Miombo Ecoregion Vision Report - Biodiversity Foundation for Africa

2.4 Hydrological Processes
Miombo Ecoregion Report, page 14
A large part of the ecoregion is drained by the Zambezi River system that discharges into the
Indian Ocean through Mozambique. Other rivers draining into the Indian Ocean are the Rufiji
and Rovuma in southern Tanzania and Save and Limpopo in Zimbabwe and Mozambique. In the
northwest the ecoregion is drained by the Congo River, the headwaters of which comprise the
Chambeshi–Bangweulu–Luapula system in northern Zambia. The gradient in the headwaters of
the Congo and Zambezi is low and extensive floodplains and swamps occur, such as the Bulozi
floodplain in western Zambia, Bangweulu swamps, Lukanga swamps and Kafue Flats in central
Zambia. The Luangwa is an exception, with its steeper gradient. Consequently there are no
significant swamps along the Luangwa River. The tailwaters of the major rivers draining into the
Indian Ocean have steeper gradients and therefore have fewer, if any, expansive wetland areas,
apart from in their deltas which are outside the ecoegion.
The distinctive drainage and hydrological characteristics are determined by three factors: the
seasonal distribution of rainfall, the spatial distribution of surface water and the gradient of the
plateau surfaces.
2.4.1 Rainfall Processes
The three main airstreams affecting the rainy season in the ecoregion are the Congo airstream,
the south-east trades and the northeast monsoons (Davis 1971). The Congo air originates partly
from the southeast trades of the South Atlantic ocean which curve inland over the Congo Basin
as they approach the equator and reach the Miombo Ecoregion from the northwest. This air
stream is very humid in its lower levels and can produce widespread rain when subjected to
convergence. The south-east trades of the south Indian Ocean hold more moisture during the
summer months (November to April) and bring rains to the northeastern portion of the ecoregion,
especially in Tanzania where rainfall tends to be bimodal. The northeast monsoon originates in
the Asiatic high pressure system and may bring rain to the eastern portion of the ecoregion in
summer.
Most of the rainfall occurs near the margins of the Inter-Tropical Convergence Zone along the
Congo Air Boundary and at the northern limit of the southeast trades. Consequently, rainfall
decreases from north to south across the ecoregion, except for areas at higher altitude and those
in the proximity of lakes and swamps, both of which receive above-average rainfall compared to
the surrounding areas. The rainy season is about 200 days in the north and 100 days in the south
and valleys, such as the mid-Zambezi. However, there are substantial annual variations in the
duration and amount of rainfall.
2.4.2 Characteristics of the Plateau Surface
The gently sloping plateau landscape that characterises the ecoregion has given rise to a sluggish,
very widely-spaced drainage system. Drainage of the low plateau interfluves is probably effected
mainly by sheet flow. Infiltration may account for a considerable proportion of the rainfall,
especially in areas of Kalahari sands in the southwestern portion. The characteristic feature of the
drainage in the headwaters of the plateau streams is the broad, shallow linear depressions known
as dambos which may retain water and maintain streamflow well into the dry season. Dambos
cover 10 to 15% of the area in the headwaters of the Zambezi and Congo, and about 5% in the
middle waters of the Zambezi (Byers 2001a). The sluggish drainage has also given rise to