Combined effects of climatic change and hydrological conditions on thermal regimes in a deep channel-type reservoir.

The thermal regime in large reservoirs plays a significant role in the water quality and ecosystem succession; however, little is known about the impacts of regional climate changes and hydrological conditions on a sizeable stratified reservoir with strong inflow conditions, i.e., the Xiangjiaba Reservoir. Using measured data from 2014 to 2018, the monthly and seasonal variations of the water temperature, thermal stability, and their influencing factors were addressed by using empirical models. The results showed substantial variability and seasonality in the reservoir water temperature, which correlated highly with the air temperature, inflow water temperature, and discharge. Correspondingly, there was a seasonal varying thermal stratification in the reservoir's yearly cycle, with its duration being up to 4 ~ 5 months, the maximum surface-bottom water temperature difference being up to 7 ~ 10 °C. There were significant positive correlations between Schmidt's stability index of the thermal structure and inflow-reservoir temperature difference and the surface-bottom temperature differences, while negative correlations with large discharge. Moreover, the inflow tends to influence thermal stability by retaining hypolimnion cold water, with its maximum bottom hysteresis residence time being up to ~ 4 months. Research findings indicated that climate warming in the recent 30 years (1988 ~ 2017) would cause a 0.213 °C/decade and 0.153 kJ/m/decade increase in reservoir surface water temperature and Schmidt's stability index, respectively. Among these variations, the inflow temperature increase caused by climate change accounted for the largest proportion, i.e., 0.16 °C/decade and 0.115 kJ/m/decade. Therefore, climate warming significantly affected the thermal regimes in this large reservoir, and the inflow water temperature increase due to warm air was the main factor altering the reservoir's thermal structure. Findings from the present study provide a fresh perspective on how to best optimize the deep channel-type reservoirs' water quality in the face of anticipated climate change.