Thermodynamic Factors in Evapotranspiration

Many of the methods discussed in this paper are refinements of procedures presented in the report 'Consumptive Use of Water and Irrigation Water Requirements' by M. E. Jensen. The use of the H. L. Penman method and later versions of combination theory such as the J. L. Monteith addition, require estimating factors such as vapor pressure, air density, atmospheric pressure and latent heat of vaporization. These factors also are required for some radiation methods. This paper summarizes estimating procedures from perspectives of: history, thermodynamics, fluid mechanics, and meteorology.