Interannual changes of urban wetlands in China’s major cities from 1985 to 2022

With global climate change and accelerating urbanization, accurate and timely extent information on urban wetlands is extremely important for sustainable urban development and conservation of ecosystem services, supporting the implementation and evaluation of the United Nations Sustainable Development Goals (SDGs). China has experienced the most dramatic urbanization process in recent decades, but accurate and comprehensive information for urban wetland changes in China's major cities are still unavailable. In this study, using 137,779 Landsat images available on the Google Earth Engine platform, we developed a novel approach (MWC-CCDC) integrating historical maximum wetness composition (MWC) and continuous change detection and classification (CCDC), and generated the first annual 30-m resolution urban wetland maps for 71 major cities with populations over 0.5 million across China from 1985 to 2022. The resultant annual urban wetland distribution dataset in China, named China_Urban_Wetland (CUW), achieved over 82.81 % overall classification accuracy. According to the CUW, cities in the Yangtze River basin, Pearl River Delta, and Hangzhou Bay cover the majority of urban wetland area in China's 71 major cities. Wuhan had the largest urban wetland area (168.00 km2), followed by Nanjing (103.19 km2). Jiujiang has the highest wetland coverage rate at 27.23 %, followed by Wuhan (20.68 %) and Xiangyang (18.89 %). Between 1985 and 2022, 52 of 71 cities lost wetland area, with Tianjin having the largest percentage loss (79.40 %), followed by Urumqi (74.86 %) and Guiyang (67.55 %). In contrast, Hohhot experienced the largest increase (90.63 %), followed by Beihai (89.21 %) and Xi'an (71.34 %). In about one-third the cities, urban wetland landscape patterns are becoming more fragmented and less connected. Such consistent annual assessments of urban wetlands is expected to benefit the implementation and evaluation of urban-related targets in the SDGs.