Crop coefficient of semi-dwarf winter wheat for different growth phases in a humid sub-tropic environment M. H. Ali * , M. R. Hoque 1 , A. A. Hassan and M. A. Khair 1 Agricultural Engg. Division, Bangladesh Institute of Nuclear Agriculture, P.O. Box-04, Mymensingh 2200,Bangladesh, E- mail: mha_bina@yahoo.com; * Corresponding author; 1 Dept. of Irrigation & Water Management, Bangladesh Agricultural University, Mymensingh 2202, Bangladesh. Citation: Ali, M. H., M.R. Hoque, A. A. Hassan and M.A. Khair (2007). Crop coefficient of winter wheat at different growth stages in a humid sub-tropic environment. Int. J. Bio-Research, 2(1): 36 – 46 Abstract Crop coefficient is one of the most important parameters, needed for estimating crop water requirement using the reference crop method, and thus needed for irrigation scheduling and water allocation. This study determined the crop coefficients of winter wheat (Triticum aestivum) based on field measured actual evapotranspiration data for three years (at Ishurdi, Bangladesh, during 2002-03, 2003-04 and 2004-05 season) and reference crop evapotranspiration (ET 0 ) using different methods. The crop coefficient (k c ) under different ET 0 methods for early, crop development, middle, and late season stages ranged from 0.54 to 0.96, 0.95 to 1.36, 1.2 to 1.62, and 0.68 to 1.05, respectively. The P– M method gave relatively higher k c value than those of other methods. No significant differences were observed by pair ‘t’ test among the k c values determined by various ET 0 methods. When the values were averaged over years and methods, a k c value of 0.77, 1.17, 1.41, and 0.89 were found for early, crop development, mid-season, and late season stages, respectively. The values were found to differ markedly from FAO k c values. For single-stage deficit, the yearly average values of k c under different ET 0 methods ranged from 0.35 to 0.48, 0.36 to 0.56, 0.43 to 0.60, and 0.58 to 0.76 for early, crop development, middle and late season deficits, respectively. When the values were averaged over years and methods, a k c value of 0.41, 0.46, 0.52, and 0.65 were found for early, crop development, mid-season, and late season stages, respectively. For deficit control, an average (over years and methods) k c value of 0.34, 0.26, 0.33, and 0.57 were found for early, crop development, mid-season, and late season stages, respectively. The developed crop coefficients would be of great use to determine water requirement for wheat, and thus in planning and management of irrigation in the region, and also in the regions having similar climate. Keywords: Reference evapotranspiration, crop coefficient, wheat, Bangladesh