Effects of Seasonal Fluctuations of Surface Heat Flux and Wind Stress on Mixing and Vertical Diffusivity of Water Column in Deep Lakes

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Advances in Water Resources



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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 < 0.25 in the water column's top 50 m. The average calculated Wedderburn number was 45 during summer stratification, indicating that local winds were not sufficiently strong to generate upwelling. Burger numbers (S < 1) show that the Coriolis force significantly affects vertical mixing in Boulder Basin over the entire annual cycle. Diffusivities seasonally varied by 1 to 1.5 orders of magnitude (typically 5 × 10−5 to 10−3 m2 s−1) in the upper water column, and typically varied by about 1.5 orders of magnitude (typically 3 × 10−6 to 10−4 m2 s−1) in the deeper layers. Increases in winter diffusivities caused deep water dissolved oxygen (DO) concentrations to increase from 6.0 to 8.5 mg L−1. Analysis of DO profiles and chloride and sulfate concentrations in the epilimnion and deep hypolimnion showed marked differences between epilimnetic and hypolimnetic concentrations during stratification. Similar epilimnetic and hypolimnetic concentrations during January and February confirm increased vertical mixing during these months. Use of hourly-based computed TKEs, and water column vertical diffusivity estimates in stratified and unstratified conditions over the entire annual cycle can help modelers to more accurately predict vertical mixing in large lakes.


Thermal stratification; Turbulent kinetic energy; Lake Mead; Surface water quality; Surface mixed layer; Internal waves


Civil and Environmental Engineering | Water Resource Management



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