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The Orange basin

The Orange basin, located in Southern Africa, covers almost 3% of the continent and spreads over four countries (Map 11 and Table 46).

Table 46 :Orange basin: 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 (%)

As % of total area of country (%)

Average annual rainfall in the basin area
(mm)






min.

max.

mean

Botswana

581730

71000

7.9

12.2

165

520

295

Namibia

824900

219249

24.5

26.6

35

415

185

Lesotho

30350

30350

3.4

100.0

575

1040

755

South Africa

1221040

575769

64.2

47.2

35

1035

365

For Orange basin


896368

100.0


35

1040

325

Rivers and discharges

The source of the Orange River is in Lesotho. The river receives water from the Makhaleng tributary just before entering South Africa. The Caledon tributary flows on the border between South Africa and the north of Lesotho and flows into the Orange River further downstream in

South Africa. The average annual runoff from Lesotho to South Africa is estimated at 4.73 km3/year, which is far in excess of the country's water requirements.

Almost the entire plateau of South Africa, representing over 48% of the area of the country, is drained by the Orange River and its tributaries, though they contribute only about 22% of the total runoff of South Africa. The Vaal is the major tributary of the Orange River and the average annual runoff in the Vaal basin area is about 4.27 km3, of which 2.15 km3 is exploitable. The average annual runoff of the Orange basin, excluding the Vaal, is estimated at 7.59 km3, of which 5.76 km3 is exploitable.

The Molopo, which forms the border between Botswana and South Africa, is a fossil river, which once flowed into the Orange River. Now it receives most of its very occasional flows from its tributaries in the northern Cape province of South Africa [61].

The Orange River forms the border between the south of Namibia and South Africa. The most important tributary entering from Namibia is the Fish River, on which the Hardap dam was constructed in 1972.

Irrigation potential and water requirements

The irrigation potential in the orange basin in Botswana is negligible because of a lack of renewable water resources [64].

Namibia currently has access to an agreed volume of at least 0.5 km3/year of water from the Orange River [162]. The gross irrigation water requirement is estimated at 8500 m3/ha per year in the present study. Literature gives figures of 33000 m3/ha per year [163]. The difference can be explained by a difference in the assumed future irrigation cropping pattern. As probably good soils alongside the rivers will be a limiting factor, an irrigation potential of 25000 ha has been retained [*].

The soils of Lesotho, which lies entirely in the Orange basin, are very poor [192]. The irrigation potential has been estimated at 12500 ha [127]. Through the Lesotho highlands water project, it is planned to transfer about 2.1 km3/year of water from Lesotho to South Africa while enabling Lesotho to generate its own electricity.

The water available for agriculture in the Orange basin in South Africa is estimated at 4.3 km3/year by the year 2010 [190]. Table 47 summarizes the water resources, irrigation potential and water requirements [190, 191, *].

Table 47: Irrigated areas, water availability, water requirements and irrigation potential in the Orange basin in South Africa

Sub-basin

Irrigated area (ha)

Actual water use (km3/yr)

Water available (km3/yr)

Irrigation water requirement (m3/ha.yr)

Irrigation potential (ha)

Vaal (C)

160000

1.366

1.770

14000

126400

Orange (D)

140000

1.413

2.488

11000

226100

Total for Orange basin in South Africa

300000

2.779

4.258


352500

For the whole of the Orange basin in South Africa, the available water of 4.258 km3/yr would lead to an irrigation potential of 352500 ha, using between 11000 and 14000 m3/ha of water per year [*]. At the moment 300000 ha already benefit from irrigation, using less than 10000 m /ha per year.

Table 48 below summarizes the irrigation potential, water requirements and irrigated areas for the Orange basin.

Table 48: Orange basin: irrigation potential, water requirements and areas under irrigation

Country

Irrigation potential (ha)

Gross potential irrigation water requirement

Area under irrigation (ha)



per ha
(m3/ha per year)

total (km3/year)


Botswana

0

4000

0.000

0

Namibia

25000

8500-33000

0.500

0

Lesotho

12500

9500

0.119

2722

South Africa

352500

11000-14000

4.257

30000

Sum of countries

390000


4.875


Total for Orange basin

<=390000


4.875


The irrigation potential of 390 000 ha for the Orange basin should be considered as an upper limit from the point of view of water availability. It would require agreed rules for the management of the river water by Namibia and South Africa.

The South Interior

The South Interior is divided into two separate basins, as shown in Figure 2. One is shared by Zimbabwe, Botswana and Namibia. A major part of the Kalahari Desert is located in this basin. The other one is shared by Angola and Namibia. Its total area represents 2.1% of the area of the continent (Map 12 and Table 49).

Table 49: South Interior: 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 (%)

As % of total area of country (%)

Average annual rainfall in the basin area
(mm)






min.

max.

mean

Zimbabwe

760

24210

3.7

6.2

465

660

550

Botswana

581760

368780

57.1

63.4

270

670

405

Angola

1246700

53118

8.2

4.3

500

905

680

Namibia

824900

199718

30.9

24.4

275

580

410

For South Interior


645826

100.0


270

905

435

Rivers and discharges

The surface water resources of Zimbabwe are estimated at 0.038 km3/year, of which 0.008 km3 is still available for irrigation development after deducting quantities already used or committed [216]. The annual runoff of the Mosupe and Mosetse rivers, located in Botswana, is estimated at 0.055 km3. Most of the rivers are ephemeral. In Angola the South Interior occupies 4% of the area of the country, but no information is available on water resources. In the Namibian part of the basin there are only ephemeral rivers.

Irrigation potential and water requirements

The area already developed or planned for irrigation in Zimbabwe is about 250 ha [216]. The combination of suitable land, which is already limited, and available water, which is even more of a limiting factor. leads to an irrigation potential of 1100 ha, considering an irrigation water requirement of 9500 m3/ha per year [*]. Should 18000 m3/ha per year of water be used, as planned in the irrigation subsector review of Zimbabwe [216], the area would have to be even less than 1100 ha.

The maximum irrigation potential for Botswana in the South Interior has been estimated at 3950 ha, all of it being located in the Makgadikgadi Pans in the eastern part of Botswana. About 1450 ha are marginal land and would need major constructions for water development and storage [64]. In this study, 2500 ha have been retained for the irrigation potential. The Central Kalahari Game Reserve in Botswana occupies a large area of the South Interior.

For Angola, no details are available on the distribution of the irrigation potential over the country. It has been estimated that 50000 ha are located in the South Interior (see the section The Congo/Zaire basin) [*].

In Namibia, in the part of the South Interior which is shared with Angola, a large area is occupied by the Etosha National Park. Irrigation potential has been estimated at 400 ha [*]. The present study estimates a water requirement of 5500 m3/ha per year. Other estimates, considering a different irrigation cropping pattern, go as high as 34000 m3/ha per year [163].

Table 50: South Interior: irrigation potential, water requirements and areas under irrigation

Country

Irrigation potential (ha)

Gross potential irrigation water requirement

Area under irrigation (ha)



per ha
(m3/ha per year)

total (km3/year)


Zimbabwe

1100

9500

0.010

250

Botswana

2500

6000

0.015

0

Angola

50000

5000

0.250

0

Namibia

400

5500-34000

0.008

0

Sum of countries

54000


0.283

250

Total for South Interior

<= 54000


0.283



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