## 3 Results
### Performance Based on Inter-Comparison
A general negative spatial correlation between PML-V2 and BESS can be observed, especially in the upper reach, the Korat Plateau and the Chuor Phnum Dangrek. Figure 7a shows comparisons of the annual ET of the four products over the entire basin in different land cover types. The four products show a comparable annual ET range of needleleaf forests, shrubs and grasslands with mean yearly ET of 717.2 mm (Figure 7(a3)), 1054.0 mm (Figure 7(a4)) and 698.9 mm (Figure 7(a6)), respectively. The annual ET range of MOD16 and PML-V2 is comparable in evergreen broadleaf forests with a mean yearly ET of around 1200 mm, while those of BESS and GLASS are much lower and higher, with their mean yearly ET of 1019.0 mm and 1359.2 mm, respectively (Figure 7(a1)). Specifically, BESS is 177.4 mm lower than the average of the four products, while GLASS is 162.8 mm higher than the average of the four products. In addition, the annual ET ranges of MOD16, BESS and GLASS are comparable in deciduous broadleaf forests with a mean yearly ET of around 1150 mm, while that of PML-V2 is much lower (Figure 7(a2)). Specifically, PML-V2 is 175.78 mm lower than the average of the four products. A large discrepancy in the annual ET range between MOD16 and GLASS on croplands can also be found (Figure 7(a5)). The mean yearly ET of PML-V2 and GLASS on cropland are around 1040 mm and are comparable, though the annual ET range of the former is much broader than the latter (Figure 7(a5)). Figure 4: Spatial ET of multi-year average of the four products. (**a1**), (**a2**), (**a3**) and (**a4**) are multi-year average ET of MOD16, PML-V2, BESS and GLASS respectively. Figure 5: Spatial ET of multi-year seasonal average of the four products. (**a**), (**b**), (**c**) and (**d**) represent spring, summer, autumn and winter respectively, and 1, 2, 3, 4 represent MOD16, PML-V2, BESS and GLASS respectively. The average yearly ET of the four products over the entire basin is around 1000 mm during the studied period (Figure 7b). The annual ET difference between MOD16, PML-V2 and BESS is relatively small, while the annual ET difference between GLASS and the other three products is significant. Specifically, the annual ET of the entire basin from GLASS fluctuates around 1100 mm during the period of 2001-2015, which is around 150 to 200 mm higher than that from the other three products (Figure 7b). The annual ET of the four products over the entire basin shows an increasing trend, with 3.8 mm per year (Figure 7b). Significant differences can be found in the trends of the annual ET of the four products. MOD16 and PML-V2 show significantly increasing trends (at a confidence level of \(\alpha=0.1\)), while BESS and GLASS do not show significant trends. PML-V2 increases by about 261.4 mm in 15 years (9.46 mm/year), and MOD16 increases by 83.69 mm (16.74 mm/year) during 2001-2015. It should be noted that the linear trends of BESS and GLASS are not presented in Figure 7b, as their Spearman's rank correlation coefficients are not significant; namely their linear trends are not reliable. Figure 6: Spatial correlation of annual evapotranspiration between MODIS and PML-V2 (**a**), MOD16 and GLASS (**b**), MOD16 and BESS (**c**), PML-V2 and GLASS (**d**), PML-V2 and BESS (**e**), and between GLASS and BESS (**f**) over the entire basin.