沙漠研究 19-1, 257- 260 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Relationship between Vegetation and Topography
for an Erosion Management in Ethiopia
Anissa GARA1, Sawahiko SHIMADA1, Hiromichi TOYODA1, Hiromu OKAZAWA1,Shinji SUZUKI1, Ahmed GHRABI2
Abstract: Ethiopia has severe problems of soil erosion lead especially by water. The mostvulnerable soils are those without a protection plant cover. That’ s why, in order to evaluate
and predict erosion issues, it is necessary to define the relationship between topographiccharacteristics and land cover that could explain the vegetation type that occupies thewatersheds. We combined classes of elevation, slope and aspect and we obtained a
topographic typology that we joined to vegetation distribution through watersheds. We foundthat there are correlations between the topographic characteristics and vegetation cover,approved by the precipitation map of Ethiopia, in the literature.
Keywords: Digital Elevation Model, Tabulation, Topographic characteristics, Vegetation
distribution, Watershed

1. Introduction
Ethiopia is an African mountainous country with highly erodible soils on steeply sloping land and islosing from its topsoil about 1.5 billion tons per year, washed away by rain according to Ademola et al.
(2008). They reported to be a strong link between vegetation cover and erosion, this explains thenecessity to detect and assess the land surface as a first step of erosion evaluation (Woreka, 2004).
That is why, in the present work we try to assess the type, extend of vegetation and the topography
over the watersheds -as a primary goal of a land investigation- in order to find out an eventual relationshipbetween topographic characteristics and land cover that could explain the vegetation type that occupies thesubwatershed.
2. Materials and Methods
In this analysis, we basically used ArcGIS 9.2 for mapping, tabulating, classifying and joining
topographic and vegetative data. In parallel we used Excel to analyze statistically the combinations
between topography and vegetation.
At first, to proceed to the vegetation assessment, we used the data of DEM (Digital Elevation Model)
and land cover to have information about the land cover with respect to its morphology.
In this study, we used SRTM (Shuttle Radar Topography Mission)-DEM data to define the elevation,
while land cover data is based on USGS (United States Geological Survey) legend of GLCC (Global Land
Cover Characterization). Furthermore, we created the subwatershed-polygons extending on all Ethiopia
surface. In order to evaluate the vegetative potential of the subwatersheds, we calculated the tabulation of
the distribution of vegetation through all subwatersheds. In the same way, we calculated the tabulation ofelevation and slope using the DEM data as well as the aspects of all the subwatersheds.
We made a classification of the topographic characteristics: elevation, slope and aspect. In order tomake a typology that combines all the 3 topographic characteristics. A subjective classification of these
topographic characteristics was made. Elevation is classified into 3 categories: Low from: -176 to 1000 ma. s. l, Middle from 1000 to 1900 m a.s.l, and High from 1900 to 4522 m a.s.l. Slope is also classified into
3 categories: Gentle from 0 to 10 degree, Middle from 10 to 30 degree and Steep from 30 to 72 degree.
We obtained 39 combinations that we pulled into 11 combinations following the similarities inelevation and in slope (Fig. 1). Then, we joined all these topographic combinations with the dominantland-cover of each subwatershed (Fig. 2).

12Tokyo University of Agriculture, Tokyo-JAPAN
Borj Cedria Institute, Soliman-Tunisia

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3. Results and Discussion
We obtained 648 watersheds whose surfaces vary between 5.8 ha and 59900 ha within whole Ethiopia
which is covered by 12 types of vegetation defined by USGS data. The major land-covers are Shrubs(40.6% of the area), Savanna (28.8%), Dryland-cropland-pasture (9.74%) and finally Barren or sparsely
vegetated (6.51%).
Through watershed assessment and vegetation distribution map, we note that Shrubland occupies theeast half of the country facing mostly East and South-east in the aspect. While Savanna andDryland-cropland-pasture faces the West and South-west, and are concentrated in the west part of Ethiopia.
Barren and sparsely vegetation dominated watersheds occupy the north of Ethiopia in the Afar region, theaspect is mainly north.
This could be explained by another topographic characteristic related to the existence of the Great Rift
Valley that cross the country as a line of mountains from the South-west to North-east and divides thecountry on two halves. The east half faces the east and this explains that in this east side we findShrubland that faces the east too. In the same way, we can explain the fact that savanna anddrylands-cropland-pasture, which face the west, are localized in the west half of the country that faces thewest too.
As well, according to Mengistu (2003) and Reynolds (2006), the map of precipitation distribution (Fig. 3)
shows that the rainfall is more important in the west half of Ethiopia than in the east one, which could
explain the spread of more vegetation in the west side especially with savanna, cropland and pasture.
As a result of the tabulation of Land-cover versus topographic typology (Fig. 5), we find out that
Shrubland has as a major topographic characteristics typology: low elevation with gentle slope (68% of thearea), followed by middle elevation and gentle slope (19% of the area). Rarely, we find middle slope withlow or middle elevation. This could explain that in the east side of the country -occupied mostly by
Shrubland- the elevation is not high and has tendency to decrease towards the east, at the Somali region, as
shown in the map of elevation (Fig. 4).
As for the savanna, 35% of its area has low elevation with gentle slope, 31% has middle elevation
with gentle slope. Few percentages only have middle elevation and slope or high elevation with gentle
slope and middle slope. So, the area of savanna seems to decrease when the elevation becomes higherand the slope becomes steeper. Drylands-croplands-pasture is found in the area of middle and highFig. 1. Distribution of the classified topographic
characteristics.

Fig. 2. Topographic typology distributed throughwatersheds.

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Elevation (m)mm

Fig. 3. Precipitation map of Ethiopia (Mengistu, 2003).
Fig. 4. Elevation map of Ethiopia.
high-elev/steep-slop
high-elev/mid-slop
high-elev/gentl-slop
mid-elev/steep-slop
mid-elev//mid slop
mid-elev/gentl-slop

B arren or SparselyV egetated

W ater B odies

M ixed ForestEvergreen
B roadleaf ForestLand
cover type
Vegetationtype

D ecidiuous
B roadleaf Forest

Savanna

Shrubland

G rassland

C roplan/w oodland
M osaic

C ropland/G rassland
M osaic

D ryland-C roplandPasturewater bodies 2
U rban-build up

Topography occupation-area (%)

Distribution of topography through vegetation
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

low-elev/steep-slop
low-elev/mid slop
low-elev/gentl-slop
water bodies 1

Fig. 5. Distribution of topography through vegetation.

elevation with gentle and middle slope. The area seems to be more important where elevation and slope
increase, besides it has an important precipitation. This would explain the spread of erosion in this aridzone, because the crop and pasture management with important elevation and slope erodes the topsoil andmakes the land fragile especially when exposed to intense rainfall as it is. Finally, the barren or sparsely
vegetated area is mainly spread (90%) in low elevation and gentle slope. Within the sub-watersheds
analysis point of view, we assessed and classified the topographic typologies into three categories, i.e.
major, second and non-usual for the main vegetative types within each sub-watershed. As a result, wefind out this sub-repartition of the topographic characteristics as shown in the Table 1.
So, the major topography combinations within the subwatersheds for the most dominant land-cover in
Ethiopia are: gentle slope with middle or low elevation, followed by gentle slope with middle or highelevation. Middle slope and high elevation are non-usual. This is confirming with the map of
topographic characteristics distribution that shows in the center of the country the two types of topography:

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Table 1. Repartition of the topographic characteristics through the four main dominant land cover in thesub-watersheds.
Subwatersheds
Dryland-Cropland
Barren or sparsely
Savanna
Shrubland-pasture
Vegetateddominant L/Cdominant L/Cdominant L/Cdominant L/CTopography
Major

low-elev/gentl-slop

low-elev/gentl-slop

mid-elev/gentl-slop

Secondmid-elev/gentl-slop

high-elev/gentl-slop

high-elev/gentl-slop

Non-usual

low-elev/midslop
high-elev/mid-slop

mid-elev//mid sloplow-elev/gentl-slop
high-elev/mid-slop

low-elev/gentl-slop

low-elev/mid slop

gentle slope with middle or high elevation. While in the periphery, we see the low elevation with gentle
slope.
4. Conclusion and recommendation
We found a relationship between vegetation and topographic characteristics (elevation, slope, aspect)
and precipitation in Ethiopia. This correlation may explain the spread of erosion in this arid zone
subjected to erratic and aggressive rainfall especially when agricultural and livestock management arepracticed in inappropriate topography and location. Meteorological data will improve our interpretationin later date. Moreover, we can deepen this research while assessing the vegetation cover by using NDVI
in order to make erosion investigations on the appropriate chosen watershed using a hydrological model.
Acknowledgment
Authors would like to thank the DT9 reviewers for comments.
References
Ademola, K. Braimoh, Paul L.G. Vlek (2008): Land Use and Soil Resources. p.253.
Mengistu A. (2003): Ethiopia, Country Pasture/Forage Resources Profiles, Ethiopia, FAO.
Reynolds S.G. (2006): Ethiopia, Country Pasture/Forage Resources Profiles, Ethiopia, FAO.
Woreka B.B. (2004): Evaluation of soil erosion in the Hararge region of Ethiopia using soil loss models, rainfall simulation and fieldtrial. Ph.D. thesis.