Hydrology of Southern Africa

There are many ways to look at the hydrology of southern Africa, but the most fundamental level is to examine the distribution of rivers and precipitation.

Southern Africa experiences variable precipitation levels ranging from low to fairly good. Most falls in the summer with the exception of the Western Cape of South Africa, which has a temperate climate. Rainfall is highly variable in distribution and intensity, particularly in the drier regions (Pallet et al. 1997).

Precipitation

Rainfall across the southern African region is generally low and prone to high variability. What rain does fall, is often episodic, arriving in short intense downpours during warm weather. Rain falling in intense downpours often runs off into river channels as it falls faster than can be absorbed into the soil and groundwater (recharge). Many areas, particularly in the south and west, receive very little rain at all (250 mm/yr) and are subject to high temperatures and high rates of evaporation. Rainfall increases in consistency and volume further north in SADC, closer to the equator.

The map below shows the average rainfall distribution across southern Africa. This does not show the range of variation.

The table below summarises the rainfall and evaporation statistics for the region, including rainfall range.

Rainfall and evaporation statistics for (selected) SADC countries .

Source: Pallet et al. 1997

Water losses from evaporation and evapotranspiration are extremely high in southern Africa, with only a small percentage of rainfall reaching aquifers through groundwater recharge or surface water through run-off (Pallet et al. 1997). The map below shows the distribution of evapotranspiration across the region.

Surface Water

The surface resources are distributed fairly unevenly across the southern African region. Namibia and most of Botswana in particular are very sparsely covered and many of the channels across the region, especially those experiencing low rainfall, high temperatures and high rates of evaporation are non-perennial, only flowing during and after intense rainfall events that are characteristic of precipitation in the region.

Surface water flows are also subject to human intervention, most in terms of streamflow reduction for irrigation, domestic and urban supply. Impoundments, such as dams, reduce streamflow and in many cases fundamentally alter river basin hydrology. Official estimates of dams in the SADC region is 746 (FAO 2000). This survey is now 10 years old and this number is likely to have increased substantially.

Groundwater

Due to the limited availability of surface water resources groundwater is critical to integrated water resources management, particularly in rural areas not close to larger rivers or urban water supply networks.

The map below, developed by the German Geological Survey and UNESCO (WHYMAP 2008), describes groundwater resources in terms of:

• Major groundwater basins;
• Areas of complex hydrogeological structure; and
• Areas with local and shallow aquifers.

Groundwater recharge potential, related to climatic conditions such as average temperatures and evapotranspiration, and geological factors such as porosity and infiltration rates, is relatively low across much of the southern African region, improving to the north, due mostly to increased precipitation.

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