Impact of intra-annual runoff uniformity and global warming on the thermal regime of a large reservoir.

Thermal stratification is common in reservoirs and greatly influences the aquatic environment. Changes in the uniformity of intra-annual runoff have been detected in several basins, but few studies have focused on the impacts that these changes have on thermal regimes. Using runoff data for Sanbanxi Reservoir, China, during 1950-2015, the long-term trends of intra-annual runoff uniformity were statistically analyzed and extrapolated for the 2050s and 2090s, and the relationship between these trends and the thermal regime of the reservoir were investigated. Moreover, the thermal regime was evaluated for future climate scenarios accounting for global warming. This study shows the following: 1) for South China, the concentration degree (Cd) for the distribution of intra-annual runoff in natural basins such as Sanbanxi Reservoir tended to be higher, but for rivers significantly impacted by human activities, Cd tended to be lower. 2) a higher Cd was associated with an increased reservoir temperature and released water temperature, and decreased thermal stability. For Sanbanxi Reservoir, a 10% increase in Cd corresponded to a change in annual average temperature, thermal stability, and released water temperature of 0.036 °C, -48.4 J m-2, and 0.153 °C, respectively. These changes were larger in summer than in other seasons; 3) global warming is predicted to increase reservoir temperature, released water temperature, and thermal stability, having a more significant influence on these parameters than intra-annual runoff uniformity; 4) future changes in thermal regimes will intensify oxygen stratification and hypolimnetic anoxia, promoting algal blooms, and delaying fish spawning. Effects of two methods aimed at controlling the thermal regime were also analyzed, including changing the operation level and intake elevation of the reservoir. This study investigated the response of the thermal regime of Sanbanxi Reservoir to climate change, and provides theoretical support for the management of water temperature and the reservoir's aquatic environment.