Surface Water Resources Across Africa under Climate Change

Water is essential to development and the importance of ensuring availability for all is outlined in United Nations Sustainable Development Goal #6. However, the emphasis of SDG6 on equity and access necessitates an underlying understanding of how climate change is altering current water supplies across regions within Africa. In the table below I have summarised findings from various studies of climate change impacts on surface waters across Africa. Immediately clear is that supply in most regions is expected to become increasingly uncertain, ultimately aggravating the situation of currently water-stressed regions. However, there is significant variation and some research reports that in certain regions of the continent the effects may be more favourable. In this blog, I explore surface water resources, in a bid to better understand how changes in its supply from precipitation and overland flow, will affect regions differently within Africa. 

Region	Project Change in Water Resources	Reference
Africa	By 2050, water stress will increase over 62%-75.8% of total river basin area.  Significant variability over the continent.	(Alcamo et al. 2007) (UNESDO, 2020)
East Africa	Runoff in eastern Africa is projected to increase by 2050.     In Tanzania, annual averages of water yield and surface runoff increase up to 61.6% and 67.8%.	(Arnell, 2003) (Näschen, 2019)­­
West Africa	River flows are expected to decline by 15-20%.	(El Vilauy et al. 2013)
Southern  Africa	Decrease in runoff 10-30% by 2050.   For Swaziland, streamflow will decrease by up to 40%	(Milley et al. 2005) (Matondo, 2012)
North Africa	All countries are exposed to strong temperature increases and a high drought risk under climate change	(Schilling et al. (2020)
Central Africa	Runoff is change is uncertain but likely to decrease.	(Arnell, 2003)

Table 1. Summary of studies on climate change impacts of surface water resources in Africa. Adapted from: (Goulden et al. 2010)

Existing Variability: The Uneven Distribution of Surface Water Resources

Across Africa, there are sixty international river basins and over 160 freshwater lakes (>25KM2), which account for 62 % of the total land area. Collectively, the continent appears reasonably furnished with water, however, these resources are distributed unevenly (Figure 1a) and this is largely due to spatial patterns of precipitation (Figure 1b). Although Thompson et al. (2010) report that other factors, such as vegetation, can impact the presence of surface water resources. Gan et al. (2016) suggest that climatological factors have the most significant effect on surface water supply as overland flow occurs when precipitation exceeds the rate at which it infiltrates the soil.  In arid northern and southern Africa, precipitation rates are significantly lower than those of humid, central Africa. When moving south from the Sahara Desert and north from the Kalahari Desert, precipitation rates increase from less than 300mm to over 2,000mm in Cameron and Gabon (Central Africa). Consequently, surface water resources are located around the equatorial region where precipitation is greatest, whereas, in North African arid regions, there is a distinct lack of surface water resources (Figure 1b). 

Figure 1a. Major river basins and stream networks in Africa, selected from the AEON Africa Database. Source: (de Wit and Stankiewicz, 2006). Figure 1b. Annual average precipitation (mm) across Africa from the Global Precipitation Climatology Centre. Source (Hydroclimate Extremes in Africa: Variability, Observations and Modelled Projections)

So, what will happen under climate change? 

Against this backdrop of existing variability, climate change will worsen the situation by altering the timing, distribution, and quantity of precipitation. A model developed by de Wit and Stankiewicz (2006) found that intensification of the hydrological cycle will affect surface water resources across 25% of Africa, with these effects varying from region to region. As highlighted in Table 1, semi-arid and arid regions, which are already vulnerable, are expected to fare worst. For example, the drought-stricken region of South Africa is expected to lose more than half its surface water supply (which already happening), and countries in the north of Africa are likely to experience “strong to moderate decreases in their water supply”. In contrast, East Africa’s future looks far more optimistic, with increases in surface water resources expected alongside increased precipitation. However, Rowell and Chadwick (2018) highlight in tropical regions like East Africa, complex global climate interactions, limited high-quality observational data and inherent uncertainties associated with the global circulation models used to model future climate can make water resource impacts hard to predict. As such, already vulnerable and ill-equipped populations become more susceptible to climate change extremes. The study clearly highlights the importance of reducing hydrological modelling uncertainties through better modelling techniques and observational data, which will ultimately to support more accurate regional climate projections and help build climate resilience. 

Final Thoughts. 

Despite the uncertainty, these findings highlight that building climate change resilience to changes in surface water resources in Africa clearly requires a regional perspective, especially as already limited surface water supplies in semi-arid and arid regions are predicted to become more stressed under future climate change. For example, for North African countries it is important to reduce dependence on rain-fed agriculture and increase adaptive capacity. However, Rockström and Falkenmark, (2015) highlight it is important to account for all freshwater resources and not just water from rivers and lakes. For example, water below the surface in Africa is expected to be 20 times larger than surface water resources, yet groundwater does not figure prominently in assessments of global, regional and national water availability. In next week's blog, I explore whether groundwater can offer a renewable source in more water-stressed regions.