Historical and future transitions between opposing UK hydrological extremes

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.