High spatial resolution mapping of African wetlands using multi-sensor data analysis

This thesis integrates optical indices and SAR polarimetric features obtained from Sentinel-1 SAR and Sentinel-2 optical imagery from different seasons using a Random Forest classification algorithm to produce a detailed wetland map, first for southern Nigeria, and then for the whole of the main continent of Africa at 10 m resolution. Wetlands of southern Nigeria cover a total area of 29,924 km2 and those of Africa cover 947,750 km2, larger than that indicated in previous coarser global wetland maps, excluding open water bodies. A wetland fragmentation and population density index was developed indicating that ~13,021 km2 of wetlands are potentially threatened by human activity within Africa. Using the new map and compiled wetland carbon inventory data, it was estimated that African wetland contains about 54.30 Gt of carbon which is around 5% to 9% of wetland soil carbon stored globally. Calculations across a range of climate zones suggest that drained peatland, mangrove and marsh in Africa could emit 260Mt C yr-1 (936Mt CO2 yr-1 equivalents) which is equivalent to 2.4% of global net annual CO2 emissions. Field sampling across Africa is required in order to include emissions from other wetland types. Long-term assessment of changes within the wetlands of the Lake Chad region for 2000-2020 showed that there was a net loss of wetland area. The greatest wetland decline occurred between 2000 – 2005 with a net loss of 277 km2 (±12 km2). However, there was wetland loss for all periods studied, including during a wetter period when it was anticipated that wetland extent would increase. The most significant change occurred around the northern pool of the lake. The tools developed by this study pave the way for ongoing high-resolution monitoring and assessment of African and global wetlands.