Use of MERIS data to detect the impact of flood inundation on land cover changes in the Lake Chad Basin.

摘要

Lake Chad Basin is a vital water source in West Africa, however, the basin experienced devastating droughts and the lake declined considerably, leading to environmental degradation and food insecurity. Stake holders now cultivate flood recessed lands as a livelihood coping strategy. Seasonal inundation of the lake occurs annually, providing irrigation water, moisture retentive soils and fish breeding environments. Harvest uncertainties due to unpredictable flooding causes crop losses and destroys infrastructure thus necessitating the need for early warning systems. Consistent and accurate data acquisition using conventional methods is difficult because the region is remote. Thus no current inventory of cultivated and cultivable lands and recessional farming activities exists. Remote sensing offers scope for environmental assessments and has been used in previous studies to monitor changes in the lake environment. In this study, the potential of MERIS for broad regional studies of Lake Chad was evaluated by investigating multi temporal patterns of change which could identify farmlands in the lake basin. MERIS data were validated with ASTER while ASTER suitability for validating MERIS was determined using high resolution IKONOS images and fieldwork. Spectral Mixture Analysis was performed on MERIS data and three endmember fractions: soil, vegetation and water extracted. Post classification threshold change detection was performed using the endmembers as indicators of land cover changes to assess temporal changes over the lake basin. A basis for an early warning system was investigated from, the relationship between altimeter data and extent of surface inundation. The spatial distribution and areal extent of annual flooding of the lake were estimated from MERIS water fraction abundance. The areal extents were related to lake height level data derived from (i) TOPEX POSEIDON data and (ii) HYDROM data. Temporal change patterns were detected using MERIS thus making it suitable for inventorying and monitoring resources on the lake basin. Farmlands smaller than the nominal pixel resolution of MERIS were not detected due to the coarse spatial resolution of MERIS. The relationship between altimeter and extent of inundation showed that external factors e.g. farming practices and the lake's physical characteristics influence the surface inundation and modify this relationship.