## 3 Results
### The Effects of ET Variations on Irrigation Estimation  
Figure 7 shows the monthly and annual irrigation water estimated using the five ET products combining the AWAP precipitation and runoff data in irrigated area in the MRC. There are significant differences between them, and the average values of annual irrigation water estimated using the AWAP, CSIRO, GLDAS, MODIS, and TerraClimate ET products are 87.0 mm, 114.2 mm, 154.6 mm, 115.7 mm, and 49.1 mm, respectively. The maximal one, estimated using MODIS ET, is more than  
Figure 5: The yearly runoff estimations using different ET products.  
Figure 6: The yearly water storage changes estimated using different ET products.  
three times of the minimal one estimated using TerraClimate ET. Compared with the annual irrigation water estimated using the AWAP, the CSIRO's are larger except in 2001, the GLDAS's are larger, the MODIS's are larger in most years, and the TerraClimate's are less except in 2010 and 2011. Although the irrigation water estimated using CSIRO and MODIS ET are close, they are different both over monthly and annual scales. The maximum values of the annual irrigation water using the AWAP, CSIRO, and MODIS ET products appeared in 2006 and were 118.8 mm, 171.8 mm, and 196.6 mm, respectively, while the maximum values using the GLDAS and TerraClimate ET products appeared in 2011 and were 256.5 mm and 110.9 mm, respectively.  
The Effects of ET Variations on the Gross Primary Productivity (GPP) and Water-Use Efficiency of the Grassland and Forest  
The annual GPPs range is from 976.8 g C m\({}^{-2}\) to 1279.5 g C m\({}^{-2}\) for forest and 397.0 g C m\({}^{-2}\) to 719.6 g C m\({}^{-2}\) for grassland, and the annual ETs range is from 384.8 mm to 766.6 mm for forest and 118.0 to 618.8 mm for grassland (Figure 8a,c). The points representing different ET products overlap each other except for the MODIS ET for forest (Figure 8a), which indicates the ET products are close. However, the points representing GLDAS ET and MODIS ET are located at the left and right respectively, and the other three ET products are located in the middle and have some overlap for grassland, which reflects the obvious differences among the ET products (Figure 8c). There are significant quadratic relationships between the yearly GPP and all ET for forest and grassland, but the quadratic functions are different from each other (Figure 8a,c). For forest, the mean water use efficiency estimated using AWAP, CSIRO, GLDAS, and TerraClimate ET are close and about 1.9 kg C m\({}^{-3}\) H\({}_{2}\)O, while the water-use efficiency estimated using MODIS ET is up to 2.3 kg C m\({}^{-3}\) H\({}_{2}\)O (Figure 8b). For grassland, the mean water use efficiency estimated using AWAP, CSIRO, GLDAS, MODIS, and TerraClimate ET are 1.5 kg C m\({}^{-3}\) H\({}_{2}\)O, 1.5 kg C m\({}^{-3}\) H\({}_{2}\)O, 1.2 kg C m\({}^{-3}\) H\({}_{2}\)O, 2.4 kg C m\({}^{-3}\) H\({}_{2}\)O, and  
Figure 7: The monthly (**a**) and annual (**b**) irrigation water estimated using different ET products.  
1.6 kg C m\({}^{-3}\) H\({}_{2}\)O (Figure 8d). The ranges of the annual water-use efficiency estimated using AWAP, CSIRO, MODIS, and TerraClimate ET for forest are less than those for grassland, and the ranges using GLDAS ET are the same.