Abstract: Grassland ecosystems are widely distributed in arid area of central Asia,but there are sparse observation sites on water and energy in this region and there have been no deep studies on the evapotranspiration in this region. Particularly observations on the actual evapotranspiration of the complex surface are still very weak and further analysis on the evapotranspiration are helpful for learning the land surface processes in this region. In this paper,based on the data of energy and meteorological which were collected in northern hilly dry steppe of Tianshan Mountains with time step of 1 hour from 2013 to 2015,by means of HL20 Bowen ratio observation systems and the Bowen ratio - energy balance method,the evapotranspiration characteristics of the growing season (April to October)are analyzed systematically. The results shows that as follows: (1)The diurnal variation of the evapotranspiration during the growing season reaches its peak at noon generally,while in the morning and afternoon, the evapotranspiration is low and weak at night. On rainy days the diurnal variation of the evapotranspiration is relatively complex,and the evapotranspiration is usually lower than that on sunny days. Annual average evapotranspiration in the growing season is 353.2 mm from 2013 to 2015. Due to the influence caused by many factors such as precipitation,temperature and vegetation phenology,the evapotranspiration during the growing season aviates sharply with the difference of the years,seasons and months. The condensations during the growing seasons of 2013,2014 and 2015 accounted for 9.7%,18.8% and 16.8% of the total precipitation,respectively. The daily average condensations were 0.177 mm,0.179 mm and 0.316 mm,respectively. Unlike the precipitation,the condensation was relatively stable. (2)Latent heat flux and sensible heat flux occupy the main body of the net radiation energy during the growing season. The variation trends of latent heat flux and sensible heat flux is similar to net radiation's. Overall,latent heat flux will drop slightly before reaching its peak at noon. Generally,latent heat flux is lower than sensible heat flux,however,in the peak season (May-July),latent heat flux may be higher than sensible heat flux except for the years with less available precipitation. The daily variation of latent heat flux and sensible heat flux is closely related to the vegetation growth which mainly depends on phenology and available precipitation. (3)Bowen ratio fluctuates sharply and complexly during the night. In the daytime,Bowen ratio increases from a negative value to a positive one (less than 1)before noon,and then increases to a positive value which is more than 1,Bowen ratio is more than 1 at noon before dropping down again at around 8 p.m. The study has brought about in-depth understanding of evapotranspiration characteristics in the hilly dry steppe of the northern slope of Tianshan Mountains and provided some significant data to the scientific management in this region.

Key words: Bowen ratio, evapotranspiration, latent heat flux, sensible heat flux, soil heat flux