The Penman-Monteith equation

The Penman-Monteith is an elegant, physical expression of the process that water vapor follows when transported from inside the leaf of a plant into the atmospheric boundary layer above. The original Penman combination equation (1948) combined the energy input of radiation from the sun and atmosphere with convective transport of temperature and humidity from the vegetation surface. The Penman-Monteith formulation (1965) included resistance terms for bulk stomates and aerodynamic transport. The equation has gained popularity through its use in estimating reference ET from a grassed surface that is then transformed into ET for specific crops through the crop coefficient.