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OBJECTIVE OF THE STUDY
the objective of this study was the development of a
methodology combining some plurithematic data , in order to
determine the zones of the steppique ecosystem near the stage of
the desertification and to establish a synthesis map which
represents the most exposed zones to this natural phenomenon.
THE ADOPTED APPROACH
A methodology was recommended in order to better understand
apprehend the problems of desertification on which the tool of
investigation is remote sensing. It treats some thematic data:
land cover, pedopaysagic and morphology. This recommended
methodology is based essentially on the crossing with the aide
of a GIS and according to a well defined matrix the following
information:
land cover; map pédopaysagic; MNT and the map of the slopes.
BIBLIOGRAPHIC STUDY ON DESERTIFICATION
The works achieved by the institutions or specialised organisms
as the CILSS’ remain partial and often oriented toward some
different objectives to ours.
The nearest works to our preoccupation's were developed by the
FAO" and PNUD™ in the setting of two studies: the world-
wide map of risk of desertification and the case studies on
desertification. The world-wide map of risk of desertification
worked out to leave some very synthetic criteria’s, it interests the
macro ecological entities. The case study on desertification
made the object of criteria’s numerous and rigorous for
evaluation . This methodology is based on the analysis of the
following elements: the deterioration of the vegetation; the hydric
erosion ; the eolienne erosion ; the deterioration of the structure
of soil.
These factors of desertification were examined according to three
aspects:
static aspect (state of medium deterioration at a given instant of
time);
dynamic aspect (speed of deterioration of the medium);
aspect natural risk (vulnerability of natural medium to the
deterioration).
PROBLEMATIC OF DESERTIFICATION
desertification is defined as being a dynamics of deterioration
tending to transform some zones before no désertic in desert,
making lose the ecosystems their faculty to relive or to
regenerate (INESG™"" | 1988). In the absence of covering plant
destruction of the plants , the hydric erosion and « eolienne »
intervene on the arable layer, the ultimate stage of the
desertification being the uncovered rock mother or some chalky
encroütements and gypsum. The eroded products are deposited
and their thin elements are taken by the erosion « eolienne ». The
intensity of the erosion depends on the topography, the character
of the precipitation’s and winds.
“ Comité International de lutte contre la Sécheresse au Sahel
“” Fond Alimentaire Organisation
ee Programme des Nations Unies pour le Développement
77" Institut Nationale d'Etudes Strategiques Globales
METHODOLOGY
1- Presentation of the zone of study
The region of study is the North of [djebels oum El gueddour
daira] of [gueltet sidi saad], [wilaya] of [laghouat]. It is a region
of high plains (steppiques] characterised by a relief more or less
injured of which the altitudes vary between 1100 and 1300 m
(see figure 1).
2- The physical medium
Generally the soils are distributed as a function of the
geomorphologic units. However a diversity [édaphique] could
exist on a same unit, as it arrives to have some very comparable
soils on some different units.
The vegetation is one of the a main components of the medium.
its role fixes and orientates the evolution of soil its absence
breaks the ecological balance of the medium and favors erosion.
The climate of the steppiques zones is a Mediterranean climate,
with two very distinct seasons the one rainy and cold and the
other dry and hot.
3 - Elaboration of the thematic maps
A) the land cover map (see figure 3 ).
| channels choice [topographic map |
colored composition
basis points choice
tsets samples
choice
supervised
classification
classifed image
geometric
correction
[4 land cover map =)
Fig 2 : Methodical organisation chart for the realisation of
the map of land cover
For this study we have chosen some TM pictures of LANDSAT
satellite 5 of 08 April of the year 1995 of the scene (196/ 36). for
this, a window of 800 columns and 600 lines was chosen,
covering a steppic zone which presents an exposed landscape to
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 767
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