Rising temperatures may increase flood risk through river ‘whiplash’, study finds

Rising temperatures may trigger a dangerous increase in “Hydroclimatic Whiplash” in rivers that would make traditional approaches to flood and drought planning insufficient, a study has found.As temperatures rise owing to the worsening Climate Crisis, rivers will experience increasingly rapid transitions between heavy downpours and long dry spells – called Hydroclimatic Whiplash events – because a warmer atmosphere holds more moisture, intensifying rainfall extremes.Sudden swings from dry to wet conditions may increase the risk of flash flooding, the study found, because intense rainfall on dry, hardened soil is less able to infiltrate the ground. Instead, water can rapidly run off the surface leading to local flooding and water quality deterioration, as well as soil erosion because intensive rainfall can flush pollutants into the rivers.In comparison, wet-to-dry shifts can make drought planning harder because preceding wet conditions may create a false sense of security before a rapid move into drought.The bed of the River Wharfe near Kettlewell in the Yorkshire Dales dried up entirely during a heatwave in 2018. Photograph: Serenity Images23/ShutterstockIn the study, published on Wednesday in Earth’s Future, researchers used climate projections and a hydrological model to simulate changes to 698 river catchments in the UK under 2C and 4C warming scenarios. Hydroclimatic Whiplash was defined in the study as being when monthly riverflow moves from unusually low to unusually high flows, or the reverse.The lead author, Dr Yi He from the University of East Anglia, said the UK is already experiencing rapid dry-to-wet and wet-to-dry shifts, making Hydroclimatic Whiplash a significant concern.The modelling, which provides the most comprehensive national-scale assessments to date of how UK rivers may respond to different levels of global heating, found that in the 2C and 4C warming scenarios, widespread increases in the frequency of both types of whiplash events – wet to dry and dry to wet – are expected.The researchers found that in some catchments, the number of whiplash events could rise from about four over a 30-year period in the 1981-2010 baseline to up to nine under the 4C warming scenario.This increase is projected across most of the UK, however, for dry-to-wet whiplash, the greatest increases are likely to occur in South Wales, Northern Ireland, northern and western England and parts of south-east England.Dr He said these rapid shifts will make water management increasingly difficult by putting pressure on flood defences and drought-response systems at the same time. “As warming increases, traditional approaches to flood and drought planning may no longer be enough,” she said. “We need to plan for sequences of extremes, not just a single event.”The study authors said the findings underscored the need for regionally tailored adaptation plans, including enhanced flood-risk management and greater capacity to store water during wetter periods.Dr He said the modelling in the UK was an important test case for temperate regions worldwide, with the findings offering insights into how rising temperatures could alter river flows, flood and drought risks across the globe.