Chapter 8 (Water Resources) of the book "East and Southern Africa" by T. Bowyer-Bower & D. Potts contributed by Richard Taylor[1], outlines some key features regarding the African orogenesis and their results on regional climate patterns with the attention on thereby evolved water resources.
The tectonic occurence of Africa
The African continent has undergone alot of different orogenesis due to its perifery to continental plate borders and merge of different craton land masses thoughout earth´s history. Therefore the basement rocks include a variety of volcanic rocks, crystalline rocks as well as sandstones and limestones.
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| Geology of the African continent [Schlüter and Trauth, 2008] [2] |
Most of the southern and eastern parts of Africa lie on a 1000 masl raised platfom, northern parts have lower elevations. The formation of the East African Rift System led to a damatical change in hydrology and climate of the entire region. Thereby formed mountain ranges influence local rainfall and atmospheric circulation pattern by forcing the moisture saturated air to raise into higher elevations and thus precipitate on the windward side of the range creating a ´rain shadow´ on the lee side. Seasonal movements of the ITCZ due to latitudinal variations in solar radiation lead to varying rainfall pattern in tropical regions providing either northern or southern tropical regions with an influx of moisture resulting in heavy rainfall. Regions in periphery of the ITCZ recieve only a minimal amount of rainfall and are thereby dependent on alternative water sources (eg. rift valley formed lakes that provide a large storage of water.
The El Niño Oscillation also has a major impact on climate variation in Africa so that intensified ENSO years will have a growing impact on heavy rainfall pattern.
Division of Water
Even if rainfall occures it is sometimes intercepted by vegetation before it reaches the soil. It either evaporates or reaches the soil more slowly by leaf drip. As soon as the amount of water exceeds the infiltration limit of the soil the remaining water creates an Hortonian overland flow which can lead in extreme cases to temporal floods.
Water that reaches the soil and has not been used by plants contributes to the groundwater recharge or proceeds as interflow to surface waters. Compared to groundwater flow, river flow is mainly influenced by rainfall variations. Thus major variations in river flow can occure over the year.
Groundwater destribution depends upon underlying rocks. Geological (weathered) materials with a high porosity and permeability have a greater capacity to store and transmit groundwater. If the amount of stored groundwater reaches scales useful for human abstraction those materials are refered to as aquifers (eg. dolomite aquifer of Lusaka). Large-scale aquifers can be found in desert environments where the climate has undergone a dramatic change over thousands of years (´fossil aquifers´).
Their sustainability highly depends upon renewed water inflow. Also the amount of abstraction and use determins if those sources are eligable in the future. Political distribution and terretorial boundaries often lead to high variations in national possession of water.
Estimations of national, renewable water resources often don´t contain all processes and limitations occuring in certain areas and therefore seldom show the real amout of available water.
Water contamination
Water is not only a limited resource in amount and time but also easily contaminated by faecals or outwashed minerals.
Though weathered rocks are generally useful for groundwater storage and abstraction the process of mass reduction via dissolved products removed by water flow has its risks for the water quality. High levels of flouride result from volcanic rock weathering, iron-rich water results from crystalline rock weathering. Both lead in large amounts to health risks for humans. Other elements like nitrogen or phospherus cause in high rates algal blossom and with that declining oxygen rates in the water which lead to restrictions of aquatic species (eg. fish).
Faecals dissolved in water due to leaking pit latrines or random faecal toss lead to a rising infant mortality and gastrointestinal illness.
Areas of water use
Water is generally used in 3 different ways: domestic (incl. houshold needs & drinking), agricultural (incl. irrigation of crop land & watering of livestock) and industrial (incl. mining & hydro-electric power).
Estimations by the IDWSSD refering to a minimium domestic water requirement in Africa of 20L/person/day are 30 times lower than the average consumption of people living in European countries or North America.
Even though only 5% of African cropland is irrigated there is a rising trend toward national independance in food growth due to rising population growth and industrialization. Large-scale dams create huge standing water basins which induce significant evaporative water loss.
With an increasing industrialization and population growth the demand for electricity and mining goods will rise and therefore the water use for industrial purposes.
Overall there are different water sources throughout the continent even though the amount of water is not destributed and accessable equally. The sustainability largely depends upon the nations and their responsible use of it.
[1] [Taylor,
R.G., 2004; Water Resources and development challenges in eastern and southern
Africa; In: T.
Bowyer-Bower and D. Potts (Eds.), East and Southern Africa .
Regional Development Text, RGS-IBG Developing
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