How to map small wetlands: a new case study on palmiet wetlands
9 October 2017
Alanna Rebelo (UAntwerpen) combined satellite imagery and aerial photographs to accurately define how palmiet wetlands in South-Africa developed over the past 80 years.
The Ramsar Convention (1971), already more than 4 decades ago, triggered strong international attention on the importance of wetlands. Wetlands harbour an enormous biodiversity worldwide, and provide important ecosystem services to humans, including flood protection, pollution filtering and carbon sequestration.
Accurate inventories of wetland occurrence and distribution of wetlands are key to their protection, which means good tools are needed to monitor wetland extent. While this is relatively easy and straightforward for large protected wetland areas (e.g. the Everglades in Florida), it is much more difficult for small, vegetation covered wetlands. Due to their relatively limited surface area, they are frequently overlooked in conservation and restoration efforts. Little is known on the evolution of their extent, or how this has been affected by humans.
This certainly holds true for palmiet wetlands in South-Africa. Palmiet is a robust, evergreen, semiaquatic plant, endemic to South-Africa, which grows in small river valleys in the Cape Floristic Region. Where palmiet grows, riparian systems are less prone to damage due to flood erosion. South-Africa puts strong focus on restoration and conservation of wetlands, e.g. through the “Working for Wetlands Programme”, but current national wetland mapping efforts (e.g. NFEPA) have been insufficient to determine exact extent of the palmiet wetlands, and to quantify whether their surface area has changed over recent decades. Therefore it is impossible to understand if these wetlands are threatened, and if so, to what extent.
Palmiet typically forms small valley-bottom peatlands in the Cape Floristic Region of South-Africa (Picture: Serban Proches).
New research by Alanna Rebelo (Global Change Ecology Centre, Research Group Ecosystem Management) combines existing freely available imagery and tools to better map these small wetlands. Alanna performed the research as a part of her Erasmus Mundus scholarship at UAntwerpen. She combined historical aerial photograph analysis with satellite imagery (LandSat8) to map both current extent and analyse how wetland surface changed over the last 80 years.
Alanna: “The satellite imagery was very well-suited for the initial detection of existing palmiet wetlands. Once these wetlands are located, more detailed aerial photographs can be used to assess wetland degradation status and determine changes in wetland surface over time.”
The Landsat series, 1970 (Landsat4-5), 1980 (Landsat 7) and 2010 (Landsat8), was too coarse to map the small wetlands. Aerial photographs have a long historical record, which can be used in digitization to determine exact wetland extent and its changes over time. In summary, the analysis revealed that 31% of the palmiet wetlands have disappeared since the 1940s (the furthest back that aerial photographs are available for South Africa). The major driver of change was gully erosion triggered by the construction of roads, creating erosion channels in this normally unchannelled ecosystem.
Beyond the importance for the conservation of palmiet wetlands in South Africa, the new combined approach also show strong promise for mapping small wetland decline in other areas around the globe that may have limited field data, but for whom historical aerial photographs exist.
Results were published in Remote Sensing Applications: Society and Environment.
Alanna (front) in a palmiet wetland (picture Byron-Mahieu van der Linde).
Helicopter view of a valley of palmiet wetlands