On the concept of equilibrium evaporation and the value of the Priestley-Taylor coefficient

An analytical expression is derived for the Priestley-Taylor coefficient, α. This coefficient is generally interpreted as the ratio between the actual evaporation rate and the equilibrium evaporation rate. The derived expression is shown to be equal to the accepted value of 1.26 for typically observed atmospheric conditions and to be relatively insensitive to small changes in atmospheric parameters. A comparison is made between the new expression and data taken over an irrigated field in California. The concept of equilibrium evaporation is called into question and it is suggested that the Priestley-Taylor expression with α equal to 1.26 is a more accurate representation of evaporation under 'equilibrium' wet surface conditions.