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
Climate Change in West Africa
17.06.2019 | Julius-Maximilians-Universität Würzburg
Determining the Earth’s gravity field more accurately than ever before
13.06.2019 | Technische Universität Graz
The quality of additively manufactured components depends not only on the manufacturing process, but also on the inline process control. The process control ensures a reliable coating process because it detects deviations from the target geometry immediately. At LASER World of PHOTONICS 2019, the Fraunhofer Institute for Laser Technology ILT will be demonstrating how well bi-directional sensor technology can already be used for Laser Material Deposition (LMD) in combination with commercial optics at booth A2.431.
Fraunhofer ILT has been developing optical sensor technology specifically for production measurement technology for around 10 years. In particular, its »bd-1«...
The well-known representation of chemical elements is just one example of how objects can be arranged and classified
The periodic table of elements that most chemistry books depict is only one special case. This tabular overview of the chemical elements, which goes back to...
Light can be used not only to measure materials’ properties, but also to change them. Especially interesting are those cases in which the function of a material can be modified, such as its ability to conduct electricity or to store information in its magnetic state. A team led by Andrea Cavalleri from the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg used terahertz frequency light pulses to transform a non-ferroelectric material into a ferroelectric one.
Ferroelectricity is a state in which the constituent lattice “looks” in one specific direction, forming a macroscopic electrical polarisation. The ability to...
Researchers at TU Graz calculate the most accurate gravity field determination of the Earth using 1.16 billion satellite measurements. This yields valuable knowledge for climate research.
The Earth’s gravity fluctuates from place to place. Geodesists use this phenomenon to observe geodynamic and climatological processes. Using...
Discovery by Brazilian and US researchers could change the classification of two species, which appear more akin to jellyfish than was thought.
The tube anemone Isarachnanthus nocturnus is only 15 cm long but has the largest mitochondrial genome of any animal sequenced to date, with 80,923 base pairs....
29.04.2019 | Event News
17.04.2019 | Event News
15.04.2019 | Event News
19.06.2019 | Physics and Astronomy
19.06.2019 | Information Technology
19.06.2019 | Materials Sciences