Is Groundwater Resilient to Climate Change?

Last week’s blog found that climate change will cause most arid and semi-arid regions in Africa to become drier, which will reduce renewable surface water resources. Furthermore, over the same period, the population of sub-Saharan Africa is expected to double to two billion. Clearly, the demand for freshwater will increase and competition amongst agricultural, industrial, and domestic uses will worsen. In dealing with this, emerging research is exploring whether groundwater (current storage estimates shown in Figure 1) can satisfy this demand. Writing in Nature, Cuthbert et al. (2019) show that reduced precipitation does not always deplete groundwater reserves in sub-Saharan Africa. This directly contrasts global assessments (e.g. Jiménez et al. 2014) that suggest reserves will decrease in response to global warming. Over this blog post, I will explore how groundwater may respond to climate change and examine whether it can raise hope for a more resilient future. 

Figure 1. Groundwater Storage in Africa calculated based on the effective porosity and saturated aquifer thickness. MacDonald et al. (2012) estimate that groundwater resources are as much as 0.66 million km3, which is much more than average rainfall (0.02 million km3; New et al. 2000) and freshwater storage in lakes 0.03 million km3 (Shiklomanov and Rodda 2003). Source: (MacDonald et al. 2012)

Groundwater accounts for as much as 33% of total water withdrawals worldwide but is often overlooked. Resources are especially significant in arid and semi-arid Northern Africa (Figure 1) - with Libya, Algeria, Sudan, Egypt, and Chad having some of the largest groundwater reserves. It can be accessed through wells, boreholes, and springs, which means it is especially valuable in rural areas that do not have access to piped water supplies (Macdonald et al. 2009). Furthermore, as it is generally of high quality, it is increasingly being used in towns and cities in Africa– with the majority of domestic water supplies in sub-Saharan Africa coming from groundwater. Despite the obvious dependence on groundwater, a historical bias toward surface water resources had led to insufficient groundwater level observations and management. However, after analysing long-term records of groundwater there is a growing consensus amongst academic research that groundwater reserves in sub-Saharan Africa are resilient to climate change. In fact, in most areas, because it has a slower response to changing meteorological conditions relative to surface water, there is potential for groundwater to support increased water and food security under increased climate variability.

So, how is groundwater replenished and how is affected by climate change?

 

A key determinant of groundwater levels is the balance between groundwater recharge and discharge. Recharge happens either directly – via effective precipitation that infiltrates at the land surface – or indirectly - via leakage from ephemeral streams and ponds. Previously, IPCC assessments (2007, 2008) have indicated that climate change could impact these recharge processes. They suggest warming will drive changes to global rainfall patterns and alter the amount of soil infiltration and deeper percolation that can take place. Ultimately the magnitude and timing of recharge will become less certain and groundwater levels will diminish (Hiscock et al. 2012).

 

However, a significant limitation of regional-scale assessments (IPCC; 2007, 2008) is that they often neglect how controls on aquifer replenishment work at smaller-scales. Taylor et al (2012) found that in the Makutapora Wellfield,  rates of aquifer replenishment were tightly linked to highly episodic precipitation. The study found these events cause temporary streams and ponds and allow for groundwater replenishment to take place via indirect recharge. This is important as it uncovers that groundwater recharge is more sensitive to the intensity of rainfall, rather than the overall amount. These findings are also consistent across other wells across sub-Sharan Africa (Cuthbert et al. 2019). The observations (shown below in Figure 2 ) highlight that despite drying climatic trends, increased precipitation extremes will probably increase or have a limited effect on replenishment rates. Moreover, this is particularly evident in regions which are naturally water-scarce and suggests they may be more resilient to climate change than expected (Healy, 2019). For example, in the water-stressed region of Modderfontein, South Africa, rainfall is expected to occur in fewer-heavier bursts, which could actually increase replenishment for groundwater aquifers. In contrast, surface water bodies, such as the Lake Chad, respond very quickly and visibly to changes in climate. Due to reduced precipitation unplanned irrigation, and population increase Lake Chad has shrunk by 90% since the 1960s. 

Figure 2. Groundwater and precipitation records across varying aridity in sub-Saharan Africa. Source: (Cuthbert et al. 2019)

The future of groundwater in sub-Saharan Africa - what to consider? 

Although these studies demonstrate that groundwater resources are resilient to climate change, there is still a lot to consider. First, accurate observations of groundwater levels and recharge rates are sparse in Africa. This inhibits modelling and management of resources and often impedes decision making. Furthermore, human activities can have demonstrable effects on the quantity and quality of groundwater. First, extensive unplanned agricultural irrigation can deplete groundwater quantities at alarming rates. Countries such as Tunisia and Algeria in Northern Africa have often been cited for their over-exploitation of non-renewable groundwater resources (e.g. Kuper et al. 2016). While these stocks are vast, without good management strategies, aquifers could shrink dramatically (OECD, 2012). Groundwater quality is also threatened by over-use and land-system change. Healy (2019) found that land-use change to agricultural, industrial, and mining activities could adversely affect groundwater quality. Furthermore, Aladejana et al. (2020) found that rising sea-levels have resulted in saline intrusion in shallow coastal aquifers in southwestern Nigeria, which is a region that is largely dependent on groundwater resources. 

 

Final Thoughts: Better Planning and Management 

This blog has been eye-opening. It has demonstrated that the future of water supply in sub-Saharan Africa lies firmly with groundwater resources. These resources will be hugely important in supporting rural and urban communities and sustaining commercial and agricultural activities. However, there are some limitations that must be addressed through better planning and management of resources. Central to this is the development of an expanded groundwater-level monitoring network to support better modelling of resources.