Research on the slope gradient effect and driving factors of construction land in urban agglomerations in the Upper Yellow River: A case study of the Lanzhou-Xining Urban Agglomerations

Abstract Analyses of the scale and structural characteristics of construction lands serve as the basis for delineating urban development boundaries and optimizing the spatial pattern of territorial planning. Existing studies have focused mainly on the horizontal expansion of urban construction lands, while few comprehensive studies have explored the slope gradient effects of urban construction land, rural settlement land or other construction land in the horizontal-expansion dimension or vertical slope-spectrum dimension. Therefore, based on the Google Earth Engine (GEE) platform, in this paper, we use high-precision land use cover data, DEM data and socioeconomic data to construct the standard dominant comparative advantage index (NRCA) using the geological mapping analysis method and systematically analyze the horizontal scale, slope spectrum characteristics, gradient effects and driving factors of construction lands in the Lanzhou-Xining urban agglomeration (LXUA) from 1990 to 2020 at four scales: the urban agglomeration, provincial area, typical city and county (district) scales. The results of the study show that urban construction land, rural settlement land and other construction land in the LXUA show "linear", inverted "U" and "J" growth patterns, respectively. The areas of rural settlements in 9 counties (districts) are decreasing, and the number of counties (districts) with decreasing areas is increasing, while the areas of other construction land in most counties (districts) are increasing. The center of urban construction gravity continues to shift northwestward, and the standard deviation ellipse is becoming flatter; the center of gravity of rural settlement lands continues to shift southeastward. The center-of-gravity distribution and standard deviation elliptical characteristics of construction land at other scales also vary to different degrees. The scale and extent of the increase in urban construction land in the LXUA is gradually decreasing over time, and the number of rural settlement lands in 2000–2010 was as high as 34 counties (districts), while the number of counties (districts) with strong degrees of other increased construction land rose to 12 from 2010 to 2020. The relative hotspots of the three types of increasing construction land have gradually expanded spatially, with Lanzhou city and Xining city as the center, and the overall spatial characteristics are “more in the east and less in the west”. The population and GDP are the main factors influencing urban construction land increases, while rural settlements are influenced mainly by natural conditions, and accessibility is the key factor affecting other construction land.