An Investigation of the Predictive Ability of Several Evaporation Equations

Abstract

A number of integrated flux-gradient type evaporation equations have been tested using very accurate evaporation and meteorological data. The test compared the estimated evaporation with the measured evaporation. Most of the equations were derived originally assuming that the ratio of the eddy diffusivity of momentum to the eddy diffusivity of water vapor is constant. Since recent developments have shown that this ratio is a function of atmospheric stability, these equations were revised to admit variability of the ratio and tested in the revised form. The momentum flux appears in all evaporation equations tested. Results show that the accuracy of the evaporation estimation is critically dependent on the accuracy of the momentum flux estimation by the velocity profile equation from which the evaporation equation is derived. In general, profile equations which produce momentum fluxes in good agreement with measured values will produce accurate evaporation equations. Of the equations here tested, an evaporation equation based on the Brooks wind velocity profile equation and a modified form of the Deacon and Swinbank evaporation equation were found to be the most accurate. A semi-empirical equation proposed by Pruitt was also found to give good results but may require calibration.