High synchronized drought risk in a multi-source water transfer project: a copula-EEMD assessment for the eastern route of the south-to-north water diversion project

The operational security of large-scale inter-basin water transfer projects relies on a clear understanding of spatiotemporal wet-dry encounters among source areas. This study analyzes precipitation data (1960-2018) for three major storage lakes—Hongze, Luoma, and Nansi—along the Eastern Route of China’s South-to-North Water Diversion Project (SNWDP-ER). An integrated framework was established by combining EEMD for multi-scale analysis with Copula methods for dependence modeling. The results reveal distinct spatiotemporal patterns. Hongze Lake exhibits strong monsoon-driven variability, whereas Nansi Lake shows persistent drought conditions. Synchronized drought events, defined as the simultaneous occurrence of drought conditions across multiple basins, are more likely than asynchronous events. The joint probability of concurrent drought across all three basins is 17.17%, representing a significant threat to water supply security. Uncertainty quantification indicates that data length and Copula selection are dominant factors, particularly for synchronous extreme events. State transition analysis further indicates persistent drought conditions. This study establishes a framework for multi-source wet-dry encounter analysis, thereby providing a basis for risk management and adaptive operational strategies for large-scale water diversion systems.