Effect of hydraulic load on thermal stratification in karst cascade hydropower reservoirs, Southwest China

Study region: The impounded Wujiang River, Southwest China. Study focus: Thermal stratification is an important factor shaping aquatic ecosystems in karst hydropower reservoirs. However, the underlying mechanism of thermal stratification formation is still unclear. Therefore, water temperature and related environmental factors were investigated to understand how reservoir hydraulic load (i.e., the ratio of the mean water depth to water retention time) affects thermal stratification in eight karst hydropower reservoirs on the Wujiang River. New hydrological insights for the region: The reservoir surface water had a higher temperature than the inflowing and released waters. The variation in the reservoir thermal stratification showed an obvious seasonal pattern, but the extents of the thermal stratification were different among the reservoirs. The rate of water temperature change is proposed to characterize reservoir thermal stratification and is significantly negatively correlated with reservoir hydraulic load, suggesting the control of both water depth and retention time on thermal stratification. This empirical relationship will be helpful in quantitatively evaluating the hydrological effect on thermal stratification, which further affects basic environmental parameters (including pH, dissolved oxygen, and chlorophyll a) in narrow and deep karst cascade hydropower reservoirs.