Hydrology of Southern Africa

The hydrology of southern Africa can be considered on several levels, but the most fundamental level is to examine the prcipitation and the distribution of rivers.

Precipitation

Southern Africa experiences variable precipitation levels ranging from low (less than 250 mm/yr) over large areas of the region, to relatively high (more than 1 200 mm/yr), which tends to be concentrated in the north of the SADC with some smaller areas along the south-east coast. Most rain falls in the summer months (December to March) 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). When rain does fall it 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.

The map below shows the average rainfall distribution across southern Africa. This map shows average values, and thus does not highlight temporal variation or local spatial variation of rainfall.

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

Rainfall and Evaporation Statistics for (Selected) SADC Countries

Country	Rainfall range	Average Rainfall		Potential evapotranspiration range	Total surface runoff
	mm	mm	km³	mm	mm	km³
Angola	25-1600	800	997	1300-2600	104	130.0
Botswana	250-650	400	233	2600-3700	0.6	0.35
Lesotho	500-2000	700	21	1800-2100	136	4.13
Malawi	700-2800	1000	119	1800-2000	60	7.06
Mozambique	350-2000	1100	879	1100-2000	275	220.0
Namibia	10-700	250	206	2600-3700	1.5	1.24
South Africa	50-3000	500	612	1100-3000	39	47.45
Swaziland	500-1500	800	14	2000-2200	111	1.94
Tanzania	300-1600	750	709	1100-2000	78	74.0
Zambia	700-1200	800	602	2000-2500	133	100.0
Zimbabwe	350-1000	700	273	2000-2600	34	13.1
Total			4665			599.27

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.)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, with Namibia and most of Botswana in particular have very few surface water resources. Many of the water channels across the region, especially those in areas of low rainfall, high temperatures and high rates of evaporation are non-perennial, only flow after the intense rainfall events that characterise precipitation in the region.

Surface water flows are also subject to human intervention, mainly in terms of streamflow reduction for irrigation, domestic and urban supply.  Dams and other impoundments stop streamflow and in many cases fundamentally alter river basin hydrology. In 2000 there was an estimated 746 dams in the SADC region (FAO 2000). 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.