Part A - Reference evapotranspiration (ETo)
Part A dealswith the evapotranspiration from the reference surface, the so-called referencecrop evapotranspiration or reference evapotranspiration, denoted as ETo.The reference surface is a hypothetical grass reference crop with an assumedcrop height of 0.12 m,a fixed surface resistance of 70 s m-1 andan albedo of 0.23. The reference surface closely resembles an extensive surfaceof green, well-watered grass of uniform height, actively growing and completelyshading the ground. The fixed surface resistance of 70 s m-1implies a moderately dry soil surface resulting from about a weekly irrigationfrequency. ETocan be computed from meteorological data. As a result of an Expert Consultationheld in May 1990, the FAO Penman-Monteith method is now recommended as the solestandard method for the definition and computation of the referenceevapotranspiration. The FAO Penman-Monteith method requires radiation, airtemperature, air humidity and wind speed data. Calculation procedures to deriveclimatic parameters from meteorological data and to estimate missingmeteorological variables required for calculating ETo are presentedin this Part (Chapter 3). The calculation procedures in this Publication allowfor estimation of ETo with the FAO Penman-Monteith method under allcircumstances, even in the case of missing climatic data. ETocan also be estimated from pan evaporation. Pans have proved their practicalvalue and have been used successfully to estimate ETo by observingthe water loss from the pan and using empirical coefficients to relate panevaporation to ETo. However, special precautions and management mustbe applied.
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