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TABLE 92 Comparison of the figures on soil and land suitability for irrigation with the figures on irrigation potential. by basin

Basin No.

Basin

Total area of basin(ha)

Land suitable for surface irrigation(ha)

Irrigation potential of basin(ha)

Irrigation potential as % of suitable land


(1)

(2)

(3)

(4)

(5)=100*(4)/(3)

01

Senegal

48 318 100

3 645 800

420 000

11.52

02

Niger

227 394 600

28 943 400

2 816 510

9.73

03

Lake Chad

238 163 500

36 524 600

1 163 200

3.18

04

Nile

311 236 900

92 019 000

8 000 000

8.69

05

Rift Valley

63 759 300

13 946 700

844 010

6.05

06

Shebelli-Juba

81 042 700

25 847 900

351 460

1.36

07

Congo/Zaire

378 905 300

109 815 500

9 800 000

8.92

08

Zambezi

135 136 500

37 632 500

3 160 380

8.40

09

Okavango

32 319 200

6 612 100

208 060

3.15

10

Limpopo

40 186 400

9 736 100

295 400

3.03

11

Orange

89 636 800

14 140 500

390 000

2.76

12

South Interior

64 582 600

15 421 800

54 000

0.35

13

North Interior

580 446 300

48 325 700

71 000

0.15

14

Mediterranean Coast

67 952 500

11 897 700

850 000

7.14

15

North West Coast

67 062 100

12 565 200

1 200 000

9.55

16

West Coast

143 019 600

29 567 400

5 112 750

17.29

17

West Central Coast

70 477 400

16 335 000

835 000

5.11

18

South West Coast

51 620 000

13 792 500

1 807 900

13.11

19

South Atlantic Coast

36 548 500

4 041 900

84 200

2.08

20

Indian Ocean Coast

66 378 500

14 853 200

1 500 000

10.10

21

East Central Coast

102 625 200

24 913 500

1 927 460

7.74

22

North East Coast

72 570 200

11 779 100

78 050

0.66

24

Madagascar

58 704 000

14 497 400

1 500 000

10.35

25

Islands

934 600

105 500

34 990

33.17

Total


3 029 020 800

596 960 000

42 504 370

7.12

While this study was based on average annual discharges, important seasonal variations do exist. In all cases full development of the whole irrigation potential would require important storage works and collaboration between countries on the management of shared waters within individual basins.

TABLE 93 Irrigation potential, irrigated areas and possibilities for irrigation expansion, by basin

Basin No.

Total area Basin

Irrigation of basin (ha)

Irrigation potential of basin (ha)

Area potential as % of basin area

Irrigation under irrigation (ha)

Possibility for as % of irrigation potential

irrigation expansion(ha)


(1)

(2)

(3)

(4) =100*(3)/(2)

(5)

(6) -100-(5)/(3)

(7) =(3)-(5)

01

Senegal

48 318 100

420 000

0.87

118 150

28.1

301 850

02

Niger

227 394 600

2 816 510

1.24

228 240

8.1

2 588 270

03

Lake Chad

238 163 500

1 163 200

0.49

113 296

9.7

1 049 904

04

Nile

311 236 900

8 000 000

2.57

5 078 604

63.5

2 921 396

05

Rift Valley

63 759 300

844 010

1.32

193 496

22.9

650 514

06

Shebelli-Juba

81 042 700

351 460

0.43

199 000

56.6

152 460

07

Congo/Zaire

378 905 300

9 800 000

2.59

35 767

0.4

9 764 233

08

Zambezi

135 136 500

3 160 380

2.34

146 869

4.6

3 013 511

09

Okavango

32 319 200

208 060

0.64

0

0.0

208 060

10

Limpopo

40 186 400

295 400

0.74

241 381

81.7

54 019

11

Orange

89 636 800

390 000

0.44

302 722

77.6

87 278

12

South interior

64 582 600

54 000

0.08

250

0.5

53 750

13

North Interior

580 446 300

71 000

0.01

232 500

327.5

- 161 500

14

Mediterran. Coast

67 952 500

850 000

1.25

1 606 700

189.0

- 756 700

15

North West Coast

67 062 100

1 200 000 1.79

1 000 750

83.4

199 250


16

West Coast

143 019 600

5 112 750

3.57

310 883

6.1

4 801 867

17

West Central Coast

70 477 400

835 000

1.18

28 450

3.4

806 550

18

South West Coast

51 620 000

1 807 900

3.50

70 000

3.9

1 737 900

19

South Atlant. Coast

36 548 500

84 200

0.23

84 000

99.8

200

20

Indian Ocean Coast

66 378 500

1 500 000

2.26

862 110

57.5

637 890

21

East Central Coast

102 625 200

1 927 460

1.88

178 610

9.3

1 748 850

22

North East Coast

72 570 200

78 050 0.11

24 574

31.5

53 476


24

Madagascar W+E

704 000

1 500 000

2.56

1 087 000

72.5

413 000

25

Islands

934 600

34 990

3.74

30 109

86.1

4 881

Total

3 029 020 800

42 504 370

1 40

12 173 461

28.6

30 330 909


(6) + (7): In estimating the irrigation potential only renewable water resources are taken into consideration. If >100% (6) or negative (7) either non-renewable water resources are already being used for irrigation or the quantities of water used per hectare are less than the quantities recommended for the potential area.

The present study concentrated on long-term averages. Climate fluctuations, however, may greatly influence the possibilities for irrigation development. In the Niger basin, for example the 1980s were much drier than earlier years: the average annual discharges of the 1980s were 20 to 40% less than those before the 1980s. The average annual discharge of the White Nile entering Sudan from Uganda during the period 1961-1980 (50 km≥/year) was nearly twice the average annual discharge during the period 1905-1960 (27 km /year). The recent drought years in Southern Africa will also lead to different averages depending on the period of reference considered.

Figure 18 and Tables 93 to 95 show the part of irrigation potential by basin which is already under irrigation at present1. As expected, it is much lower in the humid regions than in the dry regions. In a few basins, figures higher than 100% mean that the area already under irrigation is larger than the potential. This is due to the fact that either fossil water is used for irrigation (northern Africa) or that the quantity of water used per hectare is less than the irrigation potential in Africa 153 irrigation water requirement figure used in the computation of the irrigation potential (both northern and southern Africa). Figure 19 shows the remaining possibilities for irrigation expansion, which are greatest in the humid regions and smallest or even negative (for the reasons explained above) in the dry regions.

1This is the area occupied by irrigation schemes with full or partial control (11.5 million ha), spate irrigation (0.5 million ha) and wetlands and inland valley bottoms that are equipped for water control (0.2 million ha).

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