Development of a physically exact energy balance method for estimating evaporation from free water surfaces
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Based upon hydrodynamic, thermodynamic, and meteorologic principles, a formula is proposed for estimating free water surface evaporation. The proposed formula considers the density and isobaric specific heat of the moist air as a variable depending on the amount of water vapor content in the constant flux boundary layer of the atmosphere. It has been shown that if the influence of the variation of atmospheric water vapor content through density and specific heat of the air is not considered, then the sensible heat flux as computed by conventional methods in the energy-based evaporation equations will be smaller by approximately 17 per cent in comparison to the latent heat flux. Consequently, this underestimation of sensible heat flux (as calculated conventionally) results in a corresponding overestimate of the estimated evaporation. The eddy exchange coefficients for momentum, heat, and moisture have been incorporated in the prediction formula in the familiar form of turbulent Prandtl, turbulent Schmidt, and turbulent Lewis numbers. Thus the proposed prediction equation does not contain the often questioned assumption of the equality of these eddy transfer coefficients in evaporation equations. Evaporation data collected over water and well-watered short grass were used for checking the validity of the proposed prediction equation. These data were gathered under different conditions of the atmospheric stability, and under a wide range of wind velocities. The analysis of the data showed an excellent agreement between the predicted amounts of evaporation and the observations.