## 3 Results  
### Land Cover Change  
The land cover maps of the study area in 2001 (static) and 2018 (dynamic) were compared using the MCD12C1 data, as shown in Figure 2a,b below. The main land cover type in the YRB was grassland, followed by cropland. Figure 2c,d shows the grid cells with the changes in land cover types between the two years. In 2001, grassland accounted for 67.5% of the whole basin, cropland accounted for 14.3%, shrubland accounted for 8.0%, and forests (DBF, MF et al.) accounted for 2.3%. In 2018, grassland accounted for 67.3% of the basin, cropland accounted for 20.7%, forests (DBF, MF et al.) accounted for 7.4%, and shrubland accounted for 2.5%. Meanwhile, we found that the changes in land cover types were mainly concentrated in the northwest and the southeast of the YRB (Figure 2c,d). Among all of the changed pixels, grassland accounted for 34.7% in 2001 and 33.4% in 2018, which implies that more grassland pixels have been converted into other types. Cropland accounted for 7.1% of the changed pixels in 2001 and for 41.7% of them in 2018, which implies that more other types of pixels were converted into cropland. We found that the emerging grassland pixels were mainly concentrated in the northwest of the study area, and most of them were converted from other types of land (including barren, cities, etc.) and shrubland. The loss of grasslands was mainly distributed in the southeast of the YRB, and most of them have been converted into cropland. During 2001-2018, the number of shrubland pixels also decreased significantly, and most of them were converted into cropland and forests. The main transfer was characterized by the conversion from other types to grassland in the northwest of the YRB and from grassland and shrubland to cropland in the southeast of the YRB.  
Figure 3 shows the land cover change rates in the eight sub-basins of the YRB. The circular rings represent the proportion of the land cover transition for DBF, SHR, GRA, and CRO. Overall, DBF and SHR decreased slightly, and GRA and CRO decreased significantly in the YRB. DBF decreased at a rate of \(-1.05\times 10^{-3\%}/\mathrm{yr}\) (\(p<0.05\)), and SHR decreased at a rate of \(-4.68\times 10^{-3\%}/\mathrm{yr}\) (\(p<0.05\)). CRO decreased by \(-1.10\times 10^{-3\%}/\mathrm{yr}\), and GRA showed the highest decline rate of \(-2.12\times 10^{-2\%}/\mathrm{yr}\) (\(p<0.05\)) during 2001-2018. The land cover transition varied across different sub-basins. LGL and ALG are situated in the upper reaches of the YRB, located in the Qinghai-Tibet Plateau. The primary land cover type of LGL and ALG is GRA, with a low proportion of land cover pixels undergoing change. The land covers of the LHT, IZ, HTL, and LSG, which are located in the middle reaches of the YRB (i.e., the Loess Plateau), changed significantly. Similarly, the changes in  
pixel proportion in the BH and SGH, which are located in the lower reaches of the YRB (i.e., the North China Plain), were quite dramatic.  
Figure 3: Land cover change rate of sub-basins in YRB according to ESACCI. The colored sections on the spatial map represent the land cover transition types that occurred in 2018 (corresponding to the types in Figure 1 from 2001), and there are no land cover changes in the gray areas. The histograms indicate the frequency of variation tendency for DBF, SHR, GRA, and CRO. The circular rings indicate the proportion of the land cover transition for DBF, SHR, GRA, and CRO.  
Figure 2: The spatial pattern of the land cover types and changes over the Yellow River basin based on the MCD12C1 maps. The panels (**a,b**) show the geographical distribution of the different land cover types in 2001 and 2018, respectively. The panels (**c,d**) show the land cover types of the changing pixels in 2001 and 2018, respectively. Frequency histograms displaying the areal proportions (%) of corresponding differences are inset.