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. We will present an analysis of diurnal, intra-seasonal, and seasonal variations of evaporation and surface energy budget from long-term eddy covariance measurements starting from August of 2007 in the Ross Barnett reservoir (32o26'N, 90o02'W), Mississippi, U.S.A. The fetch of eddy covariance system exceeds 2 km in all directions and the water depth is about 4 m around the flux tower. The tower with its height of 4 m stands over a stationary wood platform with its size of 3 m × 3 m and height of about 1 m above the water surface. Along with sensible and latent heat 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 eight depths down to about 4 m.

Our results indicate that different environmental variables (e.g., net radiation, wind, air temperature, water temperature, relative humidity) play important roles in diurnal, interseasonal, and seasonal variations in the surface energy budget and evaporation. Using these datasets, the daytime and nighttime evaporation rates are also analyzed and nighttime evaporative water losses are substantial, contributing a significant portion to the total evaporative water loss. We investigate the influence of synoptic weather variations in controlling evaporations and the surface energy budget. For example, daily H and LE (i.e., evaporation) during the passages of cold fronts are around 2–4 times those of normal days and these cold front events lead to an increase in the seasonal H by approximately 40% and LE by 160%. These results imply that future potential changes in cold front activities (intensity, frequency, and duration) as a result of climate change may lead to substantial shifts in regional energy budget and hydrological balance in the southern regions with an abundance of open water bodies (e.g., lakes, reservoirs, swamps etc).