Large-scale wetland mapping and evaluation

Abstract

Wetlands are vital for human survival. Considering there is no common wetland definition which is accepted by all domains or sectors, the Ramsar wetland definition is recommended for large-scale wetland mapping. Three kinds of methods for developing wetland dataset at large spatial scale-wetland mapping by remote sensing, compilation of historical datasets, and simulation using physical models-are reviewed and assessed. Large-scale wetland mapping is challenging because of the complexity in types and spatial heterogeneity of wetland landscape. Wetland-related datasets suffer from major inconsistencies. Estimates of the areal extent of the remaining global wetlands range from 1.53 million to 14.86 million km2. Until recently there have been two global wetland simulation datasets which have been developed by Zhu and Gong (2014) and Hu et al. (2017) without considering influences of anthropogenic activities. Based on Hu’s simulation of global wetland and Globcover 2009 dataset, global wetland loss was at least 33% as of 2009, including 4.58 million km2 of wetlands without open water and 2.64 million km2 of open water. The areal extent of wetland loss has been greatest in Asia, but Europe has the greatest proportion. The top 100 globally large Ramsar Sites have been mapped based on MODIS time-series data in 2001 and 2013, and its changing characteristics and landscape integrity were evaluated. Results show that the global wetland area has maintained a stable state with less than 1% of wetland area loss. However, significant fluctuations exist within Ramsar sites caused by the cumulative effects of natural conditions, i.e., rainfall and temperature, and human activities. The change rate of inland wetland area is higher than that of coastal and artificial wetland. From the perspective of wetland disturbance/degradation, those Ramsar sites are still under threat at different degrees. At the same time, the effects of protecting China’s national wetland reserves between 1978 and 2008 have been evaluated. Results show that about 79% of the 91 national wetland reserves are in a poor condition with regards to protection area. These are generally located around the Yangtze River, Eastern Coast, the Three Rivers Source, and Southwest China. Only 15% of national wetland reserves are under sound protection, and these are generally located in the upper reaches of the Songhua River. Since most experiences of local wetland mapping cannot be directly learned or transferred to regional and global scales, more research is needed to conduct global thematic wetland mapping, to obtain more accurate wetland classes and more detailed ecological and environmental information on wetland ecosystems.