Zimbabwe: Landuse in Dry Tropical Savannas

Drought

Droughts lasting between one and five years may occur in isolated areas or on a regional scale. A poor year can result in large-scale crop failure, food shortages and in extreme cases, famine. Trees and grasses wilt and die and animals perish from hunger and thirst. Subsistence farming, which provides most people of the region with their food, depends on sufficient rainfall. Drought is associated with suffering and loss of valued crops, livestock and wildlife. People often have to trek for long distances to the few sources of water that may still be available. Praying for rain is not uncommon in most parts of the region, both in traditional and christian ceremonies, and the onset of the rains is often viewed as the single most important event of the year. (1) A historical overview of drought and rainfall patterns in Southern Africa since 1800 is given below:

 
 
1800-30  Southern African rivers, swamps and other water sources dried up. 
         Some well-watered plains turned to semi-arid vegetation.
 

 
1820-30  This was a decade of severe drought throughout Africa. 
 

 
1844-49  Southern Africa experienced five consecutive drought years.
 

 
1870-90  This period was humid in some areas and former Lake Ngami, in the 
         northwest of Botswana, was full.
 

 
1875-1910 There was a marked decrease in rainfall in southern Africa, 
          and 1910 experienced a severe drought.
 

 
1921-30  Severe droughts in the region.
 

 
1930-50  Southern Africa experienced dry periods alternating with wet ones, 
         and in some years the rains were very good. The 1946-47 season
 
         experienced a severe drought.
 

 
1950s    There was abnormally high rainfall in some parts of the region. 
         East Africa experienced flooding, and Lake Victoria rose by 
         several metres. Elsewhere, the equatorial region experienced below 
         normal rainfall.
 

 
1967-73  This six-year period was dry across the southern African region. 
         The equatorial region experienced above average rainfall.
 

 
1974-80  This period of six years was relatively moist over much of southern 
         Africa. In 1974, the average annual rainfall was 100 percent 
         above normal throughout the region.
 
1981-82  Most of southern Africa experienced drought.
 

 
1982     Most of sub-tropical Africa experienced drought.
 

 
1983     This was a particularly bad drought year for the entire African 
         continent.
 

 
1985     Conditions improved.
 

 
1986-87  Drought conditions returned.
 

 
1991-92  Southern Africa, excluding Namibia, experienced the worst drought in 
         living memory.
 

 
Source, State of the environment in Southern Africa, SADC, 1994 

Effect on water supplies

Drought has its greatest impact on water supplies. Lack of water affects every aspect of environmental health and human activity, including agriculture, natural areas and development projects. The 1991-1992 drought which ravaged most of southern Africa, killed more than one million cattle in Zimbabwe. During a drought, overgrazing leads to further degradation of pastures and arable areas in cattle farming areas. The deterioration of grazing capacity further reduces livestock numbers. In drier areas, scanty rainfall for a few years can kill vegetation permanently and poor land-practices only make it worse.

The drop in water supplies in dams and rivers also affects the quality of water. The cholera outbreak that affected almost every country in the region during 1992 and 1993, claiming hundreds of lives, may have been compounded by the drought.

The migration patterns of wild animals, including birds and mammals, are determined by seasonal rainfall. In the event of a drought, migrations are disrupted and wildlife numbers decrease, particularly herbivores. Severe loss of wildlife leads to ecological imbalances and economic losses. Fish population also tend to decline during drought. Rivers and lakes shrink and food sources for fish decrease, resulting in low breeding rates and smaller catches for fishermen.

Rainfall pattern, and the frequency and intensity of drought cycles, negatively affect the region's ecozones on a regular basis. The drier ecozones, especially dry savanna are particularly affected.

El Nino

Although climatologists have produced a number of plausible explanations as to why droughts occur, a single conclusive answer is yet to be found. Past occurrences of drought have been linked to such events as El Nino and volcanic eruptions. El Nino is a component of the global weather phenomenon, the Southern Oscillation, and together these are known as ENSO. During an ENSO phase, equatorial waters across the Pacific ocean get warmer. Normal airflow moves westward from the Pacific to the Indian Ocean, but during El Nino this movement is weakened or altered. This results in high rainfall in some parts of Latin America but low rainfall and even drought in southern Africa. During the devastating 1991-1992 drought El Nino lasted until the end of February 1992. Some experts say that about one-third of the droughts in the region could be attributed to El Nino.

La Nina

The opposite extreme of the ENSO cycle occurs when a cold phase known as La Nino or (anti El Nino) is experienced. The occurrence of La Nina results in unusually heavy rain in southern Africa. At this time the Pacific is cooler than the Indian ocean and wind moves from the Pacific towards the latter. El Nino means "the boy-child" in Spanish, so named because it occurs around December, when Christians are celebrating the birth of Christ. La Nina means, "the little girl".