## 4 Discussion
### Limitations and Prospects  
The surface radiative energy balance changed where the land use changes accrued. The changes of surface radiative energy balance in the area with the land transition cannot be attributed to the driving of land use change, it is also impacted by other factors such as climate change and variability. The effects of land use change can not be distinguished by only estimating the change of surface radiation energy balance in the area of land use change. Thus, in our paper, we chose a space-for-time approach to estimate the effects of cropland expansion [2; 47; 48]. The impacts of cropland expansion on the biogeophysical parameters and surface radiative energy balance are the difference between the biogeochemical parameters and surface radiative energy for the \"actual landscapes\" and \"potentially converted\" landuse types, but not the change of those in the area where cropland expansion occurred. Although there are still some uncertainties in this method [2; 47; 48], the influence of other factors, such as climate change and variability, can be excluded to the greatest extent.  
This paper was based on remote sensing data and it aimed to better describe the spatial difference in biophysical properties due to land cropland expansion at the regional scale instead of the larger global scale or the smaller local-scale. The results were also more realistic than the model simulations. However, the result still needs to be verified further by the field observation data. They can provide guidance on farm management measures related to climate change issues in large crop areas such as the North China Plain and the Northeast China Plain. For example, cropland expansion from grassland in North China causes the heat to the atmosphere to increase, which has a warming effect. From the aspect of climate change, it would not be a wise policy to promote cropland expansion from grassland in North China. However, only natural factors and parameters were considered in this study. Without consideration of the impacts of different tillage methods, the results of this study will remain relatively coarse. Therefore, future studies should attempt to provide deeper analyses by taking into account the farm management measures.  
Changes in heat flux from the surface to the atmosphere due to cropland expansion had a significant spatial distribution pattern, but the values were not significantly correlated with the area of cropland expansion. Cropland expansion alters biophysical characteristics, including not only albedo, emissivity, and latent heat fluxes but also canopy conductance, soil moisture, surface roughness, leaf area, and rooting depth. All of those factors may have played a role in changing the heat flux to the atmosphere, and they deserve more consideration in future studies.  
Due to the difference of spatial and temporal resolution of data sources, we analysised the surface radiative energy balance based on statistical results in coarse resolution, and there are some uncertainties. In addition, the background state may change due to the change of large-scale radiation forcing and climate feedback [46]. Limited by the availability of observation data, it can not be well verified at present. The change of radiative energy and heat fluxes on the unchanged land use types was influenced by both the natural and human factors. It is difficult to distinguish the factors between climate change and human factors. The crops in the new cropland and old permanent cropland are usually similar. Under human management, crop growth was slightly different due to the influence of soil fertility. The surface heat flux to the atmosphere at local scales may be with little difference between the new cropland and old permanent cropland. However, it may be significantly different at site scales. All the limitations deserve more consideration in future studies.