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The West Coast is the region grouping all the basins draining to the sea from Senegal to Nigeria. It covers 4.7% of the continent and spreads over 13 countries (Map 16 and Table 61).
Table 61: West Coast: areas and rainfall by country
Country |
Total area of the country (km²) |
Area of the country within the basin (km²) |
As % of total area of basin (km²) |
As % of total area of country (km²) |
Average annual rainfall in the basin area (mm) |
||
min. |
max. |
mean |
|||||
Senegal |
196720 |
124854 |
8.7 |
63.5 |
350 |
1630 |
870 |
Gambia |
11300 |
11300 |
0.8 |
100.0 |
800 |
1115 |
955 |
Guinea Bissau |
36120 |
36120 |
2.5 |
100.0 |
1260 |
2440 |
1700 |
Guinea |
245857 |
119502 |
8.4 |
48.6 |
1300 |
3080 |
2085 |
Sierra Leone |
71740 |
71740 |
5.0 |
100.0 |
1870 |
3395 |
2690 |
Liberia |
97750 |
97750 |
6.8 |
100.0 |
1770 |
3300 |
2370 |
Mali |
1240190 |
9496 |
0.7 |
0.8 |
545 |
1365 |
675 |
Burkina Faso |
274000 |
197379 |
13.8 |
72.0 |
555 |
1310 |
920 |
Côte d'Ivoire |
322462 |
298692 |
20.9 |
92.6 |
1050 |
2310 |
1370 |
Ghana |
238540 |
238540 |
16.7 |
100.0 |
855 |
1785 |
1265 |
Togo |
56785 |
56785 |
4.0 |
100.0 |
925 |
1550 |
1215 |
Benin |
112620 |
66236 |
4.6 |
58.8 |
915 |
1345 |
1145 |
Nigeria |
923770 |
101802 |
7.1 |
11.0 |
1090 |
2595 |
1505 |
For West Coast |
1430196 |
100.0 |
350 |
3395 |
1435 |
||
In this section two international basins in this region have been treated separately, the Gambia River basin and the Volta basin. The other basins have been regrouped and called 'the West Coast, excluding the Gambia River and Volta basins'.
The Gambia River basin occupies about 5.4% of the West Coast and is shared among three countries (Map 16 and Table 62).
Table 62: Gambia basin: areas by country
Country |
Total area of the country (km2) |
Area of the country within the basin (km2) |
As % of total area of basin (%) |
As % of total area of country (%) |
Guinea |
245 857 |
8 000 |
10.3 |
3.3 |
Senegal |
196 720 |
68550 |
75.2 |
29.8 |
Gambia |
11 300 |
11 300 |
14.5 |
100.0 |
For Gambia basin |
77 850 |
100.0 |
Rivers and discharges
The Gambia River has its sources in the high rainfall mountainous Fouta Djallon in the north of the Central Guinea region. The total quantity of water leaving Guinea for Senegal is estimated at 3 km³/year.
The river then flows northwards to enter The Gambia in the extreme east of the country. Contradictory information exists about the discharges entering The Gambia. According to different sources, they range from 4 km /year [181, average of 1951-1990] to nearly 10 km³/year, [25]. Its flow is highly seasonal: the peak discharge is about 2000 m³/s, but for six months the inflow at the Gambian border is less than 10 m³/s. In May it falls below 0.5 m³/s.
Because of the flat topography of The Gambia and the low river discharges during the dry season, salt water moves up to about 70 km upstream in the wet season and 250 km upstream in the dry season. The tidal variation at the mouth is about 1.6 m [48a].
Irrigation potential and water requirements
The higher, upstream part of the basin in Guinea is badly eroded. Irrigation would be possible in the downstream part, where the potential has been estimated at 20000 ha [*].
There are 60000 ha of suitable soils in the Gambia basin in Senegal [48a]. It is planned to construct a dam at Kekreti for hydropower and this could irrigate an estimated 15000 ha in Senegal and 55000 ha in The Gambia [181].
Soils suitable for irrigation in The Gambia are estimated at 80000 ha [48a]. There are about 104200 ha of swamps, of which 33500 ha are cultivated. Mangroves account for an additional 67000 ha [111]. In the dry season, the salt tongue moves upstream at a rate of 15-20 km/month. It is thought that an additional withdrawal of 1 m³/s would increase the penetration of the salt tongue by 1 km/month. The safe limit for irrigation from the Gambia River without major dam construction is, therefore, estimated to be no more than 2400 ha in the dry season [48a]. However, if the planned Kekreti dam on the Gambia River in Senegal is constructed, it is expected that 15000 ha can be irrigated in Senegal and 55000 ha in the Gambia [181]. Moreover, this dam could contain salt intrusion during the dry season. The development of these 55000 ha would require 0.275 km³/year of water. A further 25000 ha of mangrove cultivation would require 0.125 km³/year of water.
Table 63: Gambia River basin: irrigation potential and water requirements
Country |
Irrigation potential (ha) |
Gross potential irrigation water requirement |
|
per ha |
total (km3/year) |
||
Guinea |
20000 |
16 000 |
0.320 |
Senegal |
15000 |
7000 |
0.105 |
Gambia |
80000 |
5000 |
0.400 |
Sum of countries |
115 000 |
0.825 |
|
Total for Gambia basin |
115 000 |
0.825 |
|
Although the annual irrigation water requirement is only 10% of the discharge, any water abstraction within the basin in the dry season should be studied very carefully until the Kekreti dam is constructed, in view of the low discharges in the dry season and the danger of increasing salt intrusion from the sea.