Session 3 : Hydrological analysis of two sub-catchments of the Mareb River (Eritrea)
258
For each month the average Antecedent Moisture Condition (AMC) has been calculated,
obtaining: October to June type I; July and September type II; August, type III. The CN values
have been consequently corrected in relation with the monthly AMC, before calculating the daily
runoff. The annual runoff volume has been obtained adding daily runoff, obtained by equation
[3], of each month of the year. Annual runoff volume has been calculated for a period of 28
years, using equation [5]. Mean, maximum, minimum and 75% probability of water volume
inflow into the reservoir have been calculated (Table 5).
TABLE 5
Annual volume inflow into the reservoirs.
Annual Volume Inflow (Qv)
Unit
Shiketi
Emni-Tzellim
Initial Reservoir capacity
m 3
256000
170269
Mean volume inflow
m 3
557550
939960
Maximum volume inflow
m 3
1397617
1671806
Minimum volume inflow
m 3
66416
38998
Volume 75% probability
m 3
299078
561461
Comparing results of annual runoff estimation with reservoir sizing it comes out that Shiketi
dam is well dimensioned, while Emni Tzellim reservoir is far behind the potential water storage.
A dynamic annual water balance of the reservoirs has been calculated in order to evaluate the
effective water volume available for irrigation.
Sedimentation rate into the two reservoirs (Colombo et al., 1995) has been utilized to
estimate their effective volume. Losses due to evaporation and infiltration from the reservoirs
have been estimated roughly 10% of the total volume. The irrigable area has been calculated
considering an irrigation requirement of 10 000 m
3 of water pro hectare (Table 6).
TABLE 6
Reservoirs capacity and irrigable area
Reservoir Capacity
Unit
Shiketi
Emni-Tzellim
Dam construction
year
1983
1987
Initial Reservoir Capacity
m 3
256 000
170 269
Sedimentation Rate
% year
3
11
Actual Capacity
m 3
156 000
0
Actual Available water volume
m 3 year
141 000
0
Capacity after 10 years
m 3
180 000
0
Initial Irrigable Surface
he
25
17
Irrigable Surface after 10 years
he
18
0
CONCLUSIONS
The SCS-CN method has been applied to estimate daily and yearly surface runoff volume.
Remotely sensed data have given a helpful contribution to CN approach (Table 7): land unit map
has been derived from remotely sensed data, hydrologic condition has been obtained by SPOT
Vegetation Index.
258
For each month the average Antecedent Moisture Condition (AMC) has been calculated,
obtaining: October to June type I; July and September type II; August, type III. The CN values
have been consequently corrected in relation with the monthly AMC, before calculating the daily
runoff. The annual runoff volume has been obtained adding daily runoff, obtained by equation
[3], of each month of the year. Annual runoff volume has been calculated for a period of 28
years, using equation [5]. Mean, maximum, minimum and 75% probability of water volume
inflow into the reservoir have been calculated (Table 5).
TABLE 5
Annual volume inflow into the reservoirs.
Annual Volume Inflow (Qv)
Unit
Shiketi
Emni-Tzellim
Initial Reservoir capacity
m 3
256000
170269
Mean volume inflow
m 3
557550
939960
Maximum volume inflow
m 3
1397617
1671806
Minimum volume inflow
m 3
66416
38998
Volume 75% probability
m 3
299078
561461
Comparing results of annual runoff estimation with reservoir sizing it comes out that Shiketi
dam is well dimensioned, while Emni Tzellim reservoir is far behind the potential water storage.
A dynamic annual water balance of the reservoirs has been calculated in order to evaluate the
effective water volume available for irrigation.
Sedimentation rate into the two reservoirs (Colombo et al., 1995) has been utilized to
estimate their effective volume. Losses due to evaporation and infiltration from the reservoirs
have been estimated roughly 10% of the total volume. The irrigable area has been calculated
considering an irrigation requirement of 10 000 m
3 of water pro hectare (Table 6).
TABLE 6
Reservoirs capacity and irrigable area
Reservoir Capacity
Unit
Shiketi
Emni-Tzellim
Dam construction
year
1983
1987
Initial Reservoir Capacity
m 3
256 000
170 269
Sedimentation Rate
% year
3
11
Actual Capacity
m 3
156 000
0
Actual Available water volume
m 3 year
141 000
0
Capacity after 10 years
m 3
180 000
0
Initial Irrigable Surface
he
25
17
Irrigable Surface after 10 years
he
18
0
CONCLUSIONS
The SCS-CN method has been applied to estimate daily and yearly surface runoff volume.
Remotely sensed data have given a helpful contribution to CN approach (Table 7): land unit map
has been derived from remotely sensed data, hydrologic condition has been obtained by SPOT
Vegetation Index.