Meteorological forcing and hydrological connectivity together control the thermal regime of Xiangxi Bay, a tributary of the Three Gorges Reservoir

Abstract

Thermal conditions, such as stratification and mixing characteristics, are important for water quality and ecology in lakes and reservoirs. The prevailing strong stratification is a major factor contributing to global algal blooms, underscoring the need to understand the thermal regime and driving mechanisms. The thermal conditions of riverine/canyon-type reservoirs and tributaries are distinct from those of other districts, and hydrological connectivity and meteorological forcing play an important role in stratification and mixing. To test this concept and analyse how hydrological and meteorological conditions affect the thermal regime of Xiangxi Bay (XXB), one of the largest tributaries at the head of the Three Gorges Reservoir (TGR) in China, we conducted approximately one year of field monitoring. The results indicate that XXB is a typical subtropical continuous warm polymictic water body. Affected by the regulation of the TGR, the water level of XXB fluctuated up to 30 m. The stratification started in March and reached its maximum during the flood season; with reservoir storage in September, the stratified structure gradually collapsed. The combination of lower water level, increased shortwave radiation (SW) and warmer air temperature (AT) enhanced the stratification intensity (N). Vertical mixing produced by meteorological forcing was limited, turbulent mixing in the near-surface was dominated by the convection velocity (w∗) rather than the frictional velocity (u∗w), and high wind speeds enhanced the convection effect. During the heating period, the peak u∗w lagged behind w∗ by approximately 3 h, and near-surface stratification suppressed wind-induced turbulence. In addition, no temporal heterogeneity was observed during the cooling period. The heat budget was mainly controlled by the intrusive discharge of the mainstream of the TGR, and the annual water level fluctuation was the main factor affecting the heat imbalance (ΔQT-ΔQM). However, in the near-surface of the water column, heat distribution (indicator: mld) was mainly dominated by the water–air heat flux, Qnet (r = -0.833, p < 0.01, n = 323), and the difference in the cooling rate between the mainstream of the TGR and XXB induced a diurnal heat imbalance. The results of this study, the first long-term, muti-parameter field monitoring effort in XXB, provide new insights into the thermal regime of riverine reservoirs and their influencing factors while offering a thermodynamic perspective on eutrophication and algal blooms in reservoir tributaries.