Abstract: Evapotranspiration is a key factor in the exchange of material and energy between the land surface and the atmosphere. Accurate estimation of evapotranspiration is essential for formulating reasonable irrigation plans and improving water use efficiency. In this study, summer maize in the North China Plain in 2017 and 2018 was used as the research object. Based on the observation data of eddy covariance system and automatic weather station, the variation of evapotranspiration（ET）of summer maize and its influencing factors at different time scales were discussed. The results showed that the hourly scale ET showed a ‘几’ shape change, with a maximum value between 12:00-14:00 and a minimum value around 0:00. The ET values of heading-filling stage and filling-milk ripening stage were larger than that of other growth stages, and the ET value of sowing-emergence stage was the smallest（0.031 5 mm and 0.021 5 mm, respectively）. On the daily scale, with the growth of summer maize, ET increased first and then decreased, and the maximum value appeared at the jointing-heading stage（the average values were 3.7 mm and 4.5 mm, respectively）. The minimum daily mean value of the whole growth period was sowing- emergence stage（the average values were 2.72 mm and 1.86 mm, respectively）. On the monthly scale, the maximum value of ET appeared in August（125.10 mm and 117.80 mm respectively）, and the minimum value appeared in June（50.08 mm and 34.04 mm respectively）. The correlation between ET and average sunlight and effective radiation was the largest, and the determination coefficient R² was 0.270 4. The second was the daily average air temperature, with R² 0.074 2, and the correlation between average daily air relative humidity and ET was the smallest, with the coefficient of determination R² 0.020 7. The results of this study will provide a scientific basis for water management of farmland underlying surface.

Key words: North China Plain, summer maize, evapotranspiration, different time scales, eddy covariance system