Sheet n°303 - July 2008
w
Aerial photographs further Sahel land change
observation
continent from West to
East. Several hundred
kilometres wide, it is
bordered by the Sahara
to the North and the tropical rainforest to the
South. Its geographical
location puts the Sahel
among the most sensitive areas in the world to
global climate change.
Clearance of the woody
savannah for agriculture
since the beginning of
the last century is another major factor in the
erosion processes affecting the region. Examination of a time series
of aerial photographs
taken on a 500km2 study
area in the Niger Sahel,
selected as representative of the ecosystem
as a whole, has yielded
the first comprehensive
picture of the changes
that have taken place
from the early 1950s
to the present day. The
surveys, accomplished
by a team of IRD scientists and their partners1,
and covering the longest observation period
studied to date, provided reliable information
on the relative extent to
which deforestation is
attributable for ongoing
soil erosion and water
table changes. Such an
observation approach
could be harnessed in
support of local initiatives to control erosion
while at the same time
preserving food security
and freshwater resource
availability for farming
communities.
© IRD / Guillaume Favreau
he Sahel is a band
T
of semiarid terrain
that crosses the African
Visible effect of erosion: formation of a pond fed by rainwater runoff from the plateaux.
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 archives2 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
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CONTACT:
GUILLAUME FAVREAU
Unité mixte de recherche
HydroSciences IRD-Université Montpellier 2
Adresse :
Université Montpellier 2
Place Eugène Bataillon
34 095 Montpellier
Cedex 5
Tel : +33 (0)4 67 14 40 29
+ 0(1) 512 657 0174
guillaume.favreau@ird.fr
REFERENCE :
Marc J. Leblanc, Guillaume
Favreau, Sylvain Massuel,
Sarah O. Tweed,
Maud Loireau, Bernard
Cappelaere, Land clearance and hydrological
change in the Sahel: SW
Niger, Global and Planetary Change, 2008, 61 (3),
p.135-150
Doi:10.1016/
j.gloplacha.2007.08.011
KEY WORDS :
Sahel, deforestation,
erosion , aerial photo
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.
Grégory Fléchet - DIC
Translation - Nicholas Flay
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)
PRESS OFFICE :
VINCENT CORONINI
+33 (0)4 91 99 94 87
presse@ird.fr
© IRD
INDIGO, IRD PHOTO
LIBRARY :
Left: Series of aerial photos showing the expansion of the surface area given
over to millet crops (in green) which is a cause of the densification of the
gully network (blue); and showing their progressive widening (above).
DAINA RECHNER
+33 (0)4 91 99 94 81
indigo@ird.fr
www.ird.fr/indigo
© Elsevier Ltd
Sheet n°303 - July 2008
For further information
Grégory Fléchet, coordinator
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