Temporary water bodies as ecological indicators in West African drylands

(eng) Freshwater resources and changes in those resources have been identified as one of the most important global change issues in the arid and semi-arid regions of sub-Saharan Western Africa. In these regions, most surface waters are temporary and exhibit a high spatial and temporal variability associated with seasonal rains. Wide-field-of-view satellite sensors allow mapping and monitoring of the changing surface water extent every 10 days, at a 1 km spatial resolution, at a regional scale. The accuracy of these water body detections was assessed by a time series analysis, allowing the production of a map that represented a synthesis of 10-daily water body detections from 1999 to 2007. The map included, in particular, temporary water bodies, which showed the potential of wide-field-of-view satellite sensors to add novel information to existing surface water databases. The high accuracy obtained for the regional scale water body map was confirmed during a field survey in the Republic of Niger, West Africa. The fact that water bodies can be monitored in near-real time, at low cost, at a regional scale, with a high reliability, makes them ideal ecological indicators. Relationships and interactions exist between the seasonality and extent of temporary water bodies, rainfall, and vegetation cover, and changes in one factor lead to changes in one or more of the other factors. When these changes are quantified, localized and the relative contribution of each factor is evaluated with statistical analyses, it is possible to give indications about the ecosystem state at both a catchment and a regional scale. This integrated assessment of remotely sensed rainfall, vegetation cover, and water body extent showed the potential of detected temporary water bodies to function as ecological indicators in West African drylands.