Mapping the dynamics of urban land creation from hilltop removing and gully filling Projects in the river-valley city of Lanzhou, China

Lanzhou is located in the long, narrow valley of the Yellow River Basin, China. With the development of urbanization, the contradiction between its economic development and limited spatial expansion was very acute. To expand urban space, improve the efficiency of land use and protect farmland effectively, Hilltop Removing and Gully Filling Projects (HRGFP) around Lanzhou have been carried out over the last decades by local government. To better manage and evaluate the effectiveness of HRGFP implementation, it is necessary to map the urban newly land creation, monitor its dynamics change and access the environmental impact, which is of great significance to understand and master the past, present and future development trend of HRGFP. Based on multi-source remote sensing data and field investigation, here we use multiple monitoring indicators and spatial statistical analysis to analyze the change dynamic characteristics of newly plots from HRGFP and its impact on the local ecological environment. The main conclusions are as follows: From 1989 to 2016, the total area of the newly land creation around Lanzhou was about 203.18 km 2 . The center of newly land plots generally showed a trend of continuous transfer from south to north, from the main urban region to the low hills, gentle slopes, and gullies and shifted 15.5 km northwest. And the maximum annual average expansion rate was about 36.003 km 2 /a, which occurred in the third stage (i.e., 2012–2016). During the study period, it was found from land use and land cover change (LULCC) statistics that more new land plots from HRGFP were converted to forest land and grassland, except for construction land, which was consistent with the results of remote sensing environmental index (RSEI). Statistics showed that the area of environmental quality improvement in the study region is about 3984 km 2 , accounting for 55.73% of the total area. Our results indicated that HRGFP in Lanzhou have enlarged the urban space and increased economic development and improved ecological environment under the guidance of the local government.