Inland fresh waters (e.g., lakes, reservoirs, wetlands, etc) act differently from surrounding lands in exchanges of radiation, energy, water vapor, and trace gases between the water and the overlying atmosphere, thus providing unique spots in terms of their influence on the local, regional, and even global climate. Since large inland waters respond more quickly to climate variability and seasonal perturbations as compared with land surfaces, they would provide a robust indicator as to whether climate warming has the potential to accelerate the energy and water vapor (and thus evaporation) exchange between inland waters and the atmosphere, and what factors influence seasonal and interannual variations in the surface energy budget. The implications of changes in evaporation in response to climate warming for inland water resource management and hydrologic cycles are of major significance. Here we will present an analysis of eddy covariance data in 2009 and 2010 to study how seasonal and interannual variations in synoptic weather events affect the surface energy budget over an open water surface. The measurements were conducted in the Ross Barnett reservoir (32o26'N, 90o02'W; 117.5 m a.s.l.) (which is always ice-free), with a mean depth of 5 m and surface area of approximately 134 km2 in central Mississippi, U.S.A. The fetch for the eddy covariance system exceeded 2 km in all directions. Our results indicate that seasonal and interannual variations in sensible and latent heat fluxes were strongly regulated by synoptic weather events that brought in air masses with different meteorological properties. The year 2010 was drier and warmer than 2009. The drier and warmer 2010 increased sensible and latent heat fluxes by 22% and 29%, respectively, from April to September by enhancing vertical temperature and humidity gradients, as compared with 2009. However, the more cold front activities in 2009 led to the increased sensible and latent heat fluxes by 25% and 31%, respectively, from October to March, as compared with 2010. The detailed analysis about the environmental controls on the seasonal and interannual variations in the surface energy budget will be present. Our results suggest that future potential changes in weather and climate conditions 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).