Contents - Previous - Next


The Zambezi basin

The Zambezi basin is the fourth-largest river basin of Africa, after the Congo/Zaire, Nile and Niger basins. Its total area represents about 4.5% of the area of the continent and spreads over eight countries (Map 8 and Table 38). The Zambezi River flows eastwards for about 3000 km from its sources to the Indian Ocean.

Table 38: The Zambezi basin: areas and rainfall by country


Total area of the country (km)

Area of the country within the basis (km)

As % of total area of the basin (%)

As % of total area of the country (%)

Average annual rainfall in the basin area






(mm)






min.

max.

mean

Angola

1246700

235423

17.4

18.9

550

1475

1050

Namibia

824900

17426

1.3

2.1

545

690

630

Botswana

581730

12401

0.9

2.1

555

665

595

Zimbabwe

390760

213036

15.8

54.5

525

1590

710

Zambia

752610

574875

42.5

76.4

600

1435

955

Tanzania

945090

27840

2.1

2.9

1015

1785

1240

Malawi

118480

108360

8.0

91.5

745

2220

990

Mozambique

801590

162004

12.0

20.2

555

1790

905

For Zambezi basin


1351365

100.0


535

2220

930

Rivers and discharges

The Zambezi River rises in the Kalene hills in north-western Zambia and flows northwards for about 30 km. It then turns west and south to run over about 280 km through Angola and reenters Zambia with an annual discharge of nearly 18 km3. It then flows southwards through marshy plains. In the south-west of Zambia the river becomes the border between Zambia and the eastern Caprivi Strip of Namibia for about 130 km.

The Chobe tributary originates in Angola, crosses the Caprivi Strip with an annual discharge of about 1.3 km3, then forms the border between Namibia and Botswana and enters Botswana to flow southwards for about 75 km until it meets the Selinda spillway along which spillage from the Okavango occurs in high flood years (see section The Okavango basin). It then turns east, again forming the border between Namibia and Botswana as it flows through a swampy area and flows into the Zambezi River at the border point between Namibia, Botswana, Zimbabwe and Zambia with an annual discharge of about 4.1 km3. The discharge of the Zambezi River at this point is 33.5 km3/year.

The Zambezi River then forms the border between Zambia and Zimbabwe and reaches its greatest width, over 1.3 km, before its waters plunge over the Victoria Falls. It continues to form the border between Zambia and Zimbabwe until it enters Mozambique.

There are two major man-made lakes on the Zambezi River, Lake Kariba on the border between Zambia and Zimbabwe and Lake Cabora Bassa in Mozambique.

Downstream of Lake Kariba the Kafue River, a major tributary originating in the north of Zambia, flows into the Zambezi River with a discharge of about 10 km3/year. Still further downstream, at the border with Mozambique, the Luangwa River flows into the Zambezi River with an annual discharge of over 22 km3. This tributary originates in the north-east of Zambia. The total discharge entering Lake Cabora Bassa from Zambia is estimated at about 77.5 km3 /year.

Leaving the lake the Zambezi River flows south-eastwards and receives water from its last great tributary, the Shire, with an annual discharge of nearly 16 km3. The Shire drains Lake Malawi (also called Lake Nyasa) about 450 km to the north. The northern part of Lake Malawi forms the border between Tanzania and Malawi, the southern part the border between Mozambique anti Malawi. The total flow into the lake is estimated at about 29 km3/year of which 53% from Tanzania, 43% from Malawi and 4% from Mozambique. Total outflow from the lake in the Shire River in the south is 12.5 km3/year. The level of the lake has fluctuated 6 metres since the beginning of the century, with its lowest level in 1917 and its highest level in 1980.

At its mouth, the Zambezi River splits into a wide, flat and marshy delta. The annual discharge flowing to the sea is estimated at 106 km .

Annual rainfall in the basin decreases from almost 1800 mm in the north to less than 550 mm in the south. Both Botswana and Namibia are rather dry countries and only 2% of each of these countries is situated in the basin. However, rainfall in these parts, around 600 mm/year, is higher than the countries average, which is 400 mm/year for Botswana and only 280 mm/year for Namibia.

Irrigation potential and water requirements

For Angola the irrigation potential has been estimated at 700000 ha [*], as explained in the section The Congo/Zaire basin

The irrigation potential for Namibia has been estimated at between 45000 and 50000 ha, of which 10 000 ha for a sugar cane project in the Caprivi Strip [163]. Flood recession cropping is evaluated at 1000 ha in this area.

The irrigation potential for Botswana in the Zambezi basin ranges from 80 ha, considering identified soils and without the need for major water development works, to 11080 ha, including the need for major water development works. However, of this total area, 10000 ha are located in the Padamatenga plains outside the Zambesi basin in the north-east, to where it is planned to transfer water from the Chobe tributary [64]. In this study, 1080 ha have been retained for the irrigation potential.

According to the irrigation subsector review of Zimbabwe [216], of the total internal surface water resources of 13.1 km3/year, 3.6 km3/year is already committed for domestic, industrial, mining and irrigation use. Of the remaining 9.5 km3/year, at least 3.0 km3/year is reported to be effectively inaccessible. Of the remaining 6.5 km3/year, about half is considered as potentially available for irrigation development, of which 1.94 km3/year in the Zambezi basin. In addition, there is the flow of the Zambezi River.

The Zambezi basin in Zimbabwe has been divided into three hydrological zones. In the western and eastern zones, suitable soil is the limiting factor, while in the middle zone water is the limiting factor. At present 70045 ha have been developed or planned for irrigation [216]. Based on land and water and considering an irrigation water requirement of 10500 m3/ha per year according to this study, it would be possible to irrigate another 95355 ha, so bringing the total to 165400 ha [*]. However, taking a water requirement of 18000 m/ha per year as proposed in the irrigation subsector review would reduce the potential to 131000 ha.

For Zambia, of the irrigation potential of 523000 ha for the whole country the distribution of 355000 ha over the different sub-basins is known, but no details on location are given for the remaining 168000 ha, consisting of 100000 ha of dambos (wetlands), 60000 ha irrigated by renewable groundwater and 8000 ha of commercial farms [214, 215]. The irrigation potential in the Zambezi basin is estimated at 422000 ha as follows [215, *]:

TABLE 39: Irrigation potential in the different Zambezi sub-basins in Zambia

Types of irrigation

Upper Zambezi River basin (ha)

Kafue River basin (ha)

Luangwa River basin (ha)

Total for Zambezi River basin (ha)

Located

112000

165000

14000

291000

Groundwater

15000

15000

15000

45000

Commercial

2000

2000

2000

6000

Dambos

30000

20000

30000

80000

Total

159000

202000

61000

422000

In view of the rugged, very steep terrain of the southern highlands of Tanzania draining to Lake Malawi, no real possibilities for irrigation development exist and consumptive water use would be limited to domestic and industrial water supply. These are relatively small volumes and their quantities would not change the mean annual flow into the lake from Tanzania [136].

Malawi has abundant land where soil and topography are suitable for irrigation but only limited areas that coincide with easily obtainable water from perennial streams. An important feature of the flat lake shore and the Shire River valley landscapes are large areas of marshy land, swamps and lagoons, which are poorly drained and flood susceptible areas. The irrigation potential for the whole of Malawi has been estimated at 100000 ha plus 61900 ha of dambos (wetlands) [135]. It is estimated that 160900 ha of this total are located in the Zambezi basin [*].

The irrigation potential figure of 2 million ha given in literature for Mozambique includes the whole Zambezia province, part of which is located outside the Zambezi basin [159]. The area within the Zambezi basin is estimated at 1.7 million ha [*].

Table 40 summarizes the figures for the whole basin.

Table 40: Zambezi basin: 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)


Angola

700000

13500

9.450

2000

Namibia

11000

5000-25000

0.255

6142

Botswana

1080

5500

0.006

0

Zimbabwe

165400

10500

1.737

49327

Zambia

422000

12000

5.064

41400

Tanzania

0

11000

0.000

0

Malawi

160900

13000

2.092

28000

Mozambique

1700000

11000

18.700

20000

Sum of countries

3160380


37.303

146869

Total for Zambezi

3160380


37.303


For the Zambezi basin as a whole, the water requirements are much less than the water availability. Attention has to be paid, however, to the Chobe tributary, originating in Angola and shared by Angola, Zambia, Namibia and Botswana. The Zambezi River entering Zambia from Angola in the north has an annual discharge of 18 km3, which is twice the volume needed to irrigate the 700000 ha potential of Angola. The Chobe tributary, however, has a discharge of only 1.3 km3/year when leaving Angola, so if a large part of the irrigation potential area of 700000 ha in Angola or if part of the irrigation potential of 159000 ha in the upper Zambezi basin in Zambia is located in this sub-basin, problems may arise for Namibia and Botswana, even though irrigation potential there is very limited compared to the other countries.

Further downstream, no particular problems are expected in terms of water resources. However, water regulation would be necessary for full development of the potential.


Contents - Previous - Next