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The West Coast, excluding the Gambia River and Volta basins

Except for The Gambia, which is entirely located in the Gambia River basin, all the other countries from Senegal in the west to Nigeria in the East are partly or wholly located within this remaining part of the West Coast (Map 16 and Table 67).

Rivers and discharges

The area of Senegal in the West Coast can be divided into two parts: the area south of the Gambia basin: Casamance and Kayanga basins; the area north of the Gambia basin and south of the Senegal basin: Ferlo, Car-Car, Sine and Saloum basins.

Table 67: West Coast, excluding Gambia River and Volta basins: areas by country

The annual discharge of the Casamance River, as measured between 1968 and 1983 was 0.07 km³. In the dry season (April-July) the river may run dry. Dams to protect the area against salt intrusion are necessary. The Kanyanga River is the upper part of the Gêba River in Guinea Bissau, but no discharge figures are available. Nor are there figures available for discharges in the northern part.

Guinea Bissau is wholly situated in the West Coast. The main rivers are the Cacheu originating within the country, the Gêba originating in Senegal and the Corubal originating in Guinea. The water resources in this small country are abundant, but they are badly distributed in space and in time: 90% of the flow occurs in 6 months. The annual discharge of the largest river, the Corubal, is estimated at 13.2 km³/year. In the coastal area, problems of salt intrusion exist in the dry season and many 'anti-salt' dams have been constructed.

Two separate parts of Guinea are located in this West Coast area:

• the eastern part of the Middle Region and the Lower Region, draining to the sea;
• the southern part of the Forest Region, draining to Liberia and Sierra Leone.

The water resources of Guinea are abundant.

Sierra Leone is one of the most humid countries of Africa. It can be divided into 12 major river basins, of which five are shared with Guinea and two with Liberia.

Like Sierra Leone, Liberia is one of the most humid countries of Africa. Two types of river exist: the major basins from north-east to south-west, with rivers originating in Guinea and Côte d'Ivoire and with an average entering discharge of 15 to 20 km³/year; numerous, short, coastal watercourses.

The source of the Comoé River is in the south-west of Burkina Faso, the most humid region of the country. It is one of the few permanent rivers of Burkina Faso, with an average annual discharge leaving the country to Côte d'Ivoire of about 1.29 km³. In Côte d'Ivoire many other rivers run parallel southwards to the sea. In the west is the Cavally River, which has its source in Guinea, then enters Côte d'Ivoire and further downstream becomes the border between Côte d'Ivoire and Liberia.

In Ghana many rivers run more or less parallel southwards to the sea. The most important are the Pra, with an annual discharge of about 6.2 km³, and the Tano, with 4.5 km³.

The Mono originates in Togo and at about 100 km from the sea it becomes the border between Benin and Togo, with an average annual discharge of about 2.9 km³. In the south-west of Togo is the Lake Togo basin, with an area of about 8000 km². The Couffo originates at the bower between Benin and Togo about 200 km north of the sea. In Benin, three main rivers flow southwards to the sea. The Ouémé originates in the centre. The Okpara tributary also originates in the centre but becomes the border between Nigeria and Benin before re-entering Benin to flow into the Ouémé. The discharge close to the sea is estimated at 5.4 km³/year.

About one-third of the basin area of Nigeria is covered by tropical rain forest. Many rivers flow from north to south to the sea. The annual potential surface water resources of the basin area are estimated at 36 km³. Peak outflows occur in September-October. Many dams have been built on the rivers of the western littoral, including the Oyan dam on the Oyan River. The runoff of the Osun River is regulated by the Asejire Dam.

Irrigation potential and water requirements

In Senegal about 22000 ha in the Casamance are controlled region by small dams [179]. Considering a potential of 40000 ha, the irrigation water requirement would be 0.920 km³/year. This is much more than the average annual discharge measured in the river. Most of this area, however, consists of mangroves [*]. The potential for cereal production in the Kayanga region has been estimated at about 15000 ha, which would lead to a water requirement of 0.105 km³/year [179]. Although no detailed discharge figures exist, this is also more than the quantity available. In the part north of the Gambia basin the irrigation potential will probably not exceed 30 000 ha, based on available water resources [*]. In this region, it is planned to irrigate about 8500 ha through the Cayor Canal, which in fact takes water from the Senegal River [181].

The irrigation potential in Guinea Bissau is estimated at 281290 ha, which in fact corresponds to the total potential rice area, of which about 150000 ha are mangroves [120].

The humid land potential in Guinea is estimated at 520000 ha (140000 ha of mangroves, 180000 ha of wetland and 200000 ha of floodplains), of which about 210000 ha can be developed relatively easily for irrigated agriculture [116]. Table 68 below summarizes the different potential areas for the various regions and basins in Guinea.

Table 68: Humid land potential of Guinea by region and major basin group

The irrigation potential for Liberia is estimated at 600000 ha, consisting mainly of freshwater swamps [129]. The total area suitable for development in Sierra Leone has been estimated at 807000 ha, ignoring environmental aspects of wetland development [185].

The irrigation potential in the Comoé basin in Burkina Faso is estimated at 17460 ha [67, 69, 72, 73], of which about 2500 ha are valley bottoms and 450 ha small areas irrigated by small earth dams. Of the irrigation potential of 475000 ha for the whole of Côte d'Ivoire, 175000 ha are valley, 200000 ha are large floodplains and 100000 ha are coastal swamps [21a]. Of this total, 400000 ha are estimated to be in this part of the West Coast [*].

Of the irrigation potential of 300000 ha for the whole of Benin [57], 170000 ha are estimated to be located in this part of the West Coast [*]. For Togo it is estimated at 90000 ha [*]. Of the total potential of 1.9 million hectares of Ghana [114], 700000 ha are estimated to be located in this part of the West Coast [*].

In Nigeria, the national water resources master plan (NWRMP) estimates the total identified irrigation potential in the basin area at 50000 ha [172]. Irrigation projects in the region have not been accelerated compared to the other regions, because the hilly area would create some difficulties for the canal system. Water requirements are estimated at 16500 m³/ha per year in the present study and at 13400 m³/ha per year in the NWRMP. Water resources are abundant.

Table 69: West Coast, excluding Gambia River and Volta teas ins: irrigation potential and water requirements

Most of the countries have abundant water resources and no water shortage problems will arise in irrigation development, except for Senegal and Guinea Bissau.

Table 70 summarizes the irrigation potential for the whole of the West Coast, including the Gambia River and Volta basins.

Table 70: West Coast: irrigation potential, water requirements and areas under irrigation

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