Assessing the sensitivity of detection criteria for Drought-Flood Abrupt Alternation events in the UK

Abrupt transitions between drought and flood—known as Drought–Flood Abrupt Alternation (DFAA)—constitute an emerging class of compound hydrological extremes with significant implications for risk management. This study introduces a systematic framework for detecting DFAA events across 15 UKBN2 (UK Benchmark Network version 2) catchments (1970–2015) using daily SPEI (Standardized Precipitation–Evapotranspiration Index) and SRI (Standardized Runoff Index) time series. We evaluated 27 detection configurations, varying anomaly thresholds, minimum drought durations, and transition windows, to assess sensitivity and classification robustness.

Threshold selection exerted the greatest influence on event frequency, followed by transition-window length, while drought duration played a comparatively minor role. A baseline configuration (threshold ±0.7; drought ≥14 days; flood ≥1 day; transition ≤14 days) delivered the most hydrologically realistic and spatially coherent results. Under these criteria, SRI consistently identified more DFAA events than SPEI, reflecting rapid runoff responses driven by catchment storage and antecedent wetness. Spatial analysis revealed a pronounced west–east gradient, with higher alternation frequency in wetter, low-permeability upland catchments, while seasonal patterns indicated drought-to-flood transitions predominating during recovery from dry spells.

These findings underscore the critical role of index choice, storage dynamics, and transition timing in shaping DFAA behaviour. The proposed framework provides a reproducible basis for monitoring compound drought–flood risks and delivers essential evidence to support future modelling, operational early warning systems, and climate adaptation strategies.