Forty-year data analysis of droughts and drought-flood dynamics: impacts of cascading reservoirs

• Meteorological and hydrological droughts distributed inconsistently in the Yangtze River Basin • Lag from meteorological to hydrological droughts increased from upper to downstream. • Cascading reservoirs exacerbated hydrological drought to a certain extent. • Cascading reservoirs aggravated the alternation from floods to droughts. Significant variations of hydroclimatic extreme events, induced by both climate conditions and large-scale water resource management, especially cascading reservoirs, have been shown to profoundly impact the comprehensive management of complex large river basins. However, existing studies primarily rely on independent parameterizations of individual reservoirs or detailed extreme event monitoring data, ignoring the complex interdependencies between upstream and downstream reservoirs, which limits their ability to accurately understand the cumulative impacts of cascading reservoirs on the propagation between meteorological drought and hydrological drought, as well as drought-flood dynamics. In this study, we develop an inflow-driven cascading reservoir release framework embedded within the Soil and Water Assessment Tool+ (SWAT+). Our methodological contribution lies in dynamically simulating cascading reservoirs coordination through seasonally adaptive release rules that integrate inflow conditions, storage thresholds, and regional management objectives—thereby overcoming the limitations of static or independent parameterized models. The framework enables a basin-scale assessment of cascading reservoirs impacts on drought propagation and abrupt drought–flood alternations under two scenarios—with and without coordinated reservoir operations—in the Yangtze River Basin (YRB) during 1980–2020. Results indicate that: (1) Meteorological and hydrological droughts show inconsistent spatio-temporal patterns. (2) The lag between meteorological and hydrological droughts increased from 22 days in the upstream section to 54 days in the downstream section. (3) Cascading reservoirs aggravated the duration and frequency of hydrological droughts by about 20 %, while decreasing their intensity by about 4 %. (4) Cascading reservoirs alleviated 75 % of drought-to-flood events in the downstream section but exacerbated 21 % of flood-to-drought events in the upper and middle sections of the YRB. These findings highlight the systemic influence of cascading reservoirs on hydroclimatic extremes and demonstrate the value of embedding operational realism into large-scale hydrological models. Our framework provides a transferable tool for simulating effects of cascading reservoirs in other large basins, supporting integrated and adaptive reservoir management strategies under nonstationary hydroclimatic regimes.