# Evapotranspiration in Dynamic of Highland Barley Using PML ET Product in Tibet
## 3 Results and Discussion
### Changing Trend of ET Components during 2001-2020  
The variations in ET components and their ratios during 2001-2020 are shown in Figure 4. On average, ET of highland barley presented a decreasing trend with the rate of \(-3.79\) mm/y (Figure 4g), while it increased significantly (\(p<0.05\)) from 2001 to 2013 with the rate of 8.03 mm/y, and then sharply decreased (\(p<0.01\)) until 2020, with the rate of \(-25.61\) mm/y. ET did not have a universal trend at different altitude zones; it increased slightly at 3500-3600 and 3700-3800 m altitude zones, and a decreasing trend was observed for other altitude zones, with the most prominent changes at the 4000-4100 m altitude zone. Es decreased significantly (\(p<0.05\)) with the rate of \(-4.88\) mm/y (Figure 4a), while it increased significantly (\(p<0.05\)) from 2001 to 2013 with the rate of 5.36 mm/y, and then sharply decreased (\(p<0.01\)) until 2020, with the rate of \(-26.39\) mm/y. The changing trend was similar to that of ET. Ec at different altitude zones presented a similar increasing trend during 2001-2020 (Figure 4c), with larger fluctuation occurring at 3700-3800 m and 3500-3600 m altitude zones, while Ei (Figure 4e) generally exhibited a similar trend to Ec. The Es/ET ratio decreased significantly (\(p<0.05\)) with the rate of \(-0.44\%\)/y during 2001-2020 (Figure 4b), resulting in an 8.34% decrease in the Es/ET ratio. The decreasing trend was more pronounced after 2010. The Ec/ET ratio (Figure 4d) generally showed an opposite trend to the Es/ET ratio. It increased significantly (\(p<0.05\)) with the rate of \(0.43\%\)/y, resulting in an 8.08% increase in the Ec/ET ratio; a sharply increased trend (\(p<0.01\)) was observed during 2010-2020. The Ei/ET ratio (Figure 4f) at different altitude zones had a similar changing trend, i.e., it slightly decreased during 2001-2010 and then changed to a slightly increasing trend during 2010-2020. Many studies have been conducted to investigate the interannual variations in ET components over regional scales. [PERSON] et al. [65] used the improved SWH model to partition ET and analyzed the temporal variations in ET components in the Yellow River Basin. The results suggested that ET, Ec, and Es were significantly elevated during 1981-2010, and the air temperature was the most important factor affecting the variations. This was consistent with previous findings [66; 67]. The increasing trend in ET components was also reported for the other regions [50; 68]. Some studies revealed the increase in ET over \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Meteorological** & **Es** & **Ec** & **Ei** & **ET** & **Es/ET** & **Ec/ET** & **Ei/ET** \\
**Variables** & & & & & & & & \\ \hline SD & \(-0.417\) & \(-0.679\) * & \(-0.756\) ** & \(-0.501\) & \(0.437\) & \(-0.378\) & \(-0.713\) * \\ AP & \(0.524\) & \(0.563\) * & \(0.519\) & \(0.548\) * & \(0.187\) & \(-0.224\) & \(0.448\) \\ T & \(0.976\) ** & \(0.845\) * & \(0.801\) ** & \(0.965\) ** & \(0.413\) & \(-0.477\) & \(0.752\) ** \\ WV & \(0.262\) & \(0.271\) & \(0.217\) & \(0.270\) & \(0.022\) & \(-0.040\) & \(0.224\) \\ RH & \(0.929\) ** & \(0.969\) ** & \(0.949\) ** & \(0.964\) ** & \(0.087\) & \(-0.161\) & \(0.881\) ** \\ P & \(0.856\) ** & \(0.935\) ** & \(0.953\) ** & \(0.901\) ** & \(-0.065\) & \(-0.007\) & \(0.86\) ** \\ \hline \hline \end{tabular}  
* Significant at \(0.05\) significance level; ** Significant at \(0.01\) significance level.  
\end{table} 
Table 2: Correlation coefficients between the seasonal variations in ET components and meteorological variables. the Tibet Plateau consequent to global warming [69; 70]. For example, [PERSON] et al. [36] estimated the ET components using the PML model and found that Es, Ec, and Ei exhibited an increasing trend during 1982-2012. The ET estimated from the water balance model also presented an increasing trend for 16 catchments [71; 72]. This phenomenon of increasing trends had also been observed in Mediterranean-climate regions during almost the same period [73].