Interplay between convection and wind in driving surface mixing in the Xiangjiaba Reservoir, China

Near-surface processes in lakes and reservoirs strongly influence basin-scale circulation, mixing, and ecosystem functioning. While wind- and convection-driven mixing are recognized in lakes, their individual and interactive contributions in morphologically complex reservoirs remain insufficiently quantified. Using high-frequency temperatures in a large reservoir, we identified three mixing regimes during autumn cooling through the scalecomparisons between convection (Thorpe scale: LT) and wind (Monin-Obukhov: LMO): convective, wind-dominated mixing, and diurnal stratification. A diurnal-scale thermocline often developed under net heat input, suppressing vertical mixing, but was disrupted when either convective or wind intensified. Strong convection occurred under sustained surface heat loss (LT >> LMO), producing turbulence dissipation rates (εConvection~10⁻⁸-10⁻⁷ W/kg), exceeding wind-driven values (εWind) by an order of magnitude. Conversely, wind forcing dominated (LMO >> LT), yielding εWind~10⁻⁷ W/kg. The mixed layer deepening correlated with w*³, but not with shear velocity. These findings refine mechanistic understanding of reservoir hydrodynamics and support ecologically sustainable management.