## 3 Results
### Effects of Vegetation Changes on Evapotranspiration  
Figure 9a shows the interannual variation in ET under vegetation-change and no-change scenarios. From 2000 to 2018, the ET in the vegetation-change and no-change scenarios in the HLS showed an overall increasing trend, with an increase rate of 1.33 mm/a and 0.569 mm/a, respectively. Specifically, ET in both cases was at a relatively low level (360 mm) in 2001, 2004-2011, 2015, and 2017. The ET difference between the vegetation change and the vegetation unchanged scenario is in a state of continuous increase (0.75 mm/a) on the whole, with an average annual difference of 3.95 mm, and the maximum difference was 14.51 mm in 2015. In 2000-2003 and 2006, the ET under the vegetation-change scenario was less than the ET under the vegetation-unchanged scenario. From 2000 to 2018, Et under the scenario of vegetation change in the HLS decreased year by year, with a decrease rate of 2.52 mm/a. In contrast, Et under the scenario of unchanged vegetation showed a relatively slow growth trend (0.48 mm/a). The difference between Et in the vegetation-change and vegetation-constant scenario is in a state of continuous decrease (3.00 mm/a), and Et in the vegetation-change scenario is always smaller than Et under the vegetation-constant scenario. Under the scenario of vegetation change, the HLS experienced varying levels of increase in Ei and Es, with growth rates of 0.87 and 2.99 mm/a, respectively. However, from the time series of the two changes, Ei peaked in 2012 and then decreased, while Es remained at a high level after 2012. In the case of constant vegetation, Ei and Es in the HLS did not change much as a whole, and the growth rate was about 0.04 mm/a. During the study period, the Ei difference between the vegetation-change and the vegetation-constant scenario generally showed an increasing state (0.83 mm/a), but the fluctuation was large, especially after 2011. Compared with Ei, the increase state of Es was more stable (R\({}^{2}\) = 0.88527), and the increase rate reached 2.92 mm/a. Figure 10 shows the spatial distribution of the average ET and the difference between them in the HLS simulated by the PT-JPL model in vegetation-change and no-change scenarios. Figure 10a shows that in the vegetation-change scenario, the average ET in the HLS showed a gradual increase from north to south during 2000-2018, showing significant  
Figure 9: Effect of vegetation greening on the HLS ET. (**a**–**d**) The green line represents the regional mean annual values for actual ET, the orange line represents the regional mean annual values of ET for the vegetation-neutral scenario, and the dashed line represents their trends. (**e**–**h**) Difference between the annual values of ET for the vegetation-variable and vegetation-invariable scenarios, with the dashed line indicating their trends. spatial heterogeneity. In this scenario, the average ET in the southern region of the HLS is higher, reaching 668.5 mm, while ET in the northern and central regions is lower. The reason for this difference is that the land use type in the southern region of the HLS is dominated by forest land. Forest land has high vegetation coverage and strong transpiration, which can significantly increase water vapor transport through canopy interception and transpiration processes, resulting in an ET in the southern forest area of the HLS higher than that in the northern and central grassland areas. Figure 10b illustrates the spatial distribution of average ET in the HLS from 2000 to 2018 under a constant-vegetation scenario. Comparable to the ET variation under the vegetation-change scenario, the multi-year average ET under the vegetation-unchanged scenario exhibited a decreasing trend from south to north. In this scenario, the average ET in the southern region remains higher, though slightly lower than the value under the vegetation change scenario, at 663.8 mm. ET in the northern and central regions also remained low.