Despite of the great advancement of technologies for water supply, irrigation management remains inadequate in most areas. The lack of basic information on crop water needs is one of the causes for inadequate water use and irrigation management. The approach normally used to quantify the consumptive use of water by irrigated crops is the crop coefficient-reference evapotranspiration (Kc ETo) procedure. In this procedure, reference evapotranspiration (ETo) is computed for a grass or alfalfa reference crop and is then multiplied by an empirical crop coefficient (Kc) to produce an estimate of crop evapotranspiration (ETc). The ETo represents the non-stressed ET based on weather data. We selected three experiments with different crops in terms of physiology and planting arrangements to discuss the crop coefficient paradigm and its relation with reference evapotranspiration for highly canopy-atmosphere coupled crops. We found the Kc decreasing as ETo increased as a consequence of high plant atmosphere coupling and high crop inner resistance, which limits the amount of water the plant could supply to the atmosphere. Even for sugarcane plantation (after it completely covered the ground) Kc decreased with ETo, highlighting that trend might not be exclusive of tall sparse crops and for well coupled to the atmosphere. For these reasons, we suggested the definition of Kcb (for sparse crops) and Kc should take into account ETo ranges besides the components currently considered.