Evapotranspiration: Concepts and future trends

Past research on evapotranspiration has provided sound theoretical knowledg e and practical applications that have been validated through field measure ments. Many different approaches have been used; however, when primary conc epts and standard definitions are accepted, it is possible to find reasonab le agreement among methods. This paper reviews such approaches, from Penman to Penman-Monteith. The standard concepts of potential evaporation (EP) an d equilibrium evaporation (E-e), and the introduction of the climatic resis tance (r(e)), provide a better understanding of the role of the climate tog ether with surface and aerodynamic resistances (r(s) and r(a)). Therefore, the concept of reference evapotranspiration (ETa), particularly the new one adopted by the Food and Agricultural Organization of the United Nations, c an be better understood, as well as its limitations. Crop evapotranspiratio n (ETc) is related to both ETa and E-e. Crop coefficients (K-c) can be show n to have two components, alpha(o) and alpha(c), with K-c = alpha(o)alpha(c ). The alpha(o) is a function of the climatic resistance and of the aerodyn amic resistances of the crop and of the reference crop. The alpha(c) is a f unction of both surface and aerodynamic resistances of the crop and of the reference crop. From this analysis some ideas on future developments result that are directed toward providing compatibility between the one- and two- step calculation of ETc.