# Evapotranspiration in Dynamic of Highland Barley Using PML ET Product in Tibet
## 3 Results and Discussion  
### ET Components of Highland Barley in Tibet  
The annual mean ET of highland barley was 506.9 mm in Tibet, and the Es, Ec, and Ei were 391.9 mm, 107.6 mm, and 7.4 mm, respectively, which accounted for 77.3%, 21.2%, and 1.5% of total ET, respectively. This result suggests that Es was the most important ET component for highland barley in Tibet, and most of the water was lost through soil evaporation, which was similar to the results in arid climate regions [5; 26; 39]. The ET of highland barley was similar to that of spring maize [40] and winter wheat [4] in northern China, yet higher than that of corn under microirrigation systems in a semiarid environment [28]. However, large differences in the ET components varied widely among different crops and regions. Observations showed that Es, Ec, and Ei ranged from 82.9 mm and 98 mm, from 217.2 mm to 304.1 mm, and from 11 mm to 16 mm for wheat in northern China, respectively [4]. The measured Es and Ec were approximately 183 mm-205 mm and 172-185 mm for winter wheat in Syria, respectively [25]. The Ec was up to 469-475 mm in a multiched agriculture ecosystem [40]. These results suggest that highland barley generally had much lower Ec and Ei but much higher Es than those of other crops such as wheat and corn, which was similar to the results observed in arid climate regions [5; 28].  
Numerous studies showed that the proportion of ET components differed greatly across different crops and from region to region. Previous studies reported that Ec accounted for 20-80% of ET for row crops [41; 42]. [PERSON] et al. [43] found that the Ec/ET ratio varied substantially between 0.2 and 1.0 for a rice paddy field during the growing season in Mase, Tsukuba. [PERSON] et al. [27] observed the ET components of winter wheat in a semiarid region in Morocco, with the average Ec/ET ratio of 0.69-0.8. Many observations also revealed obvious differences in Ec/ET ratios across China [44]. For example, the Ec/ET ratio varied widely from 0.52 to 0.96 for maize in northwestern China [45; 46], which was very similar to the range between 0.51 and 0.98 for winter wheat in Beijing [2], and slightly higher than the Ec/ET ratio from 0.46 to 0.74 for winter wheat in North China Plain [47]. The Ec/ET ratios of these crops were much higher than that for highland barley in Tibet, generally indicating that highland barley has a relatively low transpiration rate in Tibet properly due to the stress from the extreme climate. Compared with Ec and Es, Ei accounted for only 1.5% of highland barley ET in Tibet, while Ei was an important component of the agricultural soil-plant-atmosphere continuum, regulating the reallocation of rainfall reaching the crop canopy [26]. Some observations and simulations demonstrated that Ei can account for a large proportion of ET [6]. For example, [PERSON] et al. [4] found that Ei can amount to 14-15% of ET for wheat in northern China, which was much higher than that of highland barley in Tibet. However, many studies on ET partitioning often neglected this component [4; 29].  
ET components varied obviously over the altitude. As shown in Figure 2, ET generally decreased with the increase in altitude, with higher ET observed at 3700-3800 m altitude zone (Figure 2g). Es and Es/ET ratio showed a decreasing trend with the increase in altitude from 3500 m to 3800 m and then changed to an increasing trend until reaching the altitude of 4100 m (Figure 2c,d), while Ec, Ei, and their ratios presented an opposite changing pattern to Es (Figure 2a,b,e,f).  
Figure 2: Relationship between altitude and Es (**a**), Es/ET (**b**), Ec (**c**), Ec/ET (**d**), Ei (**e**), Ei/ET (**f**), and ET (**g**).