Study on Evapotranspiration Coefficient of Grassland during the Growing Season in the Ejina Oasis

According to works of Food and Agriculture Organization of the United Nations,evapotranspiration coefficient is essential parameter to calculate actual evapotranspiration.Based on the definition of evapotranspiration coefficient and successive observation,evapotranspiration(ETc) of grassland in the Ejina oasis of northwest China was estimated by application of Bowen ration-energy balance method(BREB),and reference evapotranspiraiton was calculated by using FAO 56 Penman-Monteith model during the growing season in 2005.Then,according to the definition,the evapotranspiration coefficient of grassland was given in the Ejina oasis.During the growing season,total ETc was 446.96 mm.From the beginning of growing season,the ETc increased gradually,and reached its maximum value of 6.724 mm/d in the last fifteen days of June.Hereafter the ETc dropped rapidly,and reached its minimum value of 1.215 mm/d at the end of growing season.The variation pattern of crop coefficient(Kc) was similar to that of ETc.From the beginning of growing season,the Kc value increased rapidly,and reached its maximum value of 0.623 in the last fifteen days of June.Afterward,with slowing growth of grassland,the value dropped rapidly at the end of growing season.The evapotranspiration coefficient is helpful to estimate accurately ecological water demand of grassland in the Ejina oasis.