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| Lusaka, the capital of Zambia. A fast growing urban area within Africa with the fortune of an underlying plentiful and accessible aquifer. Note the plenitude of pivot irrigation to the north of the city. Credit: Google maps. |
In my last post I provided some context surrounding urban water and sanitation within Africa. What became clear was that there is a large degree of geographical variation on a multitude of scales in the provision of improved drinking water sources in urban areas.
In Layman's terms if there were adequate availability and equal distribution of fresh water across Africa, issues of urban water and sanitation, scarcity and stress would not be a problem. Now this is a gross simplification, but allows for the purpose of this blog to break down the issues at hand. This blog will focus on physical factors, recognising the importance of the availability and distribution of freshwater resources whereas my next blog will analyse the socio-economic factors which limit access in urban areas.
As eluded to already and reported by Conway et al. (2009), rainfall and river flows over the whole of the African continent display high levels of variability over a range of spatial and temporal scales with significant implications for society causing widespread acute human suffering and economic damage. Examples of variability include multi decadal anomalies in river flow regimes in parts of West Africa where long-term mean yields of freshwater fell by 18% between 1951–70 and 1971–89 relating to prolonged drying in the Sahel. During the period 1961–90 (1931–60), rainfall explains 60%– 80% (40%–60%) of the variability in river flows in West Africa.
In contrast, East Africa has experienced relative climatic stability punctuated by extreme wet years with prolonged periods of high river flows associated with the El Nino–Southern Oscillation (ENSO), particularly during the short October–December rains. This intense rainfall has been found to contribute disproportionately to recharge with suggestions that increased use of groundwater may therefore prove a potentially viable adaptation to enhanced variability in surface-water resources and soil moisture resulting from climate change (Taylor et al. 2012). As Conway et al. conclude, rainfall provides the dominant control on surface water flows and is key in determining the availability on annual renewable freshwater resources.
A study by Mahe (2009) found that although rainfall has decreased in West Africa for two river basins (Bani and Nakambe) since the 1970's the actual discharge of the rivers has altered significantly. Despite rainfall falling 16% and 19% respectively river discharge reduced significantly within the Bani river but increased within the Nakambe river. Bani low flow variability is well correlated to a decrease in the water table resulting from climatic changes. However, for the Nakambe basin increased flows result from land degradation driven by human activity creating higher runoff coefficients. This could potentially in the long run create land that is no longer suitable for agriculture. Again, physical factors determine the underlying availability of renewable freshwater resources.
The importance of distribution is also echoed by Taylor (2004) who argues that the difference between the distribution of water resource and demand for them currently impedes development within Africa and will likely continue to do so into the future even within countries that on a national scale have adequate water resources. For instance despite Namibia's relatively significant quantity of renewable water resources (fig.1), river flow is highly seasonal and confined to its borders (Orange river, Cunene and Cubango) with poor groundwater resources. This builds upon the wider debate (future blog post) that metrics that define resources fail to acknowledge physical spatial and temporal distribution which is critical for access.
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| Figure 1: Estimated renewable water resources, water use and per capita availability within Africa. Credit: Taylor (2004). |
A further physical characteristic that can determine the availability of water is the tectonic setting of a region. Evidence from Uganda shows how in central Uganda deep weathering accompanying long term tectonic stability results in a recharge-dominated regime with a large basin storage ensuring water availability despite highly seasonal rainfall. On the other hand, stripping of the land surface in south western Uganda resulting from tectonic uplift has resulted in a comparatively low storage component. Stream flow is highly variable and confined to incised channels creating problems for water availability. Again the distribution and availability of water resources is determined by the physical characteristics of the region.
The use of groundwater within Africa for domestic use and for irrigation is expected to increase significantly over coming years as a result of rising population and irrigation growth. MacDonald et al. (2012) estimated total groundwater storage in Africa to be 0.66 million km3 or more than 100 times estimates of annual renewable freshwater resources within Africa. Not only are reserves extensive but groundwater also provides an important buffer to the seasonality and unpredictability of rainfall and changes in climate. Many countries considered 'water scarce' by traditional metrics have significant groundwater reserves beneath them. However, once again water resources are not equally distributed, neither are they equally accessible. Figure 2 highlights the large groundwater volumes available within Northern Africa, recognised above as an area experiencing prolonged drying. Although these aquifers can sustain high levels of irrigation (5 l s-1_ they were found to be distributed away from population concentrations and at significant depth (fig. 2) making them costly to exploit.
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| Figure 2: Aquifer productivity for Africa as well as estimated depth to groundwater. Credit: MacDonald et al. 2012 |
However, a significant finding of MacDonald et al. was that the distribution of aquifers within Africa could facilitate widespread small scale community hand pumps (yields of 0.1-0.3 l s-1) with a smaller but significant number of intermediate boreholes sustaining small scale household and community irrigation (0.5-5 l s-1). These findings suggest a need to shift away from large scale top down responses to water scarcity to smaller scale, community based bottom up approaches which sustainably use the available resources within Africa.
The importance of physical factors is reinforced by the leading image of Lusaka. The high presence of pivot irrigation to the north of the map symbolizes how the availability of annual renewable freshwater resources resulting from physical factors, such as a highly productive aquifer in this case, can go a long way in ensuring access to safe freshwater in urban areas. In addition, the prospect of preferentially increasing temperatures, intensification of precipitation with knock on effects on river discharge and rising populations will likely further exaggerate the role of physical factors in determining access to water in urban areas. Nevertheless, it would be incredibly naive to argue that only physical factors were responsible in determining access. Water availability by no means ensures access. My next blog will analyse the socio-economic factors that are also vitally important.
