## 4 Discussion  
While this paper compared ETs derived from GF-4 and MODIS using the SEBAL model, the extent of the differences from SEBAL model ET input data needs further analysis. The reason for the difference in ET simulation results between the two sensors needs further discussion, and the validation of ET simulation results requires careful consideration as well:  
(1) Errors in SEBAL model input data lead to a decrease in ET simulation accuracy. However, in many areas, simulation is limited by available data, and often, the wind speed, LST, air temperature, and other data cannot be attained. While simulating the above parameters using the present model, various kinds of error must exist. If the above meteorological data could be replaced by observed data, the model accuracy could be significantly improved. In addition, the _ESUN_ of GF-4 is not published yet, and the subsequent _ESLIN_ simulation that was used has errors which affected both the calculation of NDVI and the simulated values of ET. (2) Besides the errors associated with SEBAL input data, the dependence on remote sensing data can lead to different ETs derived from the GF-4 and MODIS sensors. Many researchers have pointed out that the local variables of the SEBAL model would change with the scope of the remote sensing image, which could lead to additional uncertainty associated with the domain dependence of the remote sensing model accuracy [7; 28; 34; 48]. On the other hand, remote sensing images with different spatial resolutions would also cause differences in local variables, and thus could lead to completely different dry-wet conditions, dependent upon the resolution of the remote sensing models. For all established models based on remote sensing images, the model outputs may have uncertainties if the determination of variables or input parameters depends on the scope and resolution of those images. (3) The objective of this paper was not to validate whether the GF-4 or MODIS remote sensor has higher accuracy in ET retrieval, but to quantify the similarities and differences between ETs derived from both satellites by comparing their retrievals. It is clear that more efficient ground observations are essential to validate model results. Currently, widely used ET remote sensing model validation techniques include the lysimeter method, the field water balance method, the Bowen ratio method, the eddy covariance method and the large aperture scintillator method [23; 49; 50; 51]. However, in practical operation, the spatio-temporal scale of the observed data does not always match with that of the model retrievals. Therefore, we can only use KoFlux data with 2-10 minutes difference in remote sensing images for verification. The GF-4 is only the beginning of high orbit and high temporal-spatial resolution satellite technology, and as more research and data are gathered, new observational approaches will emerge. (4)It is also necessary to consider cross-validation using remote sensing data with higher spatial resolution. We tried to verify GF-4 imaged at 11:14 in regions CB, CG and CF by using Landsat 8 imaged at 10:29. Meanwhile in regions KB, KE and KD, Landsat 8 imaged at 10:04 was used to verify GF-4 imaged at 11:14, the verification results of ET were shown in Table 6.  
As can be seen from Table 6, when the simulated ET of GF-4 was verified by Landsat 8, the MRE of ET of GF-4 was very large. The imaging time of Landsat 8 was 44 min earlier than that of GF-4 in \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Region** & **Data of Verification** & **MRE (\%)** & **RMSE (mm/min)** \\ \hline CB, CG and CF & Landsat 8 & 79.21 & 0.00462 \\ KB, KE and KD & Landsat8 & 190.36 & 0.00383 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Using Landsat 8 data to cross-verify the simulated ET of GF-4. regions CB, CG and CF, and the imaging time of Landsat 8 was 70 min earlier than that of GF-4 in region KB, KE and KD. With such a long time difference, we think that the results of Table 6 can not accurately represent the actual accuracy of GF-4.