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Aerial photographs further Sahel land change observation

The Sahel region is vulnerable to drought and desertification which has been advancing increasingly rapidly over the past few decades. Scientists have been using satellite images since the 1970s to measure land-use changes in the region

Deforestation in this region is, however, a factor that dates back much further, to the mid 20th Century. To get round the absence of satellite remote sensing images, a team of IRD researchers and their partners turned to aerial photograph archives 2 used in West Africa since the 1950s for detailed mapping of the region.

The study area covered 500 km2 in south-west Niger and was chosen because its rural environment was representative of a rain-fed mode of agriculture practised by farming communities throughout the Sahel. Four time series of photographs taken in 1950, 1960, 1975 and 1992, gave the opportunity to trace highly accurately changes in such factors as deforestation, soil erosion or pond water level. A fifth time series for the study area, obtained this time by the SPOT satellite, was added to complete the date set. Additionally, field surveys conducted in which farmers were asked to recollect their experience of the changes in the Sahel landscape yielded information for comparison and integration with aerial observation data.

The photographs’ good resolution, down to a few metres, made possible quite clear identification of borders of cultivated plots or of the gullies that cut into the landscape. Image analysis revealed that between 1950 and 1992, 80% of the study area was cleared for expansion of millet cultivation and, to a lesser extent, to provide firewood for local communities Advantage was also taken of these shots to measure the intensity of the deforestation according to the type of terrain. Thus, hillslopes showed as the most strongly affected by clearance (87%), followed by the plateaux (59%) and then the valley bottoms (42%) which remained the best conserved environments.

Removal of the vegetation led to loss of soil rainfall infiltration capacity. This indirect consequence of forest cover could be visualized on aerial photographs which, as the time series progressed, depicted an increasingly extended gully network. The process ended with a landscape resembling an oilcloth. Here, strong runoff means that rainwater rushes down to the valley floors where it accumulates to form ponds which act as temporary retention ponds. They eventually empty thereby charging the groundwater. The aerial photographs gave the possibility to track the progress of this phenomenon. Between 1950 and 1992, the research team thus recorded a 2.5-fold multiplication of the drainage capacities of the regions observed.

This situation was manifest by an increase in dimensions of gullies, which now reached several metres in width, and the appearance of new temporary water points. The link between the decrease in forest cover and an observed water table rise, by an average of 4m between 1963 and 2005, was also clearly established. The aquifer storage capacities rose by 15 % during this period.

Not all the regions of the Sahel are equally vulnerable to deforestation. Yet although the processes have practically reached their maximum in most of the countries that occupy this narrow semiarid band, water erosion has not ceased, the landscape erosion has continued and the soils are still losing fertility.

Some areas of the Sahel, like the south-western part of Niger, are particularly susceptible. These areas should therefore be targeted as a priority in desertification control schemes, differently from other, flatter areas, where the degree of concern stemming from woodland-clearance induced erosion remains lower. In this situation, aerial photographs will help the local bodies responsible to spot more clearly the sectors that require priority protection. Erosion control efforts will gain in effectiveness by making farmers aware of the necessity to conserve the valleys that are still relatively spared from deforestation and concentrating efforts for reforestation and protection of useful woody species, such as the acacia Faidherbia albida, on the plateaux areas and the hillslopes already hit by deforestation.

1. These research studies were conducted jointly with scientists from James Cook University, Cairns, Australia.

2. The aerial photographs were provided by Niger’s Institut Géographique National (IGNN)

Grégory Fléchet | alfa
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