A probabilistic modeling framework for assessing the impacts of large reservoirs on river thermal regimes - A case of the Yangtze River.

Hydrological and thermal river regimes have changed greatly due to the construction of reservoirs and dams. Water temperature changes have important significance for aquatic habitats and freshwater ecosystems. To investigate the impact of large reservoirs on the water temperature regime along the middle reach of the Yangtze River, we present a probabilistic modeling framework to ascertain the joint dependence structures of air-water temperature and discharge-water temperature between pre-reservoir and post-reservoir periods based on the copula theory. The results show that the principle of maximum entropy (POME) method can better estimate the marginal distributions of temperature regimes. Reservoir operation disturbed the dependence structures of air-water temperature, especially after the Three Gorges Reservoir (TGR) was put into operation. Different shifts in the occurrence probabilities of high or low water temperatures at the downstream and upstream stations under extreme air temperature and discharge are observed, indicating the great effects of reservoirs on the dependence structures of the downstream river flow and thermal regime. Relying on the developed model, we propose the appropriate ranges of air temperature and discharge to maintain a suitable water temperature for Chinese sturgeon (Acipenser sinensis) spawning activity. The results of this study demonstrate the influence of dams on the thermal regime and can be helpful for optimizing reservoir operations to enhance biological conservation in the Yangtze River.