A vast basin in the interior of Africa south of the equator, the Kalahari reaches into parts of Namibia from its heartland in Botswana, with a mantle of parched sand to depths in excess of 300 m. Virtually devoid of rock and stones, it is the largest surface of sand in the world, with a total area of 900 000 km². It spreads into nine countries between the Orange River and the equator.

Kalahari sands cover much of eastern Namibia. They widen towards the north to include the Caprivi Strip and the Owambo Basin along with the Etosha Pan.

Within southern Africa, the Kalahari is loosely thought of as a desert, but little of it really is. Only the southernmost part, with average rainfall of 150-250mm per year inside Namibia, is dry enough to be described as a desert.* The rest is semi-desert.

Rainfall is seasonal, infrequent, patchy and highly variable. Evaporation rates far exceed actual precipitation as summers are extremely hot. Rain takes the form of fugitive thunderstorms in summer. The amount gradually increases northward and eastward, to as much as 700mm in the furthest corners of the Caprivi, but the greater part of the Namibian Kalahari is semi-arid with less than 600mm per year.

When tectonic forces uplifted the margins of southern Africa 128 million years ago, a basin of subcontinental proportions -- the Kalahari -- was left in the interior behind the highlands. Over the next 70-65 million years it was filled with waterborne sediments and windblown sand.

The sand forms a porous layer on top of clay and bedrock. It holds few nutrients. As rainwater rapidly drains down through it, little moisture remains behind to sustain vegetation. In much of the Kalahari the sand is red because a layer of iron oxide coats its quartzite grains. The colour fades to pink or grey where rainfall is sufficient to leach out the iron oxide. In riverbeds and pans the sand is nearly white. The landscape mostly consists of plains, situated 1 000-1 200m above sea level. Except for dune systems in some places, the surface is flat, with hardly a hill in sight anywhere. It is dotted with limestone and clay pans. Etosha is the largest one.

Nevertheless the Kalahari is not uniform in appearance. Its character changes largely in response to variations in vegetation, from drylands with dwarf shrubs in southern Namibia to wetlands with riverine forests in the north-east.

The Caprivi Strip in the north-east contains three out of only five rivers in the entire country that flow all year. They all originate in other countries and constitute the largest concentration of freshwater habitats in Namibia.

Away from the permanent waters, rivercourses are few and far between, shallow and generally dry, veins of white sand against a mass of red. They all run to the east or south, away from the sea, into the interior of southern Africa.

Similarly the pans hold water only if enough rain falls or if an ephemeral river flows into them. They are almost circular, their hard surface hollowed out from wind erosion, often with dunes on the edge. The deeper a pan and the higher the dunes, the greater the volume of rainwater it collects. Pans are also a source of natural salts for wildlife.

Despite the paucity of surface water the Kalahari supports the largest concentrations of wildlife in Namibia.

All of the Namibian Kalahari lies in the Savannah Biome, where woody trees and shrubs share dominance with perennial grasses. The biome is subdivided into arid and moist parts, roughly located to the south and north of the 450mm isohyet.

Easily the larger of the two within Namibia, the Arid Savannah supports fine-leaved, thorny acacia trees and shrubs. It resembles parkland where stands of trees, or solitary trees, interrupt wide expanses of grassland. The southern Kalahari and the central Kalahari both form part of it.

The desert or duneveld* in the south lies mainly in the basins of the ephemeral Nossob and Auob rivers, where stubbly grasses stabilise longitudinal dunes. Trees are sparsely distributed and stunted, more like shrubs, except along rivercourses. The grey camel thorn (Acacia haematoxylon) and the shepherd's tree (Boscia albitrunca) are typical.

In the central Kalahari or thornveld, relatively open grassland carries stands of thorn trees, predominantly the camel thorn. Other acacias such as the candle pod (A. hebeclada) and black thorn (A. mellifera) grow mostly as shrubs. The latter sometimes forms dense thickets.

Compared to the Arid Savannah, the Moist Savannah is actually only slightly moist within Namibia, except for perennial wetlands and shallow groundwater in parts of it. The difference in maximum rainfall between the two barely rises through 150mm in a year. As little as it is, however, it makes a difference.

Growth in the northern Kalahari, within the Moist Savannah, is closer to woodland in character. The trees and shrubs are taller, grow closer together and form a denser canopy. Such woodlands grow almost clear across the Namibian north from the Etosha Pan to the Caprivi Strip.

Broad-leaved species predominate in the northern Kalahari, among them the bushwillow (Combretum collinum), kiaat (Pterocarpus angolensis), marula (Sclerocarya caffra), mopane (Colophospermum mopane), purplepod terminalia (Terminalia prunioides), teak (Baikiaea plurijuga) and wild seringa (Burkea africana).

OWAMBO BASIN
A lobe-like westward extension of the greater Kalahari, the Owambo basin in north-central Namibia, supports 43% of the Namibian people on 10% of the land. It is divided into the political regions of Ohangwena, Omusati, Oshana and Oshikoto, commonly described as "the Four O's" or simply "the North".

With a rim of rocky hills in the south and west, the basin is an ancient depression full of sand, silt and clay, which was washed and blown into it from higher ground. It once lay under a shallow sea, on the shelf of a primitive continent, long before Africa was formed.

The Owambo basin straddles the border with Angola. The southern half of it in Namibia has an area of 84 600km² and a population of 780 000. Small-scale agriculture is the principal activity and outside the towns people still live in traditional homesteads.

The few towns are small and busy, with roadside markets and colourful crowds. Oshakati with 28 000 inhabitants is the largest. It is also the administrative centre. Only two other towns, Ondangwa and Ongwediva, have more than 10 000 people.

An extensive and intricate network of oshanas or shallow channels, known as the Cuvelai system, dominates the landscape in the heart of the Owambo basin. It gets its name from the principal river in a vast catchment 300 km to the north in Angola. The rivers rise in highlands where the average rainfall exceeds 800 mm per year.

Summer floods or efundja flow down the oshanas, spread over the flat landscape and, in the south of the basin, converge into an inland delta. In some years the floods reach beyond the delta and fill Etosha, the biggest pan among hundreds in the delta. The floodwaters bring fish, renew pastures, support crops and recharge groundwater.

If the Cuvelai delta were not ephemeral, it would resemble the Okavango Swamps. Instead it is a grassland dotted with makalani palms (Hyphaene petersiana). The rest of the Owambo basin consists of shrublands and woodlands where the mopane, purple-pod terminalia, red bushwillow, teak, wild seringa and acacia species are predominant.

Etosha pan .........