Wetland Monitoring in Semi-arid African Regions Using MODIS Time Series

Kurzfassung

Monitoring of wetlands and water bodies in semi-arid African regions is of high importance for the local population and for wetland ecology. The study area is situated in Burkina Faso, West Africa, and extends over an area of 500 km north-southwards, ranging from Sahelien to Savannah climates, and showing a gradient of different rainfall and land use characteristics. Surface water is an important resource for different livelihoods such as farmers or herders, as well as for water extraction by the local population, and can be scarce particularly in seasons of drought. This study demonstrates the applicability of time series of optical remote sensing data for dynamic water body mapping on a high temporal and medium resolution spatial scale. The main dataset is a 15-year time series (2000-2014) from the Moderate Resolution Imaging Spectrometer (MODIS) with temporal intervals of eight days and 250 m spatial resolution bands in the red and near infrared range (MOD09Q1 product), supported by further information from the 500 m resolution MOD09A1 product, and the digital elevation model (DEM) of the Shuttle Radar Topography Mission (SRTM). RapidEye and Landsat high resolution optical images are available for different seasons in different years, and serve for exploring trends on a higher resolution scale, as well as for validation of the results derived from MODIS. Spatio-temporal dynamics of water covered areas have been obtained, showing intra-annual variations as well as dynamics throughout the 15-year time series. Trends reveal changing flooding regimes in terms of larger water coverage while the flooding duration decreased. These findings correspond to information gained during field work, and is connected to siltation on the bottom of the water bodies. Caused by dam constructions 21 newly created water bodies larger than 0.5 km², and a number of smaller water bodies were discovered, as well as a few water bodies that have vanished. Secondary applications such as detecting irrigation trends around wetlands from MODIS vegetation indices could be verified using high resolution RapidEye and Landsat data, and vegetation indices can aid for wetland delineation of vegetation covered wetlands. The negative anomalies of both, surface water dynamics of wetlands and vegetation of the environment, were found to coincide with the occurrence of drought seasons (2000-01, 2004-05 and 2011-12). Possibilities and limitations for extending this approach over larger areas are discussed, and challenges such as mapping sediment rich waters, vegetation covered waters, or misclassifications due to cloud shadows or burnt areas are addressed. This study demonstrates the potential to detect and monitor water bodies and wetlands in semi-arid African regions, and contributes to the understanding of inter- and intra-yearly dynamics, based on a time series of medium resolution MODIS data. Additionally, the role of wetland monitoring for both, estimating water availability and indicating drought in semi-arid areas is discussed. An outlook on an adapted concept using Sentinel-3 data is presented. The study shows that the use of remote sensing time series is a suitable tool to monitor wetlands in semi-arid regions.