Spatio-temporal variability in the thermal regimes of the Danjiangkou reservoir and its downstream river due to the large water diversion project system in central China

Understanding water temperature variation in regulated rivers and reservoirs becomes increasingly important as the environment and ecosystem are approaching their thermal limits. In this paper, a multi-model approach is used to quantitatively access the spatio-temporal change in thermal structures of the Danjiangkou reservoir and its downstream river. The area is subject to thermal and hydrological alterations due to three large water diversion projects and related auxiliary projects, including a project to heighten the Danjiangkou dam and two small downstream reservoirs. It is found that the Danjiangkou dam heightening project alters water temperature seasonally, increasing it in winter and decreasing it in summer; while the three large water diversion projects and the two small downstream reservoirs mitigate the effect. Water temperature change in the downstream river is also studied from the aspects of release temperature and release discharge of the Danjiangkou reservoir. The former mainly changes the water temperature near the dam, while the latter affects the recovery rate and the recuperation distance. Ecological impact of the water temperature change is discussed based on the spawning of fish, indicating that the spawning periods may lag behind and the optimal spawning locations may move downstream.