Characteristics and mechanism of water mixing structure during the extinction of thermal stratification in southern reservoirs

Deep reservoirs with stable stratification often ' turn over' in winter, leading to a decline in overall water quality and affecting the safety of drinking water. To explore the mixing characteristics of water bodies during the thermal stratification period in southern reservoirs, long-term in-situ monitoring of physical and chemical indices of Tianbao Reservoir in Nanning was conducted from November 2019 to February 2020 during the thermal stratification fading in winter. Combined with the vertical diffusion coeffi- cient of water, the variation characteristics of water temperature (T) and dissolved oxygen (DO) in the whole process of thermal stratification of ihe reservoir were described by stages (thermal stratification, weakening, complete mixing and formation), and ihe influences of meteorological factors (air temperature, solar radiation, wind speed, etc.) and hydrodynamic factors (vertical diffusion coefficient K2) on ihe physical and chemical indices of water were discussed. The results showed thai ihe temperature stratification and ihe dissolved oxygen stratification of the reservoirs were out of sync during the extinction of ihe thermal stratification, and the destruction of ihe dissolved oxygen stratification lags behind that of the water temperature stratification, and its structural instability was the result of the combined action of air temperature and solar radiation. During ihis period, the vertical diffusion coefficient of the water body varied from 0.16 to 380.36 m2 /d, with an average value of 30.46 m2 /d. The main controlling factor of surface K2 change was air temperature change, and surface K2 was more sensitive to air temperature change than T and DO (RK = -0.44, i?T = 0.25, RDq=0.25, P2 in the water body was temporally heterogeneous. The lg K2 was positively correlated with T and DO during thermal stratification and semi-mixing (RT = 0.60, RDO = 0.76, PT = -0.19, P0.05) after complete mixing. Thermal stratification extinction provides environmental conditions for water quality deterioration, so we should pay attention to the change of meteorological factors in winter and take precautions against it in reservoir monitoring and management. © 2023 by Journal of Lake Sciences.