Thermal stratification and mixing processes response to meteorological factors in a monomictic reservoir.

Formation and extinction of thermal stratifications impact the reservoir ecosystems and have been closely influenced by meteorological and hydrological factors. However, quantifying the relative importance of these crucial environmental factors and mechanisms in reservoir regions characterized by various depths remain comparatively uninvestigated. Tianbao Reservoir is a typical monomictic warm and drinking water source reservoir in Southwest China. This study supplemented field observations with a three-dimensional numerical simulation model to quantitatively analyze mixing and turnover events. Air temperature and wind were two important meteorological factors resulting in hydrodynamics during stratification and mixing processes. Air temperature led to variations in stratification strength and wind-induced fluctuations of thermocline depth. A 10% rise in air temperature increased stratification strength by 18%, and a 3 m/s rise in wind speed induced the deepening of the thermocline by 2.09 m. Two hydrodynamics involved penetrative convection caused by temperature plummets and wind-induced mixing during winter turnover events were identified. Penetrative convection was the main driving force, and wind shear mixed the upper 21% of the mixed layer, which was contributed by convection. Response of water temperature to air temperature in shallow regions was faster (58 d), and the mixing depth caused by the wind was smaller than that in deep regions. Research on physical processes during stratification and mixing processes can provide support for further study on water quality deterioration distributions.

Thermal stratification responses of a monomictic reservoir under different seasons and operation schemes.

This study investigates the thermal stratification responses of a monomictic reservoir operated under different facilities. The analysis of 60-year long data showed that the reservoir's thermal regime varies with season and withdrawal scheme and is affected by upstream reach control through the vertical curtain. Isothermal conditions exist during winter (December-March) while stratification onsets in spring (starting April), intensifies in summer (August) and weakens during fall (October-November). Considering summer stratification, deep hypolimnetic withdrawals through the penstock intake promoted thicker epilimnion, with low values of thermal stability (Schmidt Stability Index, SSI) and thermocline strength index (TSI). Meanwhile, shallow withdrawals using selective outflow system resulted in narrower epilimnion, with larger TSI for no curtain scenario and larger SSI for with curtain scenario. Strongest thermoclines do not necessarily translate to largest magnitudes of thermal stability. Longer duration of stratification is associated with shallow withdrawals. Depending on the outflow depth and the occurrence of prolonged hot or cold atmospheric conditions, the onset of stratification could be likely shifted early or late. The 3D numerical simulation determined the individual effects of each operation, which strongly supported the results of the long term analysis. Since thermal stratification directly influences the reservoir's water quality regime, this study can be a helpful reference in optimizing the water quality management of the reservoir.