manuscript introduces a new variable, the "rapid-event ratio." The authors should define this term clearly upon its first appearance and use it consistently thereafter. In addition, several terms and units are not uniformly written; for example, the units in Line 158 are inconsistent with those in Lines 150–151.

The paragraph in Lines 250–263 should be reorganized to improve its logical flow. I suggest presenting the overall pattern first, then discussing seasonal differences, and finally explaining the possible drivers.

A proper citation for the CHIRPS dataset should be added in Line 106.
Citation: https://doi.org/10.5194/egusphere-2026-1969-RC1
RC2: 'Comment on egusphere-2026-1969', Jiabo Yin, 17 Jul 2026

This study investigated the spatiotemporal characteristics and drivers of hydrological wet-to-dry (W2D) transitions across Africa using root-zone soil moisture from ERA5-Land. The topic is timely and relevant, given the increasing concerns about hydroclimatic volatility under climate change. The sensitivity tests on threshold choices are appreciated, and the topic is within the scope of HESS. Overall, this manuscript addressed an important and interesting issue int terms of W2D in a data-sparse continent. Before considering acceptance, the following concerns should be well addressed.

(1) The study relies almost exclusively on ERA5-Land (and ERA5 for comparison) soil moisture to identify W2D transitions. While the authors cite previous studies that found these reanalyses acceptable over Africa, this does not substitute for a direct evaluation in the context of this specific application. Reanalysis soil moisture in data-sparse regions is subject to considerable uncertainties from model physics and forcing. The comparison between ERA5-Land and ERA5 merely shows inter-product consistency, not accuracy against observations or independent satellite retrievals. A lack of independent validation undermines the confidence in the reported magnitudes and trends. The authors should either provide a direct validation against in situ observations or independent remote sensing products, or, at a minimum, explicitly and thoroughly discuss how biases and uncertainties in reanalysis soil moisture could affect the identification of wet/dry states and transition characteristics (e.g., frequency, onset speed).

(2) The framework directly adopts Yuan et al. (2023) flash drought identification criteria, altering only the starting condition from a “dry-to-drier” to a “wet-to-dry” transition (i.e., from >75th to <25th percentile). While this is a straightforward extension, the manuscript lacks a clear physical justification for why this particular set of thresholds appropriately captures hydrological wet-to-dry transitions that are distinct from simple drought onset. The current definition essentially identifies a drought event that happened to start from a wet state. The authors should elaborate on how a W2D transition, as defined here, differs from a flash drought or a general drought event, beyond the initial soil moisture condition. What specific hazards or impacts justify this classification?

(3) Although a sensitivity test is presented (Appendix C), the rationale for choosing the default 75th, 25th, and 5%/pentad thresholds is insufficiently motivated. In the main text, the authors should clarify why this configuration is considered the most appropriate for African ecosystems and agricultural contexts.

(4) The attribution of changes in W2D transitionsis limited to a comparison of composite anomalies of precipitation, PET, and temperature between rapid and slow onset periods. This analysis is purely diagnostic and largely correlational. The manuscript claims that “greater precipitation deficits, higher temperature, and higher evaporative demand” are the drivers, but fails to demonstrate a causal link, nor does it account for confounding factors or land-atmosphere feedbacks that are mentioned in the introduction. Statements such as “the main driver for the occurrence of rapid W2D transition events varies spatially” are not convincingly supported by simply mapping anomaly differences. More detailed discussions should be provided.

(5) No significance testing is shown for the spatial anomaly difference maps in Figure 5. At a minimum, stippling or hatching should indicate where the differences between rapid and slow composites are statistically significant.

(6) The time series in Figure 2c shows considerable interannual variability. The smoothed trends appear to be influenced heavily by the endpoints. The robustness of the trends should be tested, for example, by varying the start and end years, and the results should be discussed with more nuance.

(7) The “predictability” statement in the abstract and elsewhere (“shift from slow to rapid W2D transitions reduces the predictability of hydrological volatility regimes”) is an unsubstantiated leap. Reducing duration does not automatically imply reduced predictability; a formal predictability analysis is needed to make this claim.

(8) The Discussion section largely repeats the results and compares them with previous studies on precipitation whiplash. It lacks a critical self-assessment of the study’s limitations. There is no discussion of how the use of pentad data may smooth out higher-frequency variability relevant to “sub-monthly” transitions. The exclusion of hyper-arid regions is sensible, but the sensitivity of results around the margins of these masked areas is not discussed.
(9) The term “sub-monthly” is used to describe the transitions throughout the manuscript. However, the identified rapid W2D events have an area-averaged mean duration of 32.2 days, which is just over a month. The authors should reconcile this description with the reported duration, perhaps by clarifying that the onset phase is sub-monthly while the total event extends longer.
(10) Some sentences should be polished. For example “Anthropogenic activities have naturally complicated…” The word “naturally” is confusing here.

Citation: https://doi.org/10.5194/egusphere-2026-1969-RC2

Megersa Adugna Gurara, Gang Zhao, Xingcai Liu, Ximeng Xu, Paul P. J. Gaffney, and Qiuhong Tang

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Short summary

Climate change accelerates hydroclimate swings, which significantly impact water supply, agriculture, and ecosystems. We evaluate the spatiotemporal characteristics of hydrological wet-to-dry transitions based on root-zone soil moisture from 1981 to 2024. We then classify these transitions into rapid and slow events based on the onset speed. Wet-to-dry transitions become increasingly rapid and frequent as their duration decreases to a sub-monthly timescale, demanding better planning.


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