An optimization and simulation framework for water-energy-environmental nexus under uncertainties: A case study in the city cluster along the middle reach of the Yangtze River, China

Water, energy and the environment are inextricably linked. Complex and volatile global situations pose serious challenges to long-term stable and sustainable social development, resulting in many uncertainties in maintaining regular water and energy supplies and avoiding environmental degradation. Based on the nexus theory, it has become an urgent global and regional need for policy makers and scientists to consider water, energy, and the environment nexus (WEEN) as a complex system in order to deal with the water and energy issues brought about by rapid urbanization, the synergistic response of the environment under climate change, and the corresponding potential risks. Aiming at these problems, a simulation and optimization framework for WEEN complex systems is proposed by combining the system dynamics model, Gaussian white noise, and NSGA-II method. A system dynamics model integrated with Gaussian white noise is used to characterize the feedback relationships among the elements within the different subsystems of water resources, energy, and environment under uncertainty. Taking the city cluster in the middle reaches of the Yangtze River (CCMRYR) in China as the research object, the evolution of the WEEN complex system is simulated under different uncertainty conditions such as climate change conditions and policy backgrounds. In addition, an optimization method based on NSGA-II algorithm is constructed for solving the optimal development strategy of WEEN complex system. The results show that: Following the current development path, by increasing the proportion of energy conservation and environmental protection expenditures by 0.033%, as well as adjusting the ratio of the primary, secondary, and tertiary industries from 5.8:29.0:59.1 to 6.9:28.3:59.7, it is possible for CCMRYR to achieve a reduction of 1.31 billion tons of total water consumption, a reduction of 12.17 million tce of total energy consumption, and a decrease of 0.13×10⁶ of total pollution equivalent in 2035.