Mapping and analyzing China's wetlands using MODIS time series data

Gathering accurate information on wetland distribution and changes is of key importance for wetland management. However, wetland mapping using single-date imagery is challenging due to high seasonal and intra-annual hydrodynamic variations, especially for seasonal wetlands. Although time series of satellite images are effective for improving the accuracy of land cover mapping, they have been rarely employed in large-scale wetland mapping. In this study, we used MODIS time series data at 250-m resolution to develop China's wetland maps, and we analyzed the changes from 2000 to 2015. The results showed that: (1) The method yields good results; the overall accuracy and kappa coefficients of all wetland maps for 2000, 2005, 2010, and 2015 were above 80% and 0.79, respectively. (2) In 2015, the total area of wetlands in China was 5.37 x 10(5) km(2). Paddy fields accounted for more than 60% of the wetlands, and these fields were mainly located in the middle reaches of the Yangtze River and Northeast China. Less than 40% were natural wetlands, which were mainly located in the Tibetan Plateau, Northwest China, and Northeast China. Seasonal marshes were mainly located in the middle reaches of the Yangtze River. (3) From 2000 to 2015, wetland area decreased by 2.50 x 10(4) km(2). Among all wetlands, permanent marshes are the most degraded and seasonal wetlands the most fragile. This is a major cause for concern. This study proposes a method to automatically generate large-scale wetlands maps, which could contribute immensely to nationwide wetland monitoring and protection.