The upper Zambezi basin

The Zambezi River originates in the northwest corner of Zambia near the Kalene hills. The river drops about 400 meters in elevation from its source at 1,500 meters to the Chavuma Falls, over a distance of about 400 kilometers. Mean annual rainfall exceeds 1225 mm, with considerably higher rainfall near the Zambezi source. The runoff efficiency (runoff produced from rainfall) for the Upper Zambezi sub-basin (0.21) is the highest in the entire Zambezi Basin. The steep channels and open terrain drain rapidly, with minimal floodplain retention – runoff rises sharply with the onset of rainfall, peaks between February and April, and then rapidly recedes to minimal flows between September and November (Figure A1). Mean annual runoff volume is 23,411 Mm3, generating an average flow rate of 742 m3/s. This pattern is repeated, but much diminished, during dry years when average flow rates fall to about 250 m3/s. Rainfall and hence runoff are highly variable from year to year, with runoff volumes exceeding 14,000 Mm3 in extremely wet years and dropping to less than 56 Mm3 in very dry years. No dams have been constructed in the sub-basin, although significant hydropower potential exists in Angola. Water withdrawals for irrigation and other purposes are insignificant compared to runoff from this sub-basin (about 3.6 Mm3).

Figure A1. Mean monthly flows in the Upper Zambezi sub-basin during average and drought years.

Kabompo Sub-Basin

Patterns of runoff from the Kabompo sub-basin in northwestern Zambia are similar to that of the comparably sized Upper Zambezi sub-basin, with peak runoff occurring between February and April followed by rapid recession to dry season low-flows (Figure A2). Runoff efficiency (0.09) is considerably lower, however. Rapid runoff from the upper reaches of the river and its tributaries (mostly confined to distinct, steep-sloped channels) is partially attenuated by riparian swamps in the lower reaches with high evaporative water loss. Mean annual runoff is 8,615 Mm3 (a 273 m3/s average flow rate), but falls to about half that level (138 m3/s) during drought years. Water withdrawals for irrigation (estimated at 4.8 Mm3 per annum) are minor relative to these flows. No dams or large infrastructure have been planned or constructed in the sub-basin.

Figure A2. Mean monthly flows in the Kabompo sub-basin during average and drought years.

Lungwebungu Sub-Basin

The Lungwebungu River, the longest tributary of the upper Zambezi region, enters the Zambezi River just downstream of its confluence with the Kabompo River. The headwaters of the Lungwebungu rise in central Angola at an elevation of around 1,400 m, and flow southeast across the Angolan plateau. Along most of its course, the river flows over swamps that attenuate flows, and runoff efficiency is low. Near its confluence with the mainstem Zambezi, the Lungwebungu river-floodplain system widens and merges with the vast Barotse plain. The mean annual runoff contribution is 3,587 Mm3, a flow rate of 114 m3/s. Runoff is extremely variable – as little as 754 Mm3 annual runoff (with a barely perceptible flood peak) may occur during drier years (Figure A3) but individual monthly runoff volumes exceed 3,725 Mm3 during wet years. Water withdrawals from the Lungwebungu are less than 3.7 Mm3 per year. No dams or hydropower plants have been planned or constructed in the sub-basin.

Figure A3. Mean monthly flows in the Lungwebungu sub-basin during average and drought years.

Luanginga Sub-Basin

The Luanginga catchment (34,600 km2) is one of the smallest sub-basins in the Zambezi system. The Luanginga drains runoff from the Angolan central plateau. Runoff follows rapidly from rainfall events, but is attenuated by the Nyengo Swamps and low-lying Barotse Plain, and runoff efficiency is low.

Extreme variability occurs from year to year, as in the other headwater basins. Mean annual runoff volume is 2,190 Mm3 (69.4 m3/s) from the sub-basin, but maximum monthly runoff volumes have exceeded 2,270 Mm3 in wet years while total annual runoff has fallen to less than 750 m3/s during dry years (Figure A4). As with the other Upper Zambezi headwater sub-basins, water off-take for irrigation and other purposes is insignificant (4.7 Mm3 per year), but water loss due to evapotranspiration may exceed 1,660 mm/annum. No dams or hydropower plants have been planned or constructed in the sub-basin.

Figure A4. Mean monthly flows in the Luanginga sub-basin during average and drought years.

Barotse Sub-Basin

The Barotse sub-basin encompasses the vast Barotse floodplain (7,700 km2), extending 200 km in length and 40 km wide along the Zambezi waterway. During the rainy season, the plains are inundated by floodwaters from the four upstream sub-basins to form a large shallow lake that significantly attenuates Zambezi runoff. During the major Zambezi flood of 1958, total storage within the Barotse Plain was estimated to be approximately 17,000 Mm3, half of the mean annual runoff (Sharma and Nyumbu 1985). Zambezi floodwaters take 1-2 months to pass through the Barotse Plain, delaying peak discharge until April or early May (Figure A5), and recede more gradually during the six-month dry season. Average annual water storage capacity on the Barotse Plain is high (8,500 Mm3), and evaporative water losses throughout the year greatly exceed local rainfall-runoff, resulting in a negative contribution (-553 Mm3) from this sub-basin to Zambezi mean annual runoff (Table 1).

Downstream of the Barotse floodplain, the Zambezi traverses another vast floodplain system that further attenuates runoff, the Chobe Swamps (part of the Cuando-Chobe sub-basin, described below). The Zambezi then cascades over the Katombora Rapids before plunging 98 m at Victoria Falls. Irrigation potential in this sub-basin is limited (though vast areas of floodplain crops are irrigated using traditional flood-recession agricultural practices). Water withdrawals are minimal (about 3.5 Mm3 per year, or 1/100th of one percent of mean annual runoff), though large-scale water transfers to thirsty cities in Namibia, Botswana, and even South Africa have been proposed (Scudder 1993). There are no dams or hydropower plants in the Barotse sub-basin. A reservoir has been proposed at Katombora to stabilize flows for improved power generation capacity at Victoria Falls, 60 km downstream.

Figure A5. Mean monthly flows in the Barotse sub-basin during average and drought years.

Cuando/Chobe Sub-Basin

The Cuando River rises in the central plateau of Angola and drains approximately 22% of the Upper Zambezi region. With rainfall of less than 800 mm the sub-basin is the driest in the Upper Zambezi region. Mean annual runoff from the headwaters is 1,100 Mm3 (32.5 m3/s), but substantially lower during dry years. As the Cuando River reaches the broad, flat plains of the Eastern Caprivi Strip, it discharges into the upper end of the Chobe River floodplain. During the early part of the flood season, the Chobe River conveys this runoff to the Zambezi River, and may contribute substantial runoff in some years. As Zambezi levels rise, however, the Chobe River reverses direction and flows back to the northwest where it discharges into Lake Liambezi (Debenham 1948). When runoff from the Cuando/Chobe is in phase with Zambezi River flooding, an area as large as 1700 km2 may be inundated. The unusual hydrograph resulting from these fluxes is shown in Figure A6. Overall, the contribution of Cuando River runoff to Zambezi River flow is counterbalanced by evaporation losses from Zambezi floodwaters that overflow into the Chobe floodplain, and net discharges to the Zambezi are negligible relative to runoff from the headwaters region (Table 1). Withdrawal for irrigation and other purposes (8.5 Mm3 per year) are higher than elsewhere in the Upper Zambezi region, but still minuscule compared to river flows (Heyns 1995). There are no existing or planned dams or hydropower plants, although water resource development planning is underway in Angola.

Figure A6. Mean monthly flows in the Cuando/Chobe sub-basin during average and drought years.