Climatic Patterns

The regional climate of the Limpopo River basin is heavily influenced by prevailing wind systems from the East, particularly Tropical Cyclones from the Indian Ocean (Ashton et al. 2001). The climatic patterns of the Limpopo River basin are discussed below in terms of:

• Precipitation
• Temperatures
• Evaporation and evapotranspiration

Precipitation

While a region of relatively low rainfall, the majority of the precipitation occurring in the Limpopo River basin falls within a short window, during the summer months (FAO 2004), with approximately 95 % falling between October and April and a rainfall concentration index for the basin of 60 % (100% indicates all rainfall occurring in one month; FAO 2004). Rainfall events are highly episodic and intense, usually associated with convective thunderstorms (SARDC 2002).

The table below summarises the annual rainfall for the basin states of the Limpopo River basin.

Table: Annual rainfall per country in the Limpopo River basin.

Source: CGIAR 2003

There is a general westward decline in rainfall from the Drakensberg mountains in South Africa, to the western portion of the basin in South Africa's Limpopo Province and eastern Botswana. The Mahalapswe and Lotsane Rivers sub-basins in Botswana and the Sand River sub-basin in South Africa exhibit average rainfall (LBPTC 2010). The highest rainfall per sub-basin can be found in the Upper Olifants River, in South Africa and the lower reaches of the main-stem Limpopo River in Mozambique (LBPTC 2010).

Average precipitation varies from as low as 200 mm per annum in the hot, dry parts of the basin (around the confluence of the Limpopo and the Shashe Rivers), to as high as 1 500 mm per annum in higher rainfall areas, such as the Drakensberg escarpment. This area experiences such high rainfall due to orographic effect where moist air is forced to cool quickly as air masses pass over areas of higher relief. This rapid cooling induces rain to fall on the windward side of the area of relief. In these circumstances, the leeward side of the relief often experiences lower than average rainfall, which is often known as a rain shadow.

The map below shows the average annual precipitation patterns for the Limpopo River basin per sub-basin (LBPTC 2010).

The majority of precipitation falls as rain, with localised snowfall on the higher peaks of the Drakensberg mountains during colder, winter months.

Due to the flat terrain, high temperatures, low humidity and rainfall, rainfall generally has to exceed 20 to 30 mm in a single event before runoff can occur. This volume of precipitation occurs fairly infrequently, therefore, run-off rarely reaches river channels. As a result, streamflow can vary considerably.

A number of severe droughts have been experienced in the region, the most recent of which occurred in the early 1990s (SARDC 2002).

Temperature

Temperatures across the Limpopo River basin follow a distinct seasonal cycle, with the coolest months in the winter (June - August) and the higher temperatures in early summer (late November to early December) (SARDC 2002; FAO 2004). Average daily temperatures of 40°C in summer months are not uncommon, with winter temperatures dropping as low as 0°C at night. These values vary with elevation and proximity to the Indian Ocean (FAO 2004).

During the winter months, frost is common at higher elevations, and in the southwestern and western basin (Botswana and South Africa). Days with frost can vary from 30 days in Tzaneen (South Africa) and Mahalapye (Botswana) to as high as 120 days in Mafeking (South Africa) and Lobatse(Botswana) (FAO 2004; CGIAR 2003).

Long-term climate trends observed by the Intergovernmental Panel on Climate Change (IPCC), indicate that average temperatures across Africa are increasing, with an average increase of 0.7°C in the last 100 years (FAO 2004). While the greatest temperature changes are predicted to take place in central Africa, such changes would almost certainly impact regional and sub-regional air mass circulations and climate.

The map below (left) shows the average temperatures for the Limpopo River basin. For maps showing the mean temperatures in January and July for the Limpopo River basin, please refer to the Climate Variability section of this chapter.

Evapotraspiration and Evaporation

Driven by low humidity and high temperatures, evaporation is a significant factor of the climate in the Limpopo River basin, with average rates of 1 600 - 1 700 mm/yr in cooler mountain regions in the southeastern portion of the basin. Warmer, western and central regions of the basin experience between 2 600 and 3 100 mm/yr, with the greatest intensity in the Limpopo River valley itself (Ashton et al. 2001; FAO 2004). These rates of evaporation, accompanied by high evapotranspiration make dryland subsistence agriculture extremely challenging in the Limpopo River basin (FAO 2004).

The map above (right) shows evapotranspiration for the Limpopo River basin. Evapotranspiration across the basin is high relative to rainfall, with values ranging from 1 000 mm/yr in the southern part of the basin to 2 000 mm/yr in the north. Overall, there is a net moisture loss through evaporation and evapotranspiration across the basin (FAO 2004).

Next: Climatic Variability >