Effects of seasonal fluctuations of surface heat flux and wind stress on mixing and vertical diffusivity of water column in deep lakes

•Main heat losses were due to evaporation in summer and longwave radiation in winter.•Local wind intensities affected convection-driven mixing in the water column.•Winter water column vertical diffusivities were 18 times greater than summer values.•Epilimnion and well-oxygenated zone depths similarly varied in October–December.•Epilimnetic and hypolimnetic SO42− and Cl− concentrations tracked winter overturns. Accurate vertical diffusivity estimates at different stratification conditions are essential to correctly model vertical mixing of discharges into lakes. This study presents calculated variations in vertical mixing at different depths in Boulder Basin, Lake Mead, a deep reservoir over a four-year period using hourly weather data and 6-hourly measured temperature, conductivity, and DO profiles. Turbulent Kinetic Energy (TKE) and mixing intensities within Boulder Basin, calculated based on surface heat flux and wind speed were compared to water column stability and diffusivity over the study period. Analysis of surface heat fluxes showed that evaporation and longwave radiation were the main heat loss mechanisms in summer and winter, respectively. The lake showed strong summer stratification with stability numbers N2 > 10−4 s−2, followed by increased water column instability during fall and eventually winter overturn, resulting in gradient Richardson numbers