Session 3 : Hydrological analysis of two sub-catchments of the Mareb River (Eritrea)
254
sand, well drained (Regosols). On alluvial terraces and fans there are moderately deep to very
deep soils, gravelly sand to gravelly loam, well to somewhat excessively drained (Fluvisols). On
cradle valleys there are deep black soils, clayey, moderately well to poorly drained (Vertisols).
On the alluvial plain there are moderately deep soils, gravelly clay to gravelly loam, well to
poorly drained (Cambisols in association with Fluvisols and Leptosols).
Geomorphology and Soils of the Emni-Tzellim catchment
The catchment area is delimited between the coordinates of 15°02'58'' and 15°01'01'' latitude ,
and of 38° 42' 47'' and 38° 47’ 06'' longitude, covering approximately an area of 1172 ha.
Elevation is between 2 000 and 2 550 m asl. Some morphometric parameters are in Table 1.
The Emni-Tzellim catchment includes Cainozoic volcanic products and a thick sequence of
welded pyroclastic flows with interbedded lava flow. The catchment is represented by units of
volcanic denudational origin, with steep terrain, and by units of alluvial-colluvial origin with
relatively flat to gently sloping topography. Rock falls, gullies, sheets and rills erosion are the
main active processes. Accumulation glacis are originated above structural volcanic terraces. The
old sheetflood plain is buried under the dissected accumulation glacis and residual alluvial
terraces, remnant of an older fluvial landscape, outcrop in the Tzellim plain.
On volcanic escarpments there are shallow to very shallow soils, very gravelly loam to very
gravelly sand, well drained (Leptosols in association with Regosols). On volcanic slopes and
footslopes there are moderately deep to deep soils, very gravelly loam, well drained (Regosols).
On footslopes there are deep soils, loam to sand, well drained (Fluvisols). On older alluvial
terraces and on sheetflood areas there are moderately deep to deep soils, gravelly clay to clay,
poorly drained (Vertisols). Distributed over the whole area there are moderately deep to deep
soils, gravelly loam, moderately to poorly drained (Cambisols).
RUNOFF ESTIMATION
The Soil Conservation Service (SCS) method (US Dept., 1985) has been used to predict the total
volume of runoff that may come from a watershed during a design flood (25-year return period)
and to predict the total annual runoff volume for daily rainfall, during a period of 28 years.
The SCS Runoff Curve Number (CN) is an empirical description for infiltration and rainfall
excess. The SCS runoff equation is:
Q
P I
P I
S
a
a
=
−
−
+
(
)
(
)
2
[1]
where Q = runoff (mm); P = rainfall (mm); Ia = initial abstraction (mm); S = potential maximum
retention after runoff begins (mm).
By removing Ia as an independent parameter:
Ia
S
=
×
0 2
.
[2]
runoff is:
Q
P
S
P
S
=
− ×
+ ×
(
.
)
(
.
)
0 2
0 8
2
[3]
254
sand, well drained (Regosols). On alluvial terraces and fans there are moderately deep to very
deep soils, gravelly sand to gravelly loam, well to somewhat excessively drained (Fluvisols). On
cradle valleys there are deep black soils, clayey, moderately well to poorly drained (Vertisols).
On the alluvial plain there are moderately deep soils, gravelly clay to gravelly loam, well to
poorly drained (Cambisols in association with Fluvisols and Leptosols).
Geomorphology and Soils of the Emni-Tzellim catchment
The catchment area is delimited between the coordinates of 15°02'58'' and 15°01'01'' latitude ,
and of 38° 42' 47'' and 38° 47’ 06'' longitude, covering approximately an area of 1172 ha.
Elevation is between 2 000 and 2 550 m asl. Some morphometric parameters are in Table 1.
The Emni-Tzellim catchment includes Cainozoic volcanic products and a thick sequence of
welded pyroclastic flows with interbedded lava flow. The catchment is represented by units of
volcanic denudational origin, with steep terrain, and by units of alluvial-colluvial origin with
relatively flat to gently sloping topography. Rock falls, gullies, sheets and rills erosion are the
main active processes. Accumulation glacis are originated above structural volcanic terraces. The
old sheetflood plain is buried under the dissected accumulation glacis and residual alluvial
terraces, remnant of an older fluvial landscape, outcrop in the Tzellim plain.
On volcanic escarpments there are shallow to very shallow soils, very gravelly loam to very
gravelly sand, well drained (Leptosols in association with Regosols). On volcanic slopes and
footslopes there are moderately deep to deep soils, very gravelly loam, well drained (Regosols).
On footslopes there are deep soils, loam to sand, well drained (Fluvisols). On older alluvial
terraces and on sheetflood areas there are moderately deep to deep soils, gravelly clay to clay,
poorly drained (Vertisols). Distributed over the whole area there are moderately deep to deep
soils, gravelly loam, moderately to poorly drained (Cambisols).
RUNOFF ESTIMATION
The Soil Conservation Service (SCS) method (US Dept., 1985) has been used to predict the total
volume of runoff that may come from a watershed during a design flood (25-year return period)
and to predict the total annual runoff volume for daily rainfall, during a period of 28 years.
The SCS Runoff Curve Number (CN) is an empirical description for infiltration and rainfall
excess. The SCS runoff equation is:
Q
P I
P I
S
a
a
=
−
−
+
(
)
(
)
2
[1]
where Q = runoff (mm); P = rainfall (mm); Ia = initial abstraction (mm); S = potential maximum
retention after runoff begins (mm).
By removing Ia as an independent parameter:
Ia
S
=
×
0 2
.
[2]
runoff is:
Q
P
S
P
S
=
− ×
+ ×
(
.
)
(
.
)
0 2
0 8
2
[3]