Crop coefficient for drip-irrigated cotton in a Mediterranean environment

A 3-year study was conducted in the eastern Mediterranean region of northern Syria to develop crop coefficient, Kc, for drip-irrigated short-season cotton (Gossypium hirsutum L.). Two sets of Kc curves were determined, the generalized Kc published by the UN’s Food and Agriculture Organization (FAO) that was adjusted for local climate, and the locally developed Kc as the ratio of measured cotton evapotranspiration to calculated reference evapotranspiration. The adjusted FAO Kc curves were the same for the 3 years. However, the locally developed Kc curves not only differed among the 3 years, but also from the adjusted FAO Kc. During the mid-season stage, the adjusted FAO Kc was 24% higher than the locally developed value of 1.05. Variations in locally developed Kc values were caused by normal year-to-year variations in irrigation timing and amount, suggesting sensitivity of Kc that cautions against the use of locally developed Kc based on limited data (i.e., a single season). On the season, the overestimation of crop evapotranspiration by using adjusted FAO Kc was substantial and equivalent to 150 mm water or about two additional irrigations per season. Results caution against blind application of published FAO Kc curve, suggesting some local or regional calibration for increased accuracy.