Wide Area Wetland Mapping in Semi-Arid Africa Using 250-Meter MODIS Metrics and Topographic Variables

Abstract

Wetlands in West Africa are among the most vulnerable ecosystems to climate change. West African wetlands are often freshwater transfer mechanisms from wetter climate regions to dryer areas, providing an array of ecosystem services and functions. Often wetland-specific data in Africa is only available on a per country basis or as point data. Since wetlands are challenging to map, their accuracies are not well considered in global land cover products. In this paper we describe a methodology to map wetlands using well-corrected 250-meter MODIS time-series data for the year 2002 and over a 360,000 km 2 large study area in western Burkina Faso and southern Mali (West Africa).A MODIS-based spectral index table is used to map basic wetland morphology classes. The index uses the wet season near infrared (NIR) metrics as a surrogate for flooding, as a function of the dry season chlorophyll activity metrics (as NDVI). Topographic features such as sinks and streamline areas were used to mask areas where wetlands can potentially occur, and minimize spectral confusion. 30-m Landsat trajectories from the same year, over two reference sites, were used for accuracy assessment, which considered the area-proportion of each class mapped in Landsat for every MODIS cell. We were able to map a total of five wetland categories. Aerial extend of all mapped wetlands (class "Wetland") is 9,350 km 2 , corresponding to 4.3% of the total study area size. The classes OPEN ACCESSRemote Sens. 2010, 2 1752"No wetland"/"Wetland" could be separated with very high certainty; the overall agreement (KHAT) was 84.2% (0.67) and 97.9% (0.59) for the two reference sites, respectively. The methodology described herein can be employed to render wide area base line information on wetland distributions in semi-arid West Africa, as a data-scarce region. The results can provide (spatially) interoperable information feeds for inter-zonal as well as local scale water assessments.

Coupled Ground- and Space-Based Assessment of Regional Inundation Dynamics to Assess Impact of Local and Upstream Changes on Evaporation in Tropical Wetlands

“…Especially images from seasonal transition times are susceptible to misclassification, since water bodies in the study area dry out heterogeneously inside of a pixel. The determination of pixels, which are covered by vegetation mixed with or completely flooded by water, is usually difficult [30,33]. Pixels can be covered by a mixture of different land types, which are all combined in one index pixel value leading to an over-or underestimation of MODIS-derived inundation [30].…”

“…Spatial and temporal extents of the inundation process are often investigated by multi-band classification [11,17,20,21,23,24,[27][28][29][30] using vegetation and humidity indices [17,[20][21][22]29,31,32]. MODIS-derived inundated areas can then be validated by Landsat [22,31,33,34], ASTER [27], and SAR [25], or with national land cover datasets [35]. Few studies examine MODIS satellite imagery with gauged water levels, such as those of Ordoyne and Friedl [20] or Pavelsky and Smith [36].…”

Mapping global tropical wetlands from earth observing satellite imagery

“…One tractable alternative is to use time series data and adjust the classification based on the local phenology of climate, surface wetness and vegetation. Landmann et al (2010) used time series MODIS data to identify West African wetlands from the phenological characteristics of vegetation. But their approach also used extensive ground data for calibrating the image classification.…”

“…The Niger Inland Delta is reported to have shrunk from 37 000 km 2 to 15 000 km 2 between 1950 and 1990 (Niasse et al 2004). Landmann et al (2010), in a recent study using moderate resolution optical satellite imagery, estimated the area of the Niger Inland Delta wetland at about 9000 km 2 . McCarthy et al (2003) showed that the flooded area of the Okavango Inland Delta (Botswana) has varied considerably in historical times, but put the present area of the Okavango wetland at about 9000 km 2 .…”

“…The study on West African wetland by Landmann et al (2010) used the LSWI (leaf wetness) phenology as a key information source for mapping wetlands.…”