derived crop coefficients for winter wheat using different reference evpotranspiration estimates methods

this paper reports the results of using three empirical methods (makkink, priestley-taylor and hargreaves) for estimating the reference evapotranspiration (et0) 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 et0 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 20 and 18 %. 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 9% and 4% for the priestley taylor and makkink methods, respectively, which led to an improvement of 55% and 76% of the obtained values compared with the original values.