Process-based aerodynamic roughness model for evaporation predictions from free water surfaces

The process of evaporation is a complex phenomenon, which includes physical processes in several areas of physics. Accurate estimation of evaporation is important for designing and planning projects in which a water balance is key factor as well as for conservation of water. Although considerable progress has been made in understanding and measuring evaporation, the methods used in the past have serious short-comings measuring evaporation and thus developing more accurate measurements and predictions is needed.

One approach to the prediction of evaporation from free water surfaces is through the use of the Borrelli-Sharif combination method which requires standard meteorological data. However, accuracy of the resulting estimation is dependent upon the interaction of wind profile and the determination of aerodynamic roughness, ZQ. It is evident that evaporation rate is influenced by the mean and turbulent flow properties near the surface of a given body of water. If the aerodynamic roughness can be estimated for the water surface, then the accuracy of predicting evaporation from that free-water surface will be greatly improved. In this study a formula is proposed for determining the aerodynamic surface roughness parameter for free water surfaces. The model is process based using the energy -transfer theory describing the reaction between wind and water surface. Wind is the main force, which causes the waves on water surfaces. Surface roughness of water is directly influenced by the velocity of the wind. The equation developed in this study predicts the wave height as a function of wind speed, and then surface roughness as a function of the wave height. This model expands the overall accuracy of evaporation prediction from free water surfaces