Development of drought-flood abrupt alternations identification method based on daily soil moisture index: Spatiotemporal characterization and risk assessment in the middle and lower reaches of Yangtze River Basin

Study region The middle and lower reaches of the Yangtze River (MLR-YR) in China, across six provinces (Hubei, Hunan, Jiangxi, Anhui, Jiangsu, Zhejiang) and Shanghai Municipality. Study focus Drought-flood abrupt alternation (DFAA) events, characterized by coexisting drought and flood extremes, pose severe threats to water resources and food security, particularly impacting crops. Aiming to investigate the impacts of DFAA events in typical agricultural regions, this paper put forward a daily-scale Standardized Soil Moisture Index (SSMI) to monitor the spatio-temporal variations of agricultural DFAA events in the MLR-YR and assessed its comprehensive risk. New hydrological insights for the region The SSMI demonstrated superior accuracy in capturing flood to drought (FTD) events than drought to flood (DTF) events. DTF events concentrated in central-northern areas during June and July, while FTD dominated southern regions in May and August. Since 2000, DFAA hotspots have expanded southward. The northern regions of Anhui and Jiangsu provinces exhibited high-hazard characteristics, while Zhejiang Province, though not classified as high-hazard, demonstrated a progressive expansion of its hazardous zones. Vulnerability correlated strongly with river network density. The northern regions of Jiangsu and Anhui Provinces, and the Yangtze River Delta exhibited pronounced DFAA risks, necessitating priority monitoring and adaptive risk management frameworks for mitigating compound threats. The SSMI-based framework provides actionable strategies to optimize water allocation in monsoon-dependent agriculture.