Spatiotemporal evolution of short-cycle drought-flood abrupt alternation based on the optimally selected index in the middle and lower reaches of the Yangtze River

Study region: Middle and lower reaches of the Yangtze River (MLRYR, 106°54′E–124°25′E, 24°30′N–34°45′N), covering 800,000 km2. Study focus: Three short-cycle drought-flood abrupt alternation (DFAA) indices (SDFAI, SDWAI, R-SDFAI) are systematically evaluated through misjudgment/omission analysis, historical event validation, and precipitation difference comparison. Using the optimal index, DFAA spatiotemporal characteristics (1970–2019) are analyzed across 75 stations. New hydrological insights for the region: R-SDFAI demonstrates optimal performance (zero error rate versus 3.01% for SDFAI; r = 0.757 with precipitation differences), effectively capturing rapid moisture regime shifts. DFAA events concentrate during May-August monsoon transitions with weak increasing frequency trends. While frequency varies minimally spatially, intensity shows strong heterogeneity: extreme events cluster in the southwestern Dongting and northeastern Poyang basins. Northern regions exhibit decreasing intensity trends; southern regions increase. Western areas show trend persistence, while eastern areas indicate potential reversals. Hence, effective DFAA assessment requires indices sensitive to abrupt hydrological transitions, with spatially differentiated management strategies needed for MLRYR.