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