Actual and Reference Evapotranspiration in a Cornfield in the Zhangye Oasis, Northwestern China

Evapotranspiration (ET) is an important component of the surface energy balance and water cycle, especially in arid and semiarid regions. The characteristics of the actual evapotranspiration (ETa), which was calculated using the eddy covariance method, and the reference evapotranspiration (ET0), which was estimated using the Food and Agriculture Organisation (FAO) Penman–Monteith method, were analysed. This work focussed on the seasonal variations in evapotranspiration and crop coefficient (Kc) above the heterogeneous canopy of an arid oasis ecosystem in a cornfield of the Zhangye oasis in northwestern China. The results showed that in 2008, the total net radiation (Rn) was 2457.73 MJ∙m−2 and that the rainfall was 117 mm. The average wind velocity, air temperature, and specific humidity, which were observed 2 m above the ground surface, were 1.23 m∙s−1, 7.07 °C, and 3.66 g∙kg−1, respectively. The total ETa and ET0 were 654.69 mm and 1039.92 mm, respectively; thus, the ET0 was higher than the ETa. The difference between the ET0 and ETa was high in summer and autumn, and low in winter and spring. The ETa was greatly influenced by irrigation events, whereas the ET0 was not influenced by irrigation. The ETa and ET0 were both greatly influenced by meteorological elements. The Kc values were less than 0.5 outside of the maize-growing stage and greater than 0.5 during the entire maize-growing stage (from 20 April to 22 September 2008). The Kc values were 0.63, 0.75, 0.78, 0.76, 0.61 and 0.71 at the seedling, shooting, heading, filling, and maturity stages and the entire growth stage, respectively.