## 3 Results  
### Future Spatial-Temporal Land Use Changes in Beijing  
During 2000-2020, the land use type of urban agglomeration was mainly forest, which accounted for more than 40% of the entire area, followed by cropland and impervious surface. Water body, wetland and bare area accounted for only a small portion of the study area (Figure 4). The total area of grassland and cropland decreased, among which the area of cultivated land decreased by 1764.68 km\({}^{2}\) in 20 years, the proportion of grassland decreased by about 2%. The land use of water body first decreased and then rose, because the closer it is to the water, the more suitable it is for production and living, and the easier it is to be exploited and utilized by human beings. The water area of Beijing is mainly distributed in Miyun, and the area around the reservoir was transformed into cropland for planting chestnut, plum and other cash crops in early years. People gradually realized the importance of water body, therefore, the area of water body increased by 87.14 km\({}^{2}\) from 2010 to 2020. In 2010, the construction land area of urban agglomeration was 3475.76 km\({}^{2}\), increasing by 1885.80 km\({}^{2}\) compared with 2000 which tended to show clear changes. The expansion area of impervious surface was mainly concentrated in the urbanization area of urban agglomeration, including Haidian district, Chaoyang District, Daxing District and Tongzhou District.  
The mountain terrain in the northwest region is not suitable for large-scale development, except Yanqing district, which is relatively flat and has more cropland and impervious surface with frequent human activities. Additionally, people's requirements for urban environment have been enhanced with the continuous development of urbanization and the continuous improvement of residents' living standards, so large amounts of mountainous land and farmland have been listed as northwest ecological conservation area. Similarly, cropland is vital. Therefore, land use simulation is divided into the following three scenarios: the baseline scenario, cropland protection scenario and ecological protection scenario.  
Under the baseline scenario in 2030 (Figure 4d), the area of cropland will decrease by 955.04 km\({}^{2}\) compared with 2000 and the area of other land use types will increase. The area of impervious surface, water body, grassland and forest will increase by 862.43 km\({}^{2}\), 48.67 km\({}^{2}\), 23.91 km\({}^{2}\) and 19.07 km\({}^{2}\), respectively, and the area of bare land and wetland will hardly rise. The baseline scenario is the basis for the simulation of urban agglomeration land use change considering other constraints, while the cropland protection scenario is to add the concept of farmland protection to the baseline scenario to prevent the basic farmland from being occupied by other land in the process of urbanization. As a result, the land use under cropland protection scenario (Figure 4e) has a little change than 2020. Specifically, the area of forest will change the most (from 7388.60 km\({}^{2}\) in 2020 to 7382.57 km\({}^{2}\) in 2030) and other land use areas will change by less than 3 km\({}^{2}\) under cropland protection scenario. The simulation results under the ecological protection scenario (Figure 4f) are similar to that under the baseline scenario. The areas of forest, grassland, wetland, impervious surface and bare land will be 7407.67 km\({}^{2}\), 1395.65 km\({}^{2}\), 12.04 km\({}^{2}\), 4338.19 km\({}^{2}\) and 4.89 km\({}^{2}\), respectively, under these two scenarios. Under the ecological protection scenario, more of the area will be converted to water body (288.61 km\({}^{2}\), accounting for approximately 1.76% of the total inundation area), as compared to the baseline scenario in 2030, the area of cropland will reduce by another 1.61 km\({}^{2}\).