Delineating three urban growth boundaries using “Dual-Evaluation” constraints and PLUS Model

Rapid urbanization has exerted the considerable pressure on land resources and infrastructure, leading to urban sprawl. The delineation of urban growth boundaries (UGBs) has been one of the most crucial tools to manage and curb the urban sprawl. The delineation can also be optimized to alleviate the land-use conflicts for the sustainable urban development. The second-largest city in Shaanxi Province, Xianyang City is similar to the most middle-sized cities in China, indicating the rapid development with the land-use conflicts and ecological crises. In this study, according to the "Dual-Evaluation" framework, the ecological protection areas of the high importance, the agricultural production and urban construction suitability areas were delineated.The "top-down" policy constraints were served as the delineation of UGBs. Concurrently, the patch-generating land use simulation (PLUS) model was employed to conduct to meet the resource requirements and development potential in the study area. Future demands were forecasted and simulated for the land use. A "bottom-up" demand-driven approach was used to promote the rational use of land resources in the sustainable development of the environment. The delineation of UGBs was also in line with the local needs of actual development. Additionally, the erosion-dilation test was also employed to remove the noise points from the UGBs, particularly for the smoother and easier to manage. The results showed that: 1) The southern region was predominantly focused on urban development, where the area of 1 368.98 km² was designated as the urban construction. Meanwhile, the northern was prioritized the ecological protection, where the area of 3 853.04 km² was unsuitable for construction. 2) The PLUS model was used to simulate the urban construction land in 2040, which was 133.86 km² under the constraints of suitability evaluation. Compared with the natural development scenario, there was a significant reduction in the unused land area, along with an increase in the areas of arable land, forest land, and construction land, in order to facilitate the efficient use of land. 3) The erosion and dilation were integrated to exclude the unsuitable area for urban construction. The bottom-line UGBs was delineated as 3 863.39 km²; The alternate UGBs was 852.28 km², which was composed of the suitable urban construction zones; The priority UGBs was 134.84 km² in the distribution range of urban construction land by 2040. The triple functional boundaries were established to promote the more compact and continuous urban pattern after morphological adjustments, thus guiding the direction of urban development. This approach can offer a novel perspective and scientific rationale to delineate the UGBs in the urbanization, in order to balance the development and protection. The conflicts among urban development, farmland protection and ecological conservation can also be determined to enable the coordinated development among population, resources and the environment.