Evapotranspiration and crop coefficient of Populus euphratica Oliv forest during the growing season in the extreme arid region northwest China

Based on successive observation, fifteen-day evapotranspiration (ETc) of Populus euphratica Oliv forest, in the extreme arid region northwest China, was estimated by application of Bowen ratio-energy balance method (BREB) during the growing season in 2005. During the growing season in 2005, total ETc was 446.96mm. From the beginning of growing season, the ETc increased gradually, and reached its maximum value of 6.724mmd-1 in the last fifteen days of June. Hereafter the ETc dropped rapidly, and reached its minimum value of 1.215mmd-1 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 P. euphratica, the value dropped rapidly to the end of growing season. According to this study, the ETc of P. euphratica forest is affected not only by meteorological factors, but by water content in soil.