Historical and future transitions between opposing UK hydrological extremes

Armitage, Rachael; Magee, Eugene; Chevuturi, Amulya; Chan, Wilson; Hannaford, Jamie

doi:10.5194/egusphere-2026-3154

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https://doi.org/10.5194/egusphere-2026-3154

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11 Jun 2026

| 11 Jun 2026

Status: this preprint is open for discussion and under review for Hydrology and Earth System Sciences (HESS).

Historical and future transitions between opposing UK hydrological extremes

Rachael Armitage, Eugene Magee, Amulya Chevuturi, Wilson Chan, and Jamie Hannaford

Abstract. Transitions between droughts and floods can exacerbate the impacts of the individual events and present a complex challenge for water resource management: sudden or frequent transitions between dry and wet conditions can negatively impact water availability, water quality, agricultural productivity, and cause damage to water infrastructure. Despite these potentially severe impacts, such transitions have, until recently, received less attention in the international literature than their component extremes. In the UK, there has been no systematic assessment of the occurrence of transitions, despite growing interest given a series of recent swings between floods and droughts.

Given this gap, we assess present-day and future transitions using national river flow and precipitation projections from the enhanced future Flows and Groundwater (eFLaG) dataset for 1989-2079 over 200 UK catchments. We identify transition events as the period between consecutive yet opposite extremes at seasonal timescales, using a threshold method to demarcate extreme wet and dry events for both river flow and precipitation to understand the magnitude, duration and frequency of both hydrological and meteorological transitions.

Our results reveal the spatial distribution of transitions in the UK, with higher intensity transitions in the north-west and longest durations in the south-east. We compare hydrological and meteorological transitions and find similar spatial patterns between the two but a stronger seasonality and generally shorter durations for meteorological transitions. Most regions of the UK are projected to see an increase in transition magnitude, a decrease in duration and therefore more intense transitions in the future. The south-east sees the largest decreases in transition duration under future projections. The frequency of hydrological transitions is projected to increase in the north-west and in all regions for meteorological transitions. Our findings demonstrate the risk of increasing hydrological volatility across the UK, with implications for water resources management and climate adaptation.

Received: 01 Jun 2026 – Discussion started: 11 Jun 2026

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Rachael Armitage, Eugene Magee, Amulya Chevuturi, Wilson Chan, and Jamie Hannaford

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RC1:
'Comment on egusphere-2026-3154', Anonymous Referee #1, 25 Jun 2026 reply
This manuscript presents the first systematic, nationally-consistent assessment of transitions between hydrological (and meteorological) wet and dry extremes across the UK, using the eFLaG dataset spanning 1989–2079. The authors define transitions as the period between consecutive opposite-sign extremes identified via SSI/SPI threshold crossings at a monthly timescale, characterizing them by magnitude, duration, intensity, and frequency for both dry-wet and wet-dry directions. Historically, the authors find a clear northwest-southeast gradient, with the NW showing higher-intensity, shorter-duration, more frequent transitions and the SE showing longer, lower-intensity transitions (linked to groundwater-dominated catchments). Under future RCP8.5 projections, most regions show increasing transition magnitude and intensity and decreasing duration, with the largest duration decreases in the SE and the largest frequency increases in the NW. With a few relatively minor revisions, I think the manuscript will present the main results of the study in a way that is easier for the reader to digest.
Major comments:
The paper repeatedly contrasts itself with “drought-to-flood” transition studies (Götte and Brunner, 2024; the companion Yang, Villarini & Yang CONUS paper; Anderson et al., 2025) but the distinction between a “state-switch” (SPI/SSI threshold crossing at seasonal timescales) and a “true” DtF/FtD transition (event-based, sub-seasonal, 90-day window) could be made more explicit and earlier in the Introduction, not just in section 4.1. This distinction is now considerably sharper given the newly available companion results: Yang et al. (2025, in review for Earth's Future) report that across 2,000+ CONUS stations, drought-to-flood (DtF) events occur nearly three times more frequently than flood-to-drought (FtD) events, with DtF transitions occurring within ~41 days versus ~57 days for FtD, and with DtF intensity significantly higher than FtD (Mann-Whitney U, P<0.05). This is a striking asymmetry. By contrast, the present UK paper finds wet-dry and dry-wet transitions to be broadly symmetric in magnitude/duration/intensity changes (though differing in seasonality). Is this symmetry a property of the seasonal SSI/SPI approach (which by construction looks at smoothed, longer-duration anomalies, merging events separated by gaps) versus the asymmetry that emerges when looking at sub-seasonal/rapid threshold-crossing events with explicit drought/flood definitions tied to the 2-year-recurrence bankfull discharge and 7-day low-flow statistics? The authors should discuss this methodological dependency as a substantive finding, not just a caveat. It may be the single most important point of contrast between the UK seasonal-switch framework and the rapid-whiplash framework, and is directly relevant to readers trying to reconcile the two literatures.

The large overlapping ensemble spreads (Figures 8–9) are acknowledged, but the paper would benefit from a more quantitative treatment — e.g., what fraction of ensemble members agree on the sign of change for each metric/region? A simple “fraction of RCMs showing increase” map or table would help readers gauge robustness more than visual inspection of overlapping boxplots. As written, statements like “transition magnitude is projected to increase for most regions” are somewhat qualitative given the stated caveat that “internal variability… dominate the projected signal.”

I think the main paper would benefit from figures showing a comparison of hydrological transition metrics of SimRCM minus SimOBS so the reader can clearly see where the RCM best matches simulated observations, ad where the two diverge.

Another set of panels on existing figures could show the difference in median NF and FF transition metrics as compared to BL metrics. It takes a lot of work looking back and forth between the boxplots to currently see this. The boxplots are very helpful for looking at the range and variability of metrics by scenario, but simple panels showing difference in medians would draw out some of the key results of the paper.

The Discussion (line 386-387) flags that temperature/evaporative demand differences could explain hydrological vs. meteorological transition differences, and suggests SPEI as future work. Given that PET-driven changes are central to the future projections (warming climate, RCP8.5), this omission is now harder to justify as a minor limitation. The Swain et al. (2025, Nature Reviews Earth & Environment) review, which the authors already cite, explicitly builds its entire “hydroclimate whiplash” framework around SPEI rather than SPI precisely because “definitions incorporating evapotranspiration-related ('demand side') variables are increasingly favored,” and demonstrates that potential evapotranspiration substantially amplifies whiplash increases over continents relative to precipitation alone, particularly at high latitudes. Given that Swain et al. report interannual whiplash increases of 52% over land at 3°C warming using SPEI, and that the present paper's RCP8.5 scenario corresponds to a comparable warming trajectory, the omission of an SPEI-based comparison is a substantive gap rather than a future-work afterthought. At minimum, the Discussion should explicitly frame the SPI/SSI-only approach as likely underestimating future transition magnitude/frequency changes relative to what an SPEI-based analysis would show, citing Swain et al.'s finding that evapotranspiration-inclusive metrics are “not detected in precipitation-only metrics.”

Minor comments:
Line 32-37: The list of “pseudonyms” (hydrological whiplash, hydroclimate volatility, drought-to-flood events) is useful, but could also mention “compound drought-flood events” (Matanó et al.) and “drought termination” (Parry et al.) explicitly here since both are discussed later. Could help orient readers to the terminological landscape upfront.

Line 66-69: The assumption that “following a flood, sufficient water would remain stored to buffer the impacts of a subsequent drought” is asserted but not cited. Is there a reference for this being a stated assumption in the literature, or is this the authors' inference?

Line 150-156: The description of SimObs vs SimRCM and their respective roles is clear, but it might help to add a sentence on the magnitude of “inherent biases” mentioned (line 155-156) — is there a citable RMSE/bias statistic from Hannaford et al. (2023) that could be quoted to give readers a sense of scale?

Line 184-185: “we use the 3-month accumulation period (SPI-3 and SSI-3)” — please justify this choice more explicitly. A sensitivity check (or at least a citation) showing that SPI-1/SSI-1 or SPI-6/SSI-6 would yield qualitatively different transition counts/durations would strengthen the methodological robustness, especially given how central the “moderate/severe/extreme” thresholds and accumulation period are to all subsequent results.

Line 257-262: “A few catchments in the southeast show notably high duration and low intensity transitions… situated in the chalk aquifer north of London” — this is a nice mechanistic explanation. Could the authors quantify this more precisely, e.g., by overlaying baseflow index (BFI) or an aquifer productivity classification on Figure 5, or computing a correlation between BFI and transition duration across all 200 catchments? This would convert an anecdotal/visual observation into a more quantitative structural finding.

Line 422-423: “Infrastructure needs to be designed not only for more extreme or intense wet and dry events than currently specified, but also have flexibility built in to adapt for transitions” — this is a good high-level recommendation, but feels slightly generic; given the SE-specific finding about a shift from multi-annual to seasonal transitions (line 314-318), could the Discussion include a more targeted recommendation specific to chalk/groundwater-dominated catchment management (e.g., implications for conjunctive use schemes, aquifer recharge management)?

Line 488-492 (Data Availability): Good that eFLaG and SPI/SSI source data are referenced, but given the extensive transition-identification methodology developed here, would the authors consider releasing the derived transition event catalogue (onset/end dates, magnitude, duration, intensity per catchment/scenario) as a data product? This could substantially increase the paper's value for follow-on UK water-management research .

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Citation: https://doi.org/10.5194/egusphere-2026-3154-RC1

Rachael Armitage, Eugene Magee, Amulya Chevuturi, Wilson Chan, and Jamie Hannaford

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Rachael Armitage, Eugene Magee, Amulya Chevuturi, Wilson Chan, and Jamie Hannaford

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Short summary

Transitions between droughts and floods can exacerbate the impacts of the individual events and provide challenges for water resource management. Here, we use river flow and climate projections to assess transitions in the UK over historical and future periods. Our results reveal the characteristics of and spatial pattern in current transitions, and how these are projected to change in the future, with wider implications for water resource management.


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