Risk analysis of water supply-hydropower generation-environment nexus in the cascade reservoir operation

Water supply, hydropower generation and environment are three main subsystems in water resources managements. Reservoir operation is an effective tool for water supply, hydropower generation and improvement of environment and ecology. During the cascade reservoirs operation, the subsystems are inextricably linked and interrelated, which can be conceptualized as a nexus. Complicated interrelationships in the nexus can lead to higher uncertainties in water resources system. Limited research has been carried out to study the water supply-hydropower generation-environment nexus in the reservoir operation processes. Research on water supply-hydropower generation-environment nexus has becoming a hot issue, which has been supported by National Natural Science Foundation of China. The purpose of this paper is to explore the interdependencies and further to evaluate the risk of water supply-hydropower generation-environment nexus of the upper Yangtze River Basin. The multi-objective optimal operation model considering the objectives of water supply, hydropower generation and environment was developed. The risk indicators concerning the nexus were identified. In order to overcome the shortcomings that the previous studies cannot consider the interrelationships in the nexus, copulas were used to quantitatively assess the multiple and conditional risk in the nexus. Furthermore, the conditional entropy was introduced for the first time to assess the systematic risk in the nexus. Case studies demonstrate that water supply upstream of reservoir has a weak correlation with the hydropower generation and river habitat downstream of the reservoir, and hydropower generation has a significant impact on river habitat downstream of reservoir. Higher hydropower generation increases the risk of river habitat downstream of reservoir in non-flood season. Conversely, higher hydropower generation reduces the risk of river habitat downstream of reservoir in flood season. The systemic risk of water supply-hydropower generation-environment system during the cascade reservoirs optimal operation process can maintain at a lower level in non-flood and flood seasons based on different combinations of guarantee rates of water supply, hydropower generation and environment subsystems.