Effects of the position and strength of thermocline on hypolimnion hypoxia in a dam reservoir.

Abstract

Hypolimnion hypoxia in reservoirs is a significant water quality issue. Thermocline formation, caused by natural solar heating and operating aeration systems, contributes to this phenomenon. However, the relationship between the position and strength of the thermocline and the progression of oxygen reduction in hypolimnion has not been fully elucidated, especially in the early stages of thermal stratification. As the position of the thermocline may be artificially controlled, strategic adjustment of the thermocline position may mitigate oxygen reduction in the hypolimnion and provide benefits such as shortening the operation time of water quality improvement devices. In this study, we investigated the relationship between the depth and strength of the thermocline and the DO decline rate of hypolimnion during the early stages of thermal stratification in Miharu Dam. Water temperature and DO data were collected using an automated water quality monitoring system. The position and strength of the thermocline were estimated from the vertical water temperature profile, while the DO decline rate below the thermocline was determined from continuous DO measurement. The results showed a significant positive correlation between the thermocline strength and the DO decline rate (correlation coefficient: about 0.8). A power function well describes this relationship, indicating that the stronger the thermocline, the faster the rate of DO decline. This finding is consistent with previous studies that showed that the vertical diffusion coefficient is inversely proportional to the square of the thermocline strength. The vertical diffusion coefficient can be used as a linear variable to simulate oxygen depletion in hypolimnion. Based on data from a single reservoir, this study provides valuable insights into the relationship between thermocline characteristics and DO decline during the early stages of thermal stratification. Future research will focus on extending this analysis to multiple reservoirs to generalize these findings.