## 3 Results
### Effects of Cropland Expansion on Surface Parameters  
Annual changes of albedo and emissivity in the cropland expansion regions from 2000 to 2015 are shown in Figure 4, respectively. The average albedo and emissivity of cropland, woodland, and grassland in different zones from 2000 to 2015 are shown in Figure 5. Land use change resulted in variable and significant changes of albedo, emissivity, and latent heat flux, as shown by the strong spatial heterogeneity characteristics. The changes of albedo and emissivity in the cropland expansion regions were not only due to the land use change but also other factors, which can have either direct or indirect effects, such as humidity and temperature. The albedo of the three targeted land use classes in northern China was between 0.10 and 0.25. The highest albedo of cropland was in arid NWC, and it amounted to approximately 0.196, whereas the lowest was in humid NC, where it only amounted to approximately 0.159, respectively. The largest albedo of forest was 0.170 in arid NWC, whereas the smallest was only 0.111 in humid NC. The albedo of grassland was between 0.229 in semiarid IM and 0.134 in humid NC. The emissivity of the three targeted land use classes in northern China was between 0.967 and 0.973. The comparative values of emissivity of cropland and grassland were in the order of NEC > NC > IM > NWC. The emissivity values of cropland and grassland in arid NEC and humid NWC were the largest, and the values were approximately 0.9722 and 0.9721, respectively, whereas those of humid NWC were the smallest and the values were only approximately 0.970 and 0.967, respectively. The largest emissivity of woodland was detected in semiarid IM, and the value was approximately 0.971; the smallest emissivity of woodland was in arid NWC, and the value was only approximately 0.967. In other words, compared with the other zones, the albedo of the three land use classes in humid NC was always the smallest, and the smallest emissivity of the three land use classes was always in arid NWC. Because of the higher vegetation cover, the albedo in the humid regions was usually lower than the amid regions. Albedo changes due to cropland expansion are shown in Figure 6a. In northern China, cropland expansion resulted in a general increase of albedo. Spatially, the increase of albedo was mainly distributed in the middle and east of NEC and the west of NWC. The increase varied between 0 and 0.04 in different zones. The largest increase was in humid NC, and the increase was approximately 0.037. Cropland expansion from different land use classes resulted in variable amounts of albedo increase. Cropland expansion from woodland increased the albedo in all of the zones; the albedo increase was between 0.02 and 0.04, with not much difference between the zones. However, the change of albedo due to cropland expansion from grassland was significantly different between the zones. Except for humid NC and humid NEC, albedo decreased in the other two zones. This was because cropland albedo was undoubtedly larger than woodland albedo, but the gap  
Figure 3: Area of cropland expansion in different climate zones of northern China. between cropland albedo and grassland albedo was small. In the humid regions, cropland albedo was larger than grassland albedo, and the opposite was true in the arid regions. Emissivity changes due to cropland expansion are also shown in Figure 6b. In northern China, cropland expansion caused the emissivity to increase in general. Spatially, the increase of emissivity was mainly distributed in NEC and NWC. While the increase varied in different zones, it was between 0 and 0.003. Cropland expansion resulted in the largest increase in arid NWC, and the increase was approximately 0.0025. Cropland  
Figure 4: Annual changes of albedo (**a**) and emissivity (**b**) in the cropland expansion regions during 2000–2015. Figure 5: Average albedo (**a**) and emissivity (**b**) of each land use class in different zones.