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The Orange River catchment varies dramatically both in climate and topography
from east to west. To the east, at the source of the Orange River high in the
Lesotho Highlands, the precipitation, some of which occurs as snow, can exceed
2 000 mm/a in places, which together with the relatively shallow soil cover and
low evaporation (+/- 1 200 mm/a) results in significant run-off. As the river
progresses towards the west, the lush pastures of Lesotho are gradually
transformed into harsh but impressive desert areas where only the most drought
resistant plants can grow. The desert areas of the lower Orange basin are
amongst the driest in the world with an average rainfall of less than 50 mm/a
and an annual potential evaporation exceeding 3 000 mm in some areas.
Click on image for full size photo
The mean annual precipitation (MAP) for the Orange River catchment is
approximately 400 mm compared with a world average of 860 mm. Although by
southern African standards there are some well watered areas in the Orange River
catchment, by world standards the catchment as a whole must be classified as
arid.
The average daily temperature ranges from approximately 12°C
in the Lesotho Highlands to more than 22°C in the
Richtersveld region near the river mouth. Extreme temperatures in excess of 50°C are often experienced along the banks of the lower
Orange River while in the Lesotho Highlands temperatures below -10°C are common with some areas experiencing more than
200 days of frost per year.
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