The spatial distribution and expansion of subsided wetlands induced by underground coal mining in eastern China

A large number of subsided wetlands have formed in eastern China in areas with high-intensity mining. However, data are not currently available to indicate their spatial distribution and expansion in the past thirty years. This paper uses a modified normalized difference water index (mNDWI) and a maximum between-cluster variance (OTSU) image segmentation algorithm to extract the subsided wetlands in mining areas with high ground-water levels of eastern China from 1988 to 2018 based on Google Earth Engine. The results show that the overall accuracy of the extraction of subsided wetlands is 98%; the Kappa coefficient reached 0.81. The total area of subsided wetland in 2018 was 26,034.88 ha, of which 14,290.97 ha was in Anhui Province, accounting for 54.89% of all such wetlands. The spatial extent of subsided wetlands has grown rapidly in the past three decades with the area of subsided wetlands expanding by 11.86 times from 1988 to 2018. The total area of subsided wetlands in the winter of 2018 was 25,296.25 ha, which was smaller than in summer. This indicates that seasonal precipitation affects the spatial extent of subsided wetlands. Although some restoration activities have been successful, most of the subsided wetlands still need active development and management. In conclusion, mNDWI and OTSU image segmentation algorithms could quickly and accurately allow the extraction of the spatial extent of subsided wetlands. Subsided wetlands have strong potential for development in future ecological restoration. The ecosystem services of wetlands and availability of dynamic monitoring technology should be considered important in the future.