Télédétection et ressources en eau/Remote sensing and water resources
253
STUDY AREA
The study area is composed of two sub-catchments of the Mareb river, situated on the Eritrean
highland: Shiketi and Emni-Tzellim (figure 1).
Climate and Land Cover
The two watersheds have almost the same climate. Asmara (2360 m asl) is the nearest
climatological station, located about 30 km north of the study area. Mean annual temperature is
16.3 °C and annual potential evapotranspiration is 770 mm. Mean annual rainfall is 541 mm.
There are two rainy seasons and a long dry season. The so-called small-rains (113 mm) fall
between April and June; the so-called great-rains (381 mm) from July to the beginning of
September; during Fall and Winter there is a long period with very scarce rainfall (47 mm).
Land cover is extremely variable due to differences in morphology and land use. Natural
vegetation is mainly an open formation of shrubs and small trees of Acacia etbaica, utilized for
grazing and for fuelwood collection. Percentage of vegetation cover is very poor, ranging from 5
to 30%. Main crops are wheat, barley, teff, maize, sorghum cultivated with traditional low inputs
techniques (animal traction). Irrigated agriculture (mainly horticultural crops) is confined to small
areas. Most of the Shiketi catchment is covered by natural vegetation, while in the Emni-Tzellim
catchment arable land is about 70% of the surface.
Both sub-catchments are affected by severe erosion (Colombo et al., 1995).
Geomorphology and Soils of the Shiketi catchment
The catchment area is delimited between the coordinates of 15°12'04'' and 15°10'10'' latitude, and
of 38°50'43'' and 38°51'45'' longitude, covering approximately 585 ha. Elevation is between 2050
and 2300 m asl. Some morphometric parameters are in Table 1.
TABLE 1
Some morphometric parameters of the two sub-catchments.
Parameters
SHIKETI
EMNI-TZELLIM
Basin area (he)
585
1172
Basin order (Strahler's system)
5
5
Drainage pattern
dendritic
subdendritic
Drainage density (km/km 2 )
4.8
4.8
Bifurcation ratio
3.9
3.5
Relief ratio
0.059
0.025
Gravelius Index
1.2
1.7
Concentration Time (min)
55
130
The Shiketi catchment includes Late Proterozoic rocks and Cainozoic volcanic products,
separated by a lateritic formations. The catchment is mainly represented by units of denudational
and structural origin with steep to very steep rolling to hilly topography. Rock falls, debris slides,
and gullies are the main active processes. Structural terraces in lateritic formation reproduce the
oldest peneplain. Valley bottom is characterized by alluvial terraces buried under a variable thick
of colluvial sediments.
On the volcanic plateau, on colluvial areas and on laterite there are shallow soils, very
gravelly loam to very gravelly sand, well to excessively drained (Leptosols). On volcanic slopes
and escarpments there are moderately deep to deep soils, very gravelly loam to very gravelly
253
STUDY AREA
The study area is composed of two sub-catchments of the Mareb river, situated on the Eritrean
highland: Shiketi and Emni-Tzellim (figure 1).
Climate and Land Cover
The two watersheds have almost the same climate. Asmara (2360 m asl) is the nearest
climatological station, located about 30 km north of the study area. Mean annual temperature is
16.3 °C and annual potential evapotranspiration is 770 mm. Mean annual rainfall is 541 mm.
There are two rainy seasons and a long dry season. The so-called small-rains (113 mm) fall
between April and June; the so-called great-rains (381 mm) from July to the beginning of
September; during Fall and Winter there is a long period with very scarce rainfall (47 mm).
Land cover is extremely variable due to differences in morphology and land use. Natural
vegetation is mainly an open formation of shrubs and small trees of Acacia etbaica, utilized for
grazing and for fuelwood collection. Percentage of vegetation cover is very poor, ranging from 5
to 30%. Main crops are wheat, barley, teff, maize, sorghum cultivated with traditional low inputs
techniques (animal traction). Irrigated agriculture (mainly horticultural crops) is confined to small
areas. Most of the Shiketi catchment is covered by natural vegetation, while in the Emni-Tzellim
catchment arable land is about 70% of the surface.
Both sub-catchments are affected by severe erosion (Colombo et al., 1995).
Geomorphology and Soils of the Shiketi catchment
The catchment area is delimited between the coordinates of 15°12'04'' and 15°10'10'' latitude, and
of 38°50'43'' and 38°51'45'' longitude, covering approximately 585 ha. Elevation is between 2050
and 2300 m asl. Some morphometric parameters are in Table 1.
TABLE 1
Some morphometric parameters of the two sub-catchments.
Parameters
SHIKETI
EMNI-TZELLIM
Basin area (he)
585
1172
Basin order (Strahler's system)
5
5
Drainage pattern
dendritic
subdendritic
Drainage density (km/km 2 )
4.8
4.8
Bifurcation ratio
3.9
3.5
Relief ratio
0.059
0.025
Gravelius Index
1.2
1.7
Concentration Time (min)
55
130
The Shiketi catchment includes Late Proterozoic rocks and Cainozoic volcanic products,
separated by a lateritic formations. The catchment is mainly represented by units of denudational
and structural origin with steep to very steep rolling to hilly topography. Rock falls, debris slides,
and gullies are the main active processes. Structural terraces in lateritic formation reproduce the
oldest peneplain. Valley bottom is characterized by alluvial terraces buried under a variable thick
of colluvial sediments.
On the volcanic plateau, on colluvial areas and on laterite there are shallow soils, very
gravelly loam to very gravelly sand, well to excessively drained (Leptosols). On volcanic slopes
and escarpments there are moderately deep to deep soils, very gravelly loam to very gravelly