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| Remote detection allows one to study the evolution of the vegetal coverage as well as the evolution of the environment. Comparison of airborne and satellite data over a long period (more than thirty years) makes possible a diachronic analysis of the landscape. Thus, comparison of scanned aerial photographs and a SPOT satellite image highlights the environmental degradation in a semi-arid zone of the Sahel (department of Zinder in south-east Niger).
Table: Evolution of some landscape elements in the Makaoratchi area(S-E Niger) This region is characterised by an average annual rainfall on the order of 350 mm (period 1950-1990). On the aerial photography of 1957-1958 the
vegetation is relatively dense and wind-caused soil shifting is
virtually non-existent. The image deduced from the aerial photography of 1975, after the first great drought of 1968, reveals the disappearance of the greater part of the vegetation, which essentially remains only in the wadi beds. The now-moving sands affect ever more extensive surfaces, in particular involving the summits of the dunes and the areas surrounding the villages. Following the terrible drought in the early 1980's (1987 SPOT image), the sector swept by deflation expanded significantly, thus reflecting the progressive environmental degradation. Vegetation has been cut to one-half that of 1975. In addition, the substantial increase in the volume of the delta (red zone) of the wadi feeding a lake located in the south of the zone under study attests to increased water erosion. Finally, on this same image, note that the zone indicated by an arrow corresponds to the disappearance of soil, revealing a lateritic layer precisely where the soil was covered by relatively dense vegetation at the end of the 1950's. This specific example proves that irreversible degradation can take place very quickly (in less than thirty years).
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| The comparison between aerial photographs and a SPOT satellite image emphasises the environmental degradation in the semi-arid area of Makaoratchi-Gayi, Zinder department, south-east Niger(period 1950-1990).
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The early warning systems |
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| In the Sahel region, agriculture is the major activity. Subject to widely varying meteorological conditions, the population is confronted with serious food problems during years with sharp rainfall deficits or when the rains are poorly distributed during the growing season. This type of situation must be foreseen well in advance in order to permit decision-makers and financial sources to react swiftly enough to prevent or limit the effects of these potential famines. Among the tools employed to forecast this type of disaster, the combination of the AVHRR low-resolution spatial sensors of NOAA and the Meteosat sensors has until now served to monitor the planet's vegetation. A new Vegetation sensor aboard of the SPOT-4 satellite has now made it possible to improve the spatial and temporal accuracy of the earlier satellite sensors, thus furnishing decision-makers with a more powerful tool within the specific framework of the food shortage rapid warning systems. This technique makes use of satellite information furnished at ten-day intervals. It allows one to obtain accurate information (1 km2) in real time on the condition of vegetation over a very wide territory (several million km2). As an example, an animation of the Vegetation images is presented for a zone extending longitudinally from the extreme west of Mali to the extreme east of Niger. This animation shows the seasonal evolution of the vegetal coverage with, starting in June, the vegetation front's advance towards the north.
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