ABSTRACT
Water requirement of crops in the form of Evapotranspiration (ETc) is the largest and one of the most basic components of the hydrologic cycle. It plays a very important role in the water and energy balance on earth's surface and also has a major role in agricultural and irrigation practices. It is the combination of soil evaporation and crop transpiration process. The crop coefficient approach is for calculating the crop evapotranspiration under standard conditions. The standard conditions refer to crops grown in large fields under excellent agronomic and soil water conditions. The evapotranspiration rate from a cropped surface can be directly measured by the mass transfer or the energy balance method. ETc is the largest and one of the most basic components of the hydrologic cycle. Accurate estimation of ETc is of vital importance for many studies, Such as hydrologic water balance, irrigation system design and management, water resources planning and management, etc.
