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The West Coast

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 (km)

Area of the country within the basin (km)

As % of total area of basin (km)

As % of total area of country (km)

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

Cte 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

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 km/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 km/year, [25]. Its flow is highly seasonal: the peak discharge is about 2000 m/s, but for six months the inflow at the Gambian border is less than 10 m/s. In May it falls below 0.5 m/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 m/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 km/year of water. A further 25000 ha of mangrove cultivation would require 0.125 km/year of water.

Table 63: Gambia River basin: irrigation potential and water requirements

Country

Irrigation potential (ha)

Gross potential irrigation water requirement



per ha
(m3/ha per year)

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.


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