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The West Central coast

The West Central Coast covers 2.3% of the continent and spreads over seven countries (Map 17 and Table 71).

TABLE 71 West central 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 (%)

As % of total area of country (%)

Average annual rainfall in the basin area







(mm)







min.

max.

mean

Nigeria

923770

58493

8.3

6.3

1420

2740

2 070

Cameroon

475440

239021

33.9

50.3

1365

2830

1835

Gabon

267670

267670

38.0

100.0

1320

2595

1800

Equat. Guinea

28050

28050

4.0

100.0

1695

2585

2050

Congo

342000

95023

13.5

27.8

1125

1940

1475

Angola

1246700

7150

1.0

0.6

775

1280

1110

Zaire

2344860

9367

1.3

0.4

785

1290

1190

For West Central Coast


704774

100.0


775

2830

1785

Rivers and discharges

Rising in the Cameroon highlands, an area of dense rain forest, the Cross river, enters Nigeria with an annual discharge estimated at 17 km. Annual runoff to the sea is estimated at almost 52 km. Another important river in Nigeria is the Imo River, with an average annual discharge of 4 km. The total surface water resources in the basin area are estimated at 69 km/year. About 85 % of the annual runoff of the Cross River and 70 % of the annual runoff of the Imo River are concentrated in five months, from June to October with the peak in September.

In Cameroon many rivers flow directly to the sea. The most important one is the Sanaga River, with an average annual discharge of almost 63 km. Other important rivers are the Nyong, the Wouri and the Ntem Rivers, with a total annual discharge of over 32 km.

Also in Gabon many rivers flow directly to the sea. The most important one is the Ogoou with an annual discharge of more than 148 km/year. Its basin occupies about 75% of the country. Another important river is the Nyanga to the south, with an annual discharge estimated at 17.1 km.

In the mainland part of Equatorial Guinea several watercourses, most of which originate within the country, cross the country while flowing to the sea. The renewable water resources are estimated at 30 km/year for the mainland and the island together.

Of the many rivers flowing to the sea in Congo, the most important one is the Kouilou-Niari River. Its basin covers nearly 60% of the area of Congo in the West Central Coast. Its annual flow to the sea is estimated at about 28 km.

Cabinda, the part of Angola lying in the West Central Coast, is separated from the rest of Angola by the Congo/Zaire River and a strip of land to the north of the river belonging to Zaire. Its area corresponds to only 0.6% of the total area of Angola. The most important river is the Chiloango, the upstream part of which forms the border between Zaire and Angola. The part of Zaire lying in the West Central Coast, only 0.4% of the total area of Zaire, corresponds to the basin of the Chiloango River.

Irrigation potential and water requirements

The identified irrigation potential in the West Central Coast in Nigeria is 100000 ha according to the national water resources master plan (NWRMP) [172]. The irrigation water requirements are estimated at 15000 m/ha per year in the present study and at 10400 m/ha per year in the NWRMP. Water resources are abundant.

The irrigation potential in Cameroon has been estimated at 120000 ha [*]. For Gabon it is 440000 ha [17]. For Equatorial Guinea no figures on irrigation potential are available. It is estimated in the present study at 30000 ha, corresponding to 1% of the total area of the country [*]. The irrigation potential figures of 85000 ha for Congo, 50000 ha for Angola and 10000 ha for Zaire in the West Central Coast are explained in the section The Congo/Zaire basin [*].

TABLE 72 West Central Coast: irrigation potential, water requirements and areas under irrigation

Country

Irrigation potential (ha)

Gross potential irrigation water requirement

Area under irrigation (ha)



per ha (m/ha per year)

total (km/year)


Nigeria

100000

10400

1.040

20000

Cameroon

120000

15000

1.800

3000

Gabon

440000

13000

5.720

4450

Equat. Guinea

30000

12500

0.375


Congo

85000

14000

1.190

0

Angola

50000

14500

0.725

1000

Zaire

10000

14500

0.145

0

Sum of countries

835000


10.995


Total for W. Cent. Coast

835000


10.995


Many figures are arbitrary estimates as reliable information is lacking. It is a very humid region and neither land nor water is a limiting factor for the estimation of the irrigation potential.

The south west coast

The South West Coast covers 1.7% of the continent and spreads over two countries (Map 18 and Table 73).

TABLE 73 South 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 (%)

As % of total area of country (%)

Average annual rainfall in the basin area (mm)






min.

max.

mean

Namibia

824 900

17 549

3.4

2.1

90

515

350

Angola

1246700

498651

96.6

40.0

10

1600

960

For S. West Coast


516 200

100.0


10

1600

940

Rivers and discharges

Almost 97% of the area of the South West Coast is covered by Angola, the remaining part by Namibia, that shares the border river, the Cunene, with Angola. This river originates in the central highlands of Angola and its annual discharge reaching the border is about 5 km. Many other rivers originate within Angola. Annual rainfall in the South West Coast decreases considerably from the north-east to the south-west (from 1600 mm to 10 mm).

Irrigation potential and water requirements

Namibia has access to an agreed volume of 0.18 km/year of water from the Cunene River, of which 0.13 km is for agricultural purposes. Considering a gross irrigation water requirement of 5500 m/ha per year [*], this would lead to an irrigation potential of 23600 ha. However, literature gives estimates of gross irrigation water requirements of 16500 m/ha per year [163], which would lead to an irrigation potential of 7900 ha, using the 0.13 km/year of water.

The irrigation potential of Angola in the South West Coast is estimated at 1.8 million hectares, as explained in the section The Congo/Zaire basin [*].

TABLE 74 South West Coast: irrigation potential, water requirements and areas under irrigation

Country

Irrigation potential (ha)

Gross potential irrigation water requirement

Area under irrigation



per ha (m/ha per year)

total (km/year)


Namibia

7900

16500

0.130

0

Angola

1800000

13500

24.300

70000

Sum of countries

1807900


24.430

70000

Total for S. West Coast

1807900


24.430


The South Atlantic coast

The South Atlantic Coast, located in South-Western Africa, covers 1.2% of the continent and spreads over two countries (Map 19 and Table 75).

TABLE 75 South Atlantic 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 (%)

As % of total area of country (%)

Average annual rainfall in the basin area (mm)






min.

max.

mean

South Africa

1221040

101325

27.7

8.3

45

555

200

Namibia

824900

264160

72.3

32.0

0

485

190

For S. Atl. Coast


365485

100.0


0

555

190

Rivers and discharges

The South Atlantic Coast is the driest region in southern Africa. In Namibia a few ephemeral rivers exist, on which dams have been constructed. In South Africa three main basins are located in this region and the total surface water resources are estimated at 3.37 km/year, of which 1.62 km/year is exploitable and less than 1.00 km/year available for irrigation purposes [190].

Irrigation potential and water requirements

In South Africa the irrigation water requirements are estimated at 10000 m/ha per year in this region [*]. Using 0.83 km/year of water for irrigation, this would lead to an irrigation potential of 83200 ha. About 84000 ha already benefit from irrigation, which means that from a point of view of water availability the maximum area has already been brought under irrigation.

The irrigation potential of Namibia in this region does not exceed 1000 ha [*]. The present study estimates the water requirements at 7500 m/ha per year, literature gives figures of 33000 m /ha per year, as different irrigation cropping patterns are assumed.

TABLE 76 South Atlantic Coast: irrigation potential, water requirements and areas under irrigation

Country

Irrigation potential (ha)

Gross potential irrigation water requirement

Area under irrigation



per ha (m/ha per year)

total (km/year)


South Africa

83200

10000

0.832

84000

Namibia

1000

20000

0.020

0

Sum of countries

84200


0.852

84000

Total for S.

<=84200


0.852



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