Original Penman-Monteith actual crop evapotranspiration and standardized Penman-Monteith Reference evapotranspiration models

As a worthwhile effort to implement and/or enhance the utilization of the Penman-Monteith evapotranspiration equation in water resources management, especially in water resources engineering and agricultural water management, the Food and Agriculture Organization (FAO) of the United Nations Expert Consultation recommended using the FAO56 Penman-Monteith method as a “standardized” (not “standard”) method for the estimation of reference evapotranspiration (ETref) either for hypothetical grass-or alfalfa-reference surface (ETo and ETr, respectively). Among several important reasons and even necessities, the transferability of crop coefficients between the regions/locations and establishing some kind of standardized method of calculating ETref in a more physically based manner and more accurately are among the reasons. Since the publication of the FAO56 Penman-Monteith as FAO Paper No. 56, this “standardized” method has gained considerable acceptance by the scientific communities as well as practitioners and water management, irrigation, and other professionals. To contribute toward further promotion of the utilization of the “standardized” method, this chapter provides some of the basic and fundamental principles of the evaporative loss process and presents some of the essential structural aspects of the Penman and Penman-Monteith equations (Penman; 1948, Original 1965 Penman-Monteith and 1998 FAO56 Penman-Monteith). The chapter also presents some of the applications of the equations in practice and scientific platforms and investigations, provides some suggestions of current and future potential challenges, and makes recommendations for further research in evapotranspiration and related topics.