Understanding the environmental controls on evaporation over lake/reservoir is important for water resource management as well as predicting variations in hydrology as a result of climate change. This study presents an analysis of diurnal, intra-seasonal, and seasonal variations of evaporation in the Ross Barnett reservoir (32o26'N, 90o02'W), Mississippi, U.S.A. Eddy covariance measurements of surface energy budget, evaporation, and CO2 flux are made together with other microclimate variables. The eddy covariance tower was set up in the middle of the reservoir with its fetch exceeding 1.5 km in all directions. The water depth is about 4 m around the tower location. The tower with its height of 4 m stands over a stable wood platform with its size of 3 m × 3 m and height of 1 m above the water surface. The eddy covariance system on the tower is used to measure fluxes of sensible heat, latent heat, and CO2. Besides the fluxes, microclimate data are also measured, including wind speed, wind direction, relative humidity, solar radiation, net radiation, air temperature at four levels, water surface temperature, and water temperature at five depths. Using these datasets, the daytime and nighttime evaporation rates are analyzed and the importance of nighttime evaporation in the diurnal totals is presented. The influence of different environmental variables (e.g., net radiation, wind, air temperature, water temperature, relative humidity) on the evaporation rates is examined. We also investigate the influence of synoptic weather variations in controlling evaporations.