Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Aerial photographs further Sahel land change observation

04.09.2008
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
Further information:
http://www.ird.fr/
http://www.ird.fr/fr/actualites/fiches/2008/fas303.pdf

More articles from Earth Sciences:

nachricht Radioactivity from oil and gas wastewater persists in Pennsylvania stream sediments
22.01.2018 | Duke University

nachricht World’s oldest known oxygen oasis discovered
18.01.2018 | Eberhard Karls Universität Tübingen

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Artificial agent designs quantum experiments

On the way to an intelligent laboratory, physicists from Innsbruck and Vienna present an artificial agent that autonomously designs quantum experiments. In initial experiments, the system has independently (re)discovered experimental techniques that are nowadays standard in modern quantum optical laboratories. This shows how machines could play a more creative role in research in the future.

We carry smartphones in our pockets, the streets are dotted with semi-autonomous cars, but in the research laboratory experiments are still being designed by...

Im Focus: Scientists decipher key principle behind reaction of metalloenzymes

So-called pre-distorted states accelerate photochemical reactions too

What enables electrons to be transferred swiftly, for example during photosynthesis? An interdisciplinary team of researchers has worked out the details of how...

Im Focus: The first precise measurement of a single molecule's effective charge

For the first time, scientists have precisely measured the effective electrical charge of a single molecule in solution. This fundamental insight of an SNSF Professor could also pave the way for future medical diagnostics.

Electrical charge is one of the key properties that allows molecules to interact. Life itself depends on this phenomenon: many biological processes involve...

Im Focus: Paradigm shift in Paris: Encouraging an holistic view of laser machining

At the JEC World Composite Show in Paris in March 2018, the Fraunhofer Institute for Laser Technology ILT will be focusing on the latest trends and innovations in laser machining of composites. Among other things, researchers at the booth shared with the Aachen Center for Integrative Lightweight Production (AZL) will demonstrate how lasers can be used for joining, structuring, cutting and drilling composite materials.

No other industry has attracted as much public attention to composite materials as the automotive industry, which along with the aerospace industry is a driver...

Im Focus: Room-temperature multiferroic thin films and their properties

Scientists at Tokyo Institute of Technology (Tokyo Tech) and Tohoku University have developed high-quality GFO epitaxial films and systematically investigated their ferroelectric and ferromagnetic properties. They also demonstrated the room-temperature magnetocapacitance effects of these GFO thin films.

Multiferroic materials show magnetically driven ferroelectricity. They are attracting increasing attention because of their fascinating properties such as...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

10th International Symposium: “Advanced Battery Power – Kraftwerk Batterie” Münster, 10-11 April 2018

08.01.2018 | Event News

See, understand and experience the work of the future

11.12.2017 | Event News

Innovative strategies to tackle parasitic worms

08.12.2017 | Event News

 
Latest News

Thanks for the memory: NIST takes a deep look at memristors

22.01.2018 | Materials Sciences

Radioactivity from oil and gas wastewater persists in Pennsylvania stream sediments

22.01.2018 | Earth Sciences

Saarland University bioinformaticians compute gene sequences inherited from each parent

22.01.2018 | Life Sciences

VideoLinks Wissenschaft & Forschung
Overview of more VideoLinks >>>