Arid conditions in East Africa have been brought about by a number of factors. The basic causes of aridity have been physical while human factors have increased or contributed to further aridity in East Africa
Physical causes of aridity
- Prevalence of dry/desiccated winds. Some areas in East Africa have been influenced by dry winds for instance the N.E trade winds which emanate from the Arabian Desert. Those winds pick some moisture as they blow Southwards towards Africa however these winds tend to loose their moisture in the Ethiopian highlands. Since they are dry they do not bring in rain.They even absorb the little moisture that exists in the regions in which they blow and even warm up such areas. This explains the dry conditions experienced in Northern Kenya.
- Limited water masses: Several areas in East Africa that experience aridity such as Northern Kenya and Central Tanzania lack large water bodies that could otherwise contribute to atmospheric moisture through evaporation. This therefore results into limited atmospheric moisture in such areas and therefore dry conditions result.
- Highland relief causing the rain shadow effect on the leeward side of the highland. Relief has contributed to aridity in East Africa because of the rain shadow effect produced on the leeward side of mountains. The prevailing winds that continue onto the leeward side from the Windward side are desiccated or dry and do not bring in rainfall but instead may even absorb the little moisture that may exist in the leeward areas. Arid areas in East Africa that are due to the rain shadow effect include Northern Kenya, the Masai steppe on the leeward side of the Pare and Usambara mountains ranges in N.East Tanzania. The western rift valley zone area on the leeward side of the Rwenzori mountains. In addition, the absence of highlands or mountains to trap high level winds bearing moisture may also contribute to aridity, this is because winds gather momentum and blow away to other areas.
- Continentality: This refers to the remoteness from the sea. Areas far from the Indian Ocean and whose climate is continental or affected by land conditions have tended to suffer from aridity. Coastal areas are influenced by maritime conditions such as land and sea breezes that lead to high rainfall. However, continental areas such as central and N.Eastern Tanzania tend to be dry because of the long distance from the sea
- Coastal configuration: this refers to the shape or alignment of the E. African coast. The coast is aligned in a N.E or S.W direction. Due to this alignment winds from the N.E such as the N.E trades tend to blow parallel to the coast especially along the Kenyan coast in a south westerly direction and hardly blow inland. Therefore these moisture-laden winds which may not blow inland deprive much of northern, central and southern parts of Kenya of rainfall. This therefore partly explains the prevalence of arid conditions in these parts of Kenya.
- Corriolis force effect: this is a drag force as a result of the earth’s rotation and has effect in that any object moving in the northern hemisphere from the southern hemisphere is deflected to the right. This force accounts for the prevalence of arid conditions in the Ankole – Masaka corridor and other parts to the N.West of Lake Victoria. This is because when the S.E trade winds blowing through Tanzania cross the Equator, they are deflected eastwards i.e. to the right leaving the North Western parts of Lake Victoria without moist winds. This explains the semi-desert/arid conditions experienced in the Ankole-Masaka corridor and the neighbouring areas.
- Perturbation: This is a situation where low pressure conditions due to high temperatures are created on the Indian Ocean and as a result air from the land or air that would have blown on shore is instead redirected into this low pressure belt. Air will therefore blow from the land to the Indian ocean thereby becoming offshore winds and as a result rain is formed in the Indian ocean while parts of the East African mainland and including Northern Kenya are left dry. Perturbation that may occur during certain seasons contributes to aridity and especially extended drought in East Africa.
Human causes of aridity
These include mans’ environmentally unfriendly activities such as the following:
- Deforestation: The removal of vegetation by man is a cause of aridity. This has been due to mans’ activities in the clearance of forests and other forms of natural vegetation. The main activities involved include cultivation, lumbering, industrialisation etc which have led to the destruction of natural forests that contribute to atmospheric moisture.Destruction of this source of atmospheric moisture results into aridity. Deforestation also contributes to soil erosion, which in turn leads to poor plant growth consequently leading to poor rates of transpiration thereby compounding the problem of aridity.
- Overstocking: The rearing of a big number of animals i.e. more than what the pastureland can accommodate can lead to aridity. In case the carrying capacity of the land is exceeded, the pastures are depleted very fast and the large number of animals trample the ground to create bare patches of land and loosening the soils thereby promoting erosion. This results into poor vegetation growth and low levels of transpiration and consequently leading to aridity.
- Overgrazing: This may be as a result of continuous grazing by herbivorous animals without leaving the land to rest. Overgrazing depletes the vegetation cover and may lead to low rainfall because of limited transpiration.
- Bush burning: This may also be responsible for aridity because it leads to the degeneration of the grass and other plants and reduces transpiration. Traditional farmers normally burn grass with the aim of ensuring growth of fresh pastures for the animals but this may have adverse effects on the climate.
- Reclamation of wetlands: Wetlands like swamps, swamp forests, grass swamps, marshlands, dambos etc are major sources of atmospheric moisture through evapotranspiration and their reclamation greatly reduces the process. In addition, the water table is lowered. In the final analysis, humidity and rainfall are reduced and this leads to aridity. Reclamation in East Africa has been due to the search for land, for cultivation, settlement as well as industrialisation.
- Borehole drilling: The sinking of boreholes to provide underground water resources for humans and animals may lead to the lowering of the water table. As the water table falls, plant roots may fail to access the soil moisture and as a result the plants wither. This therefore reduces the capacity of the natural vegetation to recharge the atmosphere with water vapour through evapotranspiration and this may increase on the problems of aridity.
- Industrialisation: Industrial development has also been a cause of aridity or desertification in East Africa. Industrial plants or factories emit exhaust fumes or clouds of smoke containing pollutants such as carbon dioxide, carbon monoxide, sulphurdioxide etc which tend to be green house gases. Such gases are good absorbers of solar radiation thereby contributing to increase in temperatures.In addition, gases such as carbon dioxide and sulphurdioxide may dissolve in water leading to acid rains. Acid rains lead to forest damage in that the plants loose their leaves, their growth stagnants and may finally die. This in turn will also reduce on the ability of the natural vegetation trecharge the atmosphere with moisture through transpiration and hence aridity.
- Mining: The extraction of minerals and more so through open cast method leads to the destruction of surface vegetation meaning that the ability of the vegetation to contribute to the atmospheric moisture is greatly reduced and thereby compounding the problem of aridity.
- Poor methods of cultivation: Primitive or non-scientific methods of cultivation that expose soils to erosion have also contributed to aridity. With erosion the ability of the soil to support plant growth is reduced meaning that there would be poor vegetation and consequently low levels of evapotranspiration. Such methods include shifting cultivation, cultivating up and down slope and other forms of subsistence cultivation. In addition, the use of machinery such as tractor ploughs that carry out deep cultivation tend to loosen soil particles making them prone to erosion.
- Political conflicts/Wars: These may lead to destruction of vegetation through burning, cutting down of trees, demolition of vegetation by armored vehicles as well as emission of dangerous chemicals and gases through explosives and bombs. Such explosives tend to harm the natural vegetation. Consequently, transpiration is reduced and rainfall also reduces.
All these human environmentally unfriendly activities may result in reduced atmospheric moisture and an increase in temperature. It is important to note that human causes of aridity increase desert conditions. They are also, the causes of desertification. Otherwise the naturally existing desert areas of East Africa are basically as a result of physical factors.