OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

Soil formations across the Olifants basin reflect the strong influence of underlying parent rock material, climatic
features and biological activity. The dominant soil types in the basin are moderately deep sandy to sandy-clay
loams in the west and south, grading to shallower, sandy or gravely soils in the north and east. The deeper
loam soils are extremely important for agricultural activities and support extensive irrigation developments along
the Olifants River as well as many of its tributaries. A few areas of black vertisols in the southern and western
parts of the basin also support important agricultural developments.
The valley bottom soils along all of the tributary rivers and the Olifants main channel are generally of colluvial or
alluvial origin and support extensive areas of commercial and subsistence agriculture. In contrast, hilly or
steeply sloping areas tend to have fragile, shallow, stonier soils with less agricultural potential. In the endorheic
areas, most soils have a relatively high sodium and clay content and are dispersive.
5.1.2 Climatic features
Because of its geographic position, the prevailing wind systems, including tropical cyclones from the Indian
Ocean, have a strong influence on the climate of the Olifants basin. The most important of the rain-bearing
winds are the southeasterly wind systems that bring rainfalls from the Indian Ocean. In some years, unusual
southward movements of the Inter-Tropical Convergence Zone (ITCZ) have been sufficient to influence rainfalls
in the northern parts of the Olifants basin for short periods of time.
Air temperatures across the Olifants basin show a marked seasonal cycle, with hottest temperatures recorded
during the early Austral Summer months and lowest temperatures during the cool, dry winter months. Rainfalls
are also highly seasonal, falling predominantly as intense convective thunderstorms during the warmer summer
months. Rainfalls vary from as little as 400 mm per annum in the eastern parts of the Olifants basin, along the
border with Mozambique, to over 1,000 mm per annum on the Drakensberg Mountains. A sketch map showing
the distribution of mean annual rainfall over the Olifants and Limpopo basins is shown in Figure 4.1; this map
also shows the main tributaries of the Olifants River.
Evaporation rates across the Olifants basin are both high and variable, ranging from some 2.4-2.6 metres per
annum in the eastern areas of the basin to some 1.7 metres in the cooler, mountainous regions in the
southwestern portion of the basin. In view of these evaporation rates, and the quantity of rainfall received each
year, several portions of the Olifants basin show clear evidence of the dominance of physical weathering
processes (with Weinert N values greater than 5.0). These areas are located predominantly in the eastern and
northeastern portions of the basin and along the lower reaches of the Olifants valley. Virtually all of the rest of
the Limpopo basin is subject to chemical weathering processes, either seasonally (Weinert N values below 4.0)
or continually (Weinert N values below 2.0) (Weinert, 1964).
5.1.3 Population and land use patterns
The total population of the Olifants basin is estimated to be approximately 10.5 Million, with South Africans
comprising some 85% of the basin population (Table 5.2). The Olifants basin also contains a large proportion
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