## 4 Results  
### Validation of Modeled Daily ET with Ground Observations, PML_V2 ET, and SEBAL ET  
The daily simulation outputs of TSEB-SM ET, PML_V2 ET, and SEBAL ET at Arou, Daman, Huazhaizi, and Sidaoqiao were compared with ground-based observations from 2014 to 2018, as shown in Figures 2 and 3. Figure 2 depicts the scatterplot of the three ET products compared to ground-based observations, and the validation metrics are shown in Table 1. Figure 3 illustrates that the products, which are derived from three distinct models, can all represent the intra-year single-peak trend of surface ET at the four ground observation sites. This trend displays ET at its lowest point at the beginning of the year, rising gradually with warmer temperatures and more precipitation until peaking in July and August. After September, when the crops are harvested and the leaves begin to wither, ET gradually declines until it reaches its lowest point at the end of the year, when it cycles again in the following year. However, the accuracy of the three products varied over the four sites. At the Arou site, the subsurface is dominated by alpine meadows with abundant rainfall, and the surface evapotranspiration fluctuates, with a high and pronounced peak that can reach up to 6 mm/d. The TSEB-SM ET product (R\({}^{2}\) of 0.79, RMSE of 0.76) was able to accurately capture the peak in comparison to the other two ET products. On the other hand, both the PML-V2 ET (R\({}^{2}\) of 0.76, RMSE of 0.78) and SEBAL ET (R\({}^{2}\) of 0.22, RMSE of 0.88) products displayed some underestimation, which was more noticeable for SEBAL ET, particularly during the middle of the growing season, when it occurred more frequently. But from January to March in the beginning of the year, SEBAL ET was overestimated. Furthermore, a number of 0 values in SEBAL ET did not match the real circumstances. In general, the SEBAL ET product was not very reliable at the Arou site, where there is a lot of rainfall, while the TSEB-SM ET product matched the ground-based data better. Figure 3: Temporal variations in the TSEB-SM ET, PML-V2 ET, and SEBAL ET (mm/d) against the observations for Arou (**a**), Huazhaizi (**b**), Daman (**c**), and Sidaoqiao (**d**). Figure 2: Scatterplot of TSEB-SM ET, PML-V2 ET, and SEBAL ET compared with ground observations. The Huazhaizi site is situated in a desert area with irregular rainfall and low surface evapotranspiration, peaking at 4 mm/d. While the PML-V2 ET product underestimated the surface observations in 2017, it matched the surface observations to a greater extent in 2014, 2015, and 2016, and the TSEB-SM ET product better represented the real variability in surface ET at the Huazhaizi site. In the midst of the growing season, the SEBAL ET was more noticeable and still exhibited some values of 0, continuing to be overstated from January to March. The validation metrics are listed in Table 1.  
The Daman site is in a highly irrigated agriculture area with great crop growth; therefore, its surface evapotranspiration fluctuates widely during the year, peaking at roughly 8 mm/d. The trajectory of the TSEB-SM ET product was the closest to that of the site observations (R\({}^{2}\) of 0.78, RMSE of 0.57), and it also had a peak at about 8 mm/d. PML-V2 ET followed the same trend as the site measurements to some extent (R\({}^{2}\) of 0.73, RMSE of 0.62), but it peaked significantly lower, at roughly 6 mm/d. During the growing season, SEBAL ET indicated a significant underestimate (R\({}^{2}\) of 0.34, RMSE of 0.74), with peak values of barely 4 mm/d. The TSEB-SM model adequately captured the large amount of water loss from the surface of the irrigated area during the early part of the growing season, allowing for a rapid response during that time and avoiding underestimation. The Sidaoja site is located in the downstream riparian forest, and the surface ET intensity was moderate, with a peak around 6 mm/d.