## 3 Results  
When the NDVI of GF-4 is taken as the input of SEBAL model, the spatial resolution of ET is 50 m, while when NDVI of MODIS is taken as the input of SEBAL model, the spatial resolution of ET is 250 m. Figures 2-4 were the comparison of ET at 11:36 (T\({}_{8}\)) of GF-4 and ET at 11:35 (MT\({}_{0}\)) of MODIS in CB, CF and CG regions, respectively. Figure 2: ET of GF-4 at 11:36(**a**) and ET of MODIS at 11:35 (**b**) in area CB. As seen in Figures 2-4, since the spatial resolution of NDVI of GF-4 is higher than that of MODIS, the derived texture of ET is more obvious than that from MODIS. In order to cross-verify ET obtained from MODIS and GF-4, the bilinear method was used and the GF-4 ET data were resampled to 250 m, which was identical to the spatial resolution of MODIS. As shown in Figure 5, 420 total sampling points were randomly selected in areas CB, CG and CF (Figure 5a), and 870 total sampling points were randomly selected in areas KB, KE and KD (Figure 5b) to find variability between the two ETs. Meanwhile we verified the simulated ET with the measured ET data of KoFlux as shown in Figure 6.  
Figure 4: ET of GF-4 at 11:36 (**a**) and ET of MODIS at 11:35(**b**) in area CG. Figure 3: ET of GF-4 at 11:36(**a**) and ET of MODIS at 11:35 (**b**) in area CF. As can be seen from Figure 5, most ET simulation values of MODIS are less than that of GF-4's, but there is a strong correlation between the ET derived from GF-4 and MODIS, R = 0.581 (\(p<0.001\)) in areas CB, CG and CF, and R = 0.810 (\(p<0.001\)) in areas KB, KE and KD. While the difference in imaging time of MODIS and that of GF-4 is 1 to 5 min, it is certain that the ETs derived from both satellites have a significant linear correlation at the same imaging time. In addition, as can be seen from Figure 6, the difference between the simulated and measured values of ET was not obvious. The following two methods were used to verify the simulated ET. Firstly, we used the field measured data of KoFlux to verify the simulated ET of MODIS and GF-4. Then, we took the ET calculated by MODIS as the measured value, and the ET calculated by GF-4 as the simulated value. The verification results were shown in Table 5.  
Figure 5: Cross-validation of simulated ET of GF-4 and MODIS; (**a**) ET at 11:35 of MODIS and 11:36 of GF-4 in areas CB,CG and CF; (**b**)ET at 13:45 of MODIS and 13:50 of GF-4 in areas KB,KE and KD). Figure 6: Comparison of simulated and measured ET at KoFlux Sites. (Black squares are ET values simulated by GF4, while black triangles are ET values simulated by MODIS). As can be seen from Figure 6 and Table 5, RMSE = 1.04 (mm/day) and the MRE was 16.95% when validating simulated ET values using KoFlux data, which showed that it was feasible to simulate ET using the methods in this paper, while most GF-4 simulated ET values were higher than those of MODIS's, the MRE of ET when validating GF-4 using MODIS was less than 50%. In order to further analyze the variation of ET in all study areas at time T\({}_{1}\)-T\({}_{15}\) in areas CB, CG and CF, T\({}_{1}^{\prime}\) - T\({}_{2}^{\prime}\)in areas KB, KE and KD, the minimum, maximum, and average values, and the standard error of ET at above time was calculated by ArcMAP 10.3 for all pixels, as shown in Figures 7 and 8. As shown in Figure 7, the trends of the average, minimum, maximum and average ET in different study areas have significant differences, but the minimum ET in areas CF at time T\({}_{1}\)-T\({}_{15}\) does not. The minimum ET does not vary in study area at time T\({}_{1}\)-T\({}_{15}\), and the pixel with ET = 0 always exists. As shown in Figures 7 and 8, the maximum value trend in all areas is the same as that of the average values, while the fluctuation in trend of the maximum and minimum value in all areas is different. This is mainly due to the difference among soil types and meteorological conditions in the six areas, causing the impacts on ET to vary [3; 45].