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The use of the FAO-56 reference evapotranspiration (ETo) approach for irrigation scheduling has been recommended worldwide because it provides reasonable results under a wide range of climatic conditions. The crop ET (ETc) can be obtained from ETo using a stage-dependent crop coefficient (Kc). The latter depends on canopy height and crop�soil surface resistance and albedo. The objective of this study was to evaluate the FAO-56 Kc procedure under different deficit drip irrigation treatments for cotton grown in a humid climate. A cotton experiment was conducted in 2005 in three automated rainout shelters located at the University of Georgia Campus in Griffin, GA, USA. When the modeled soil water content in the effective root zone dropped below a specific threshold of the available water content (AWC), irrigation was applied until the soil water reached 100% of AWC. The irrigation treatments were 40, 60, and 90% irrigation thresholds (IT). For instance, for the 40% IT treatment irrigation was applied when the modeled soil water content dropped to permanent wilting point plus 40% of the total available soil water. Thus, the least irrigated treatment corresponded to the 40% IT and the most irrigated corresponded to the 90% IT. The observed daily ETc was estimated from daily soil moisture readings. The length of the initial, development, and mid-stages were the same among the different treatments. However, the length of the late-stage for the 40 and 60% IT was 5 and 3 days less than that for the 90% IT, respectively. The observed Kc for the initial stage was 0.51, 0.9, and 0.99 for the 40, 60, and 90% IT, respectively. The FAO-56 Kc for this stage was within a few percent from the observed Kc. For the mid-stage, the 60 and 90% IT had a similar value for Kc (about 1.2) which was higher than the 40% IT (0.92). The end of season Kc for the 40% IT (0.1) was lower than for the 60% IT (0.38), which in turn was lower than the 90% IT (0.58). The FAO-56 proved to be an accurate method for estimating cotton ETc under deficit irrigation. The values for the initial, mid, and end Kc and the length of the different developmental stages for the 90% IT can be used in humid climates for fully irrigated cotton. The use of such information will help in effective irrigation planning and more precise water management
