Distribution of groundwater resources thoughout Africa

The paper that I just had a look at today was "Quantitative maps of goundwater resources in Africa" by A.M. MacDonald et al. [1]   
They based their studies on data collected and presented in a variety of small-scale reports from different local areas all over Africa and tried to give a general statement about groundwater reservoirs and aquifers throughout the country.

Groundwater storage for Africa based on the effective porosity and saturated aquifer thickness. This is a map of groundwater storage expressed as water depth in millimetres with modern annual recharge for comparison (Döll and Fiedler 2008).

They didn´t only estimate the amout of groundwater (~ 0.66 billion km³) stored beneith the surface, which is compared to Europes´ total water appropriations for public water supply of 3,5 km³/year [EEA, 2002] sufficiant for the next 1,8 Ma, but also pointed on the fact that not all theoretical available groundwater is accessable for abstraction and, however, not always of proper quality to provide a secure water source for people.
Even though groundwater is natuarally protected against pathogenic contamination, elevated amounts of dissolved elements eg. iron, arsen or fluorides or faecal coliforms prevent some reservoirs from common use for people.
These reservoirs are furthermore not evenly distributed thoughout the continent due to varying geology, geomorpholocial features/processes and effective rainfall over decades.
Groundwater has not only potential to move people out of poverty and enlarge irrigation of arable land but it is also one of few resources that responds much slower to meteorological conditions and longterm climate variability [Calow et al 2010]. It gathered over thousands of years into sometimes >70m deep basins which often still contain enough space to recharge them with water sufficient for decades.

Aquifer productivity for Africa showing the likely interquartile range for boreholes drilled and sited using appropriate techniques and expertise. The inset shows an approximate depth to groundwater (Bonsor and MacDonald 2011).

 
Another difficulty in groundwater accessability is given through different rates at which these can be abstracted from drilling boreholes. Custome handpumps need a rather small supply of
>0.1 L/s. For large scale irregation and urban town supplies a much steadier supply of  >5 L/s is required. Depending on basement rocks (crystalline or sediemtnary) there is a naturally given yield limitation for each aquifer.
This as well as the depth in with those groundwater reservoirs are located determins the cost and technology necessary to tap them. Water levels deeper than >50m are not accessable for handpumps and require more sophisticated equipment.
Even if most of those obstructions were overcome, the mayor issue of local distribution of aquifers is something that cannot be managed easily.

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[1] [MacDonald, A., Bonsor, H.C., O Dochartaigh, B.E. and Taylor, R.G., 2012. Quantitative maps of groundwater resources in Africa. Environmental Research Letters, Vol. 7, 024009]