A Phased and Graded Drought Limited Water Level Strategy for Mitigating Flood Drought Abrupt Alternation Events: A Case Study of the Three Gorges Reservoir

In recent decades, flood drought abrupt alternation (FDAA) events have intensified markedly in the middle and lower reaches of the Yangtze River Basin (MLYRB), exposing limitations of the conventional single flood-limited water level (FLWL) operation of the Three Gorges Reservoir. To better address drought risk during the flood season, this study develops a phased and graded drought-limited water level (DLWL) operation framework. FDAA events were identified using a hybrid method combining the Short-term Flood-Drought Abrupt Alternation Index and the Standardized Runoff Index. A multi-objective optimization model solved by NSGA-III was employed to determine staged DLWLs across five operational periods with tiered thresholds prioritizing urban, ecological, and irrigation water demands. Results show that FDAA events are mainly concentrated in June–October and have intensified significantly since 2010. Compared with conventional operation, the optimized DLWL framework substantially improves irrigation water supply reliability and reservoir fullness, while maintaining urban and ecological water supply security. Validation during typical wet years indicates that the proposed strategy introduces no evident reduction in flood control safety.