Evapotranspiration is the combination of soil evaporation and crop transpiration. Weather parameters, crop characteristics, management and environmental factors affect evapotranspiration. Reference, potential and actual evapotranspiration are distinguished. These terms are commonly used, although some differences of their definitions can be found among researches. The potential evapotranspiration of a given crop is defined as soil evaporation and plant transpiration under unlimited soil water supply and actual meteorological conditions. According to Brutseart (1982) the potential evapotranspiration is a maximum intensity of evapotranspiration from a large surface covered completely and homogeneously with actively growing plants under conditions of unlimited availability of soil water. Allen et al. (1998) call it the crop evapotranspiration under standard conditions and define as the evapotranspiration from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions and achieving full production under the given climatic conditions. The actual evapotranspiration is the amount of water transpired from plants and evaporated from soil surface under actual meteorological conditions and under non-optimal soil, biological, management and environmental conditions. It differs from the potential evapotranspiration due to soil water shortage or waterlogging, diseases, soil salinity, low soil fertility. According to Allen et al. (1998) the evapotranspiration from crops grown under management and environmental conditions that differ from the standard conditions defined for the potential evapotranspiration can be called the crop evapotranspiration under nonstandard conditions. The evapotranspiration from a reference surface is called the reference evapotranspiration and is denoted as ETo. A large uniform grass (or alfalfa) field is considered worldwide as the reference surface. The reference grass crop completely covers the soil, is kept short, well watered and is actively growing under optimal agronomic conditions. Reference evapotranspiration (ETo) is an important agrometeorological parameter for climatological and hydrological studies, as well as for irrigation planning and management. There are several methods to estimate ETo. The FAO Penman-Monteith (FAO PM) method has been considered as a universal standard to estimate ETo (Allen et al., 1989, 1994, 1998;