The first factor is the evaporative power of the atmosphere or the reference evapotranspiration (ETo) (Allen et al., 1998). The second factor is the crop coefficient (Kc) which includes at least three parameters: (1) the transpiration coefficient or basal coefficient, Kcb, (2) the water stress coefficient, Ks, and (3) the evaporation coefficient of the bare soil fraction (Ke). In this approach, plant transpiration is limited only by water stress