Food and Agricalture Organization of the United Nations, FAO Irrigation and Drainage Paper No. 56, Version 1998, 326 pp.
This publication presents an updated procedure for calculating reference and crop evapotranspiration from meteorological data and crop coefficients. Since the publication of FAO Irrigation and Drainage Paper No. 24 in 1977, advances in research and more accurate assessment of crop water use have revealed the need to update the FAO methodologies for calculating ET. The FAO Penman method was found to frequently overestimate ET while the other FAO recommended equations, namely the radiation, the Blaney-Criddle, and the pan evaporation methods, showed variable adherence to the grass reference crop evapotranspiration.
The FAO Penman-Monteith method uses standard climatic data that can be easily measured or derived from commonly measured data. All calculation procedures have been standardized according to the available weather data and the time scale of computation. The calculation methods, as well as the procedures for estimating missing climatic data, are presented in this publication. In the 'K -ET ' approach, differences in the crop canopy and aerodynamic resistance relative to the reference crop are accounted for within the crop coefficient. The K coefficient serves as an aggregation of the physical and physiological differences between crops. Two calculation methods to derive crop evapotranspiration from ET are presented. The first approach integrates the relationships between evapotranspiration of the crop and the reference surface into a single Kc coefficient. In the second approach, K is split into two factors that separately describe the evaporation (K ) and transpiration (K ) components. The selection of the K approach depends on the purpose of the calculation and the time step on which the calculations are to be executed.Contents.
Introduction to evapotranspiration.
Part A. Reference evapotranspiration (ETo).
FAO Penman-Monteith equation.
Meteorological data.
Determination of ETo
Part B. Crop evapotranspiration under standard conditions.
Introduction to crop evapotranspiration (ETc).
ETc - single crop coefficient (Kc).
ETc - dual crop coefficient (Kc = Kcb + Ke).
Part C. Crop evapotranspiration under non-standard conditions.
ETc under soil water stress conditions.
ETc for natural, non-typical and non-pristine conditions.
ETc under various management practices.
ETc during non-growing periods.
Annexes.
Bibliography.
