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
||Difference 1957-1987 (%)
|Depressions and végétation
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
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).
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.