S. Er-Raki - Cadi Ayyad University, Institute for research and development, City Prince Moulay Abdellah, BP ۲۳۹۰, Marrakech, ۴۰۰۰۰, Morocco.
A. Chehbouni - CESBIO- Center for the Study of the Biosphere from Space, Toulouse, France, ۱۸ City Edouard Belin, bpi ۲۸۰۱, ۳۱۴۰۱ Toulouse cedex ۹, France.
J. Ezzahar - Department of Physics, FSSM- Faculty of Sciences Semlalia Marrakech, Avenue Prince Moulay Abdellah, BP ۲۳۹۰ Marrakech ۴۰۰۰۰, Morocco.
S. Khabba - Department of Physics, FSSM- Faculty of Sciences Semlalia Marrakech, Avenue Prince Moulay Abdellah, BP ۲۳۹۰ Marrakech ۴۰۰۰۰, Morocco.
E. K. Lakhal - Department of Physics, FSSM- Faculty of Sciences Semlalia Marrakech, Avenue Prince Moulay Abdellah, BP ۲۳۹۰ Marrakech ۴۰۰۰۰, Morocco.
B. Duchemin - CESBIO- Center for the Study of the Biosphere from Space, Toulouse, France, ۱۸ City Edouard Belin, bpi ۲۸۰۱, ۳۱۴۰۱ Toulouse cedex ۹, France.

This paper reports the results of using three empirical methods (Makkink, Priestley-Taylor and Hargreaves) for estimating the reference evapotranspiration (ET۰) in the semi-arid region of Tensift Al Haouz, Marrakech (center of Morocco). The Penman-Monteith equation, standardized by the Food and Agriculture Organization (FAO-PM), is used to evaluate the three empirical methods. The obtained ET۰ data were used to estimate crop water requirement (ET) of winter wheat using the crop coefficient (Kc) approach and results were compared with ET measured by the Eddy Covariance technique. The result showed that using the original empirical coefficients a,  and Cm in Hargreaves, Priestley-Taylor and Makkink equations, respectively, the Hargreaves method agreed fairly well with FAO-PM method at the test site. Conversely, the Priestley-Taylor and Makkink methods underestimate the ET by about ۲۰ and ۱۸ %. After adjustment of the original values of two parameters  and Cm coefficients in Priestley-Taylor and Makkink equations, the underestimation of ET was reduced to ۹% and ۴% for the Priestley Taylor and Makkink methods, respectively, which led to an improvement of ۵۵% and ۷۶% of the obtained values compared with the original values.

Crop water requirement, Eddy covariance, Empirical method, Semi-arid Environment