The concept of reference ET (ET0) was introduced [53, 54] to avoid ambiguities that existed with the definition of potential ET. ET0 refers to the ET rates from vegetation over which weather measurements are made. Adopting this concept and grass as the reference crop, Pruitt and Doorenbos  were able to develop and validate methods to predict ET0 on the basis of actual field measurements in several locations.
Potential evaporation (Ep) is the evaporation from a surface when all surface-atmospheric interfaces are wet (saturated) so that there is no restriction on the rate of evaporation from the surface, except for atmospheric conditions and energy available at the surface [1, 2]. Similarly, potential ET (ETp) is the rate at which water would be removed from a wet (saturated) complex of vegetated surface and soil so that, without restriction, ETp depends on atmospheric conditions and energy available at the evaporative surface but is influenced by the geometric characteristics of the vegetated surface, such as aerodynamic resistance, vegetative structure, and density. An updated discussion on the above concepts in reference to actual and crop ET is given by Pereira et al. .
Numerous researchers (e.g., [56-58]) have compared measured ETof grass crops with estimates of ET computed from meteorological measurements and have contributed to the identification of sources of inaccuracy of standardized methods and of improvements that could contribute to the consistency of estimation. It has been possible to test and validate improvements to the reference methodology because it was simple, clear, and based on solid, knowledgeably made measurements. Meanwhile, research has provided abundant information that has permited appropriate revision and updating of the reference ET concept as well as calculation procedures [2, 59, 60].
The reference ET takes into consideration the effects of climate on CWR, but refers to ET from vegetation over which weather measurements are made and provides for a consistent set of crop coefficients to be used to determine ET for other crops. Relating ET0 to a specific crop has the advantage of representing the biological and physical processes involved in the energy balance at the cropped surface. Adoption of ET0 has made it easier to select consistent crop coefficients and to make reliable crop ET estimates in new areas. The use of the crop coefficient-ET0 approach has been enormously successful in obviating the need to calibrate a separate ET equation for each crop and stage of growth. It also has provided a working model  that can be used until more sophisticated methods become available for direct estimation of actual crop ET. Therefore, it is important to retain the use of ET0 and to promote its use in making routine estimates of crop ET.
Adopting a living reference grass crop as defined by Doorenbos and Pruitt  provides for a height ranging between 0.08 and 0.15 m, and thus a large variation in bulk surface resistance rs (100 to 50 s m-1). Therefore, the average rs and the associated aerodynamic resistances may vary appreciably with time between clippings and locations, depending on the structural characteristics and regrowth rates of the grass variety and management schedules. Difficulties with a living grass reference result from the fact that the grass variety and morphology can significantly affect the rate of ET0, mainly during peak periods of water consumption. Maintenance of a living reference crop for use in computing crop coefficients for other crops is difficult.
The definition and concept of ET0 must be one-dimensional with respect to evaporation and energy exchange processes. This means that all fluxes within the energy balance (net radiation, sensible heat, soil heat, and latent heat) must be uniformly vertical along the horizontal surface so that the reference surface completely represents the one-dimensional ET processes of large plantings. The requirement that the measurement of ET0 be one-dimensional often is violated in lysimeter studies, in which the crop in the lysimeter or measurement area is taller or shorter than that outside the area or extends beyond the horizontal dimensions of the lysimeter. Various problems and effects of the maintenance of the necessary environmental, site, and equipment conditions have been discussed in the literature [57, 61-66]. In the discussion by Allen et al. , errors of 20% and up to 30% in measuring ET are reported.
Boundary-layer measurements of ET also can be beset with operational problems. These systems, which include eddy correlation and Bowen ratio systems, require long fetch in the upwind direction. In addition, boundary-layer measurement-equipment components are delicate and require special maintenance. These requirements often limit eddy correlation and Bowen ratio systems to research studies. A number of researchers have analyzed the requirements for and measurements errors in ET boundary-layer measuring systems [47, 56, 67-69].
The measurement of ET by performing a continuous soil water balance from data collected through the monitoring of soil water has never received the preference of researchers mainly because of difficulties in instrumentation and spatial variability. An analysis of the performance of neutron probe readings in estimating ET is given by Carrijo and Cuenca .
In summary, despite progress in measuring ET, it is preferable for irrigation practice, to compute ET0 from weather data, and hence to use a climatic reference ET definition, and to adopt a reference grass crop with constant height.
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