Abstract
The state of knowledge on evapotranspiration under extremely arid climates is relatively poor, in spite of worthwhile scattered efforts. The main impediments are vagueness in the definitions of aridity, shortage in climatic and direct measurements data, little knowledge on the effect of siting and advection in climatic measurements, and the controversial definitions of evapotranspiration terms. There is a need for a more detailed definition of aridity based on evapotranspiration concept and more oriented towards irrigation water requirements. Awareness on the importance of climatic data for many purposes has recently led to the establishment of a vast number of climatic stations in many areas under extremely arid climates. The effect of advection on irrigation, especially under these extreme conditions, is poorly understood and requires special attention. The concept of reference crop evapotranspiration seems to overshadow the older term of potential evapotranspiration. It seems, however, that a standardization of the reference crop and the corresponding crop factor is a matter that needs urgent contribution from one of the concerned international authorities to eliminate the current controversies. (See also W90-10104) (Mertz-PTT)