The influence of wind stress, temperature, and humidity gradients on evaporation from reservoirs

Evaporation from fetch-limited water bodies has been investigated for the first time using a coupled atmospheric boundary layer-water body model. The model incorporates a simplified atmospheric boundary layer in which heat and moisture are advected horizontally and diffused vertically. The wind field evolves over the water body through the formation of an internal boundary layer, which is initiated by the change in roughness from the land to the water surface. The wind also responds to local stability through the inclusion of Monin-Obukhov similarity functions. This system is coupled to a dynamically active water body based on primitive equations with full thermodynamics. This is achieved through continuity of stress and heat flux through the air-water interface. The model results reveal that along-wind gradients in wind stress, humidity, and temperature can all significantly influence evaporation. The most important effects are growing wind stress and increasing humidity as we move downwind across the water body. However, there is a tendency for these effects to cancel, so that the behavior can range from areally averaged evaporation weakly decreasing with fetch for very smooth land surfaces to weakly increasing with fetch for relatively rough land terrain. For most situations of interest, such as typical agricultural settings, evaporation is essentially independent of fetch. All the model results have been summarized in a simple empirical expression for evaporation based exclusively on meteorological data from over the upwind land surface. This is in good agreement with detailed measurements from a small lake in southeastern Australia.