Investigating Africa's Future Water Supply

My last post highlighted how the African agricultural sectors huge reliance on highly variable rainwater resources constrains agricultural production in turn perpetuating food insecurity and poverty. As a consequence of climate change, rainfall will become increasingly unpredictable and variable. It is therefore crucial that Africa explores and adopts alternative means of providing adequate water supplies. Especially because, as is widely noted, Africa has sufficient water supplies and the continents water crisis isn’t one of volume but of spatial and temporal distribution (The Africa Water Vision for 2025). I will now briefly explore some of Africa's potential options and what role they may play in Africa’s ambitions (and need) to achieve water and food security.

Dams and Reservoirs

One option for combatting the increasingly varied and unreliable atmospheric provision of water is to store water when it is available, for using when it is not available i.e. in large reservoirs and smaller rain-water harvesting systems. As McCartney and Smakhtin highlight, water storage increases water security, agricultural productivity and adaptive capacity to climate change (McCartney and Smakhtin 2010). Formed by the creation of a dam, reservoirs have the capacity to store large amounts of water and therefore facilitate provision of a more reliable and consistent water supply.

Cahora Bassa Dam in Mozambique is one of the three major dams on the Zambezi river system
Source: trekearth.com

However, with all of the great positive aspects of dams and reservoirs there are some equally negative aspects of these engineering projects. Firstly, the construction (and maintenance) of dams is hugely expensive and environmentally disrupting and damaging. They also cause (often irreversible) upheaval to people living both near the site and far downstream from it. As well, because so many of Africa's river basins are transboundary, construction of dams has international implications. Most reservoirs have a large surface area and relatively shallow depths which means a significant proportion of the water (sometimes 90%) is lost to evaporation. The capacity of reservoirs to hold a large volume of water is both an advantage and disadvantage. With the water held in one area reservoirs are a centralised resource. As a result, in addition to the implications of the dam/reservoir construction, a large amount of infrastructure is necessary to transport the water from where it is stored to where it is needed. This adds further disruption, destruction, cost and maintenance.

As well, because the construction of dams and reservoirs is so expensive they are often funded / constructed by aid from developed countries. McCartney and Smakhtin highlight that both the European Union and China are investing significantly in water storage infrastructure throughout Africa. Foreign involvement in the affairs of African countries further implicates these engineering projects in controversy and political tensions.

Rain-water Harvesting Systems

Rain-water harvesting systems (RWHS’s) or small water storage systems are significantly smaller than reservoirs. Crucially, instead of being centralised stores of water they are at (or at least close to) the site of the water need. Therefore, RWHS’s require far less infrastructure compared to dams and reservoirs and thus they are far less disruptive and a more sustainable water storage option. Aside from increasing water security, agricultural productivity and adaptive capacity RWHS’s can also significantly improve the local people’s livelihoods and health.

Diagram of a Rainwater Harvesting System
Source: http://armfielddesign.com/

Indeed, stored water in reservoirs and rain-water harvesting systems provides a means of adapting to increasing rainfall variability and crop failure can be prevented and yields sustained (and potentially increases) if the water is used for irrigation. However, investing in water storage won’t immediately bring these benefits; context is also important.

At present, less than 5% of the cultivated area in SSA is equipped for irrigation. Therefore, achieving food security in Africa will not be as simple as just increasing water storage capacity; investment in irrigation infrastructure will also be crucial. This sentiment is reflected in the fact that one of the goals of Africa Water Vision 2025 is to at least double the area of Africa that is under irrigation by 2025.

Groundwater Extraction

Groundwater is already heavily relied upon as a source of drinking water in Africa. Increasing its extraction is another option for tackling Africa’s water/food insecurity and responding to climate change and population growth. In 2012, MacDonald et al. presented maps that showed African freshwater stored as groundwater is well-distributed across the continent and there is an estimated 0.66 million km3 of water; more than 100 times the annual renewable freshwater resources. The maps also show that many of the countries designated as ‘water scarce’ have substantial groundwater reserves.

Groundwater storage for Africa based on the effective porosity and saturated aquifer thickness. Groundwater storage is expressed as water depth in millimetres with modern annual recharge for comparison.
Source: MacDonald et. al., 2012

One of the advantages of aquifers compared to reservoir stored water is that groundwater is not nearly as affected by evaporation. As well, because Africa’s groundwater resources are well-distributed, water can be found close to the site of need and therefore extensive water transportation infrastructure is not necessary (McDonald et. al 2012). Additionally, McDonald et. al highlight that groundwater often doesn’t require treatment or processing.

Despite the great abundance and presence of groundwater, the extent to which groundwater can facilitate water/food security and adaptation to climate change will be determined by the accessibility of said water. Abstraction of most groundwater requires drilling a borehole. However, the characteristics of the rock above the water (e.g. its permeability) determines the yield and abstraction rate of the borehole. Therefore, ability to access the water is equally as important as its abundance (McCartney and Smakhtin, 2010. McDonald et. al, 2012).

In their paper, McDonald et. al conclude that: 

·      The potential for borehole yields exceeding 5 l s−1 (required for commercial irrigation) is not widespread and higher yielding boreholes may only be successful in some areas.

·      The potential for boreholes yields of 0.5–5 l s−1, which could be suitable for small scale household and community irrigation, or multiple use water supply systems, is much higher.

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 2012 in MacDonald et. al 2012).  
  

Therefore, it seems that despite its huge potential, exploitation of groundwater resources will not
necessarily be simple or a universal panacea. However, this isn’t to say that groundwater extraction
should be disregarded as an option because there are certainly cases where it can make a considerable
contribution to combatting African water and food security and poverty levels.

Additionally, because the boundaries of groundwater stores are not synonymous with national boundaries, over 80 aquifers and aquifer systems in Africa are shared internationally (Villholth and Altchenko, 2014). This creates legal issues surrounding ownership and sharing of groundwater and further questions to what extent groundwater abstraction will form part of Africa’s future.  

All of the water storage options discussed have strengths and weaknesses which depend, in part, upon their inherent characteristics but they also affected by site-specific conditions and the way in which each option is installed and managed. Indeed, none of the storage options will be a panacea. However, in the correct geographic, cultural and political location they all have important contributions to make toward achieving Africa’s ambitions (and need) to achieve water/food security and reduce poverty. I guess it's a case of watch this space...

Africa Water Atlas. (2010). 1st ed. Nairobi, Kenya: United Nations Environment Program.

MacDonald, A., Bonsor, H., Dochartaigh, B. and Taylor, R. (2012). Quantitative maps of groundwater resources in Africa. Environmental Research Letters, 7(2), p.024009.

McCartney, M. and Smakhtin, V. (2010). Water Storage in an Era of Climate Change: Addressing the Challenge of Increasing Rainfall Variability. Available at: http://www.iwmi.cgiar.org/Publications/Blue_Papers/PDF/Blue_Paper_2010-final.pdf?galog=no

The Africa Water Vision for 2025. (2009). 1st ed. Addis Ababa: Economic Commission for Africa.

Villholth, K. and Altchenko, Y. (2014). Transboundary Aquifer Mapping and Management in Africa. Iwmi.cgiar.org. Available at: http://www.iwmi.cgiar.org/Publications/Other/PDF/transboundary_aquifer_mapping_and_management_in_africa.pdf?galog=no