By George Blake, Kingfisher Writer

Published October 4th, 2021

Groundwater abstractions within sub-Saharan Africa must increase in order to satisfy the projected growth in water demand. Yet little is known about the renewability of groundwater resources, particularly under climate change, as limited observational data and modeling uncertainty is preventing any robust estimations.

Groundwater provides a vital and perennial source of high-quality water across sub-Saharan Africa, representing the primary domestic water source for some 400 million people and accounting for 20% of total irrigation water. The largely ubiquitous nature of groundwater is critical in meeting the spatially distributed water demand that characterises many poorer, more rural regions, where human well-being and economic development are largely dependent on the availability of natural resources and agriculture.

Population growth, urbanisation, and the expansion of irrigated agriculture are projected to significantly increase water demand across sub-Saharan Africa, with any increase in reliable water supplies likely to be dependent on the further development of groundwater; especially given its intrinsic ability to buffer against heightened hydrological variability associated with climate.

The ability of groundwater to satisfy future demand is dependent on the rate at which it is replenished by precipitation (also known as recharge). Yet, despite its importance, our understanding of key recharge processes is limited, especially in data-sparse areas such as sub-Saharan Africa.

Recharge is commonly defined as diffuse or focused, where diffuse refers to the direct infiltration of water in response to rainfall occurring over a large spatial area. Focused refers to the leakage of water from ephemeral (temporary) water bodies, such as streams and ponds, at a given location.

Broadly speaking, the importance of focused recharge is thought to increase with aridity, as high temperatures and evapotranspiration prevent water from infiltrating through the soil.

In these environments, recharge is heavily dependent on the presence of ephemeral surface water, which is formed after heavy rainfall, and the presence of preferential flow pathways, such as wormholes, root holes and cracks. These preferential flows allow water to bypass the main soil matrix and reach the water table far quicker than would normally be expected, allowing water to escape evapotranspiration.

Yet, most groundwater modelling studies assume recharge to be diffuse in semi-arid areas of sub-Saharan Africa, despite this being at odds with our conceptual understanding of recharge. Ignoring focused recharge and preferential flows has profound implications for regional water assessments, particularly under climate change.

Higher temperatures are increasing the moisture-holding capacity of the near-surface atmosphere, which is driving an intensification of the global hydrological cycle, particularly within the tropics and sub-tropics. This intensification is characterised by a transition from a rainfall regime of mainly light and medium rainfall events to one dominated by more extreme or heavy rainfall events

‘Ignoring focused recharge and preferential flows has profound implications for regional water assessments, particularly under climate change.’

How groundwater resources respond to this intensification is dependent on the relationship between recharge and rainfall intensity. There is increasing evidence that recharge is closely associated with heavy rainfall events (greater than 10 millimetres per day) rather than annual rainfall totals, especially in the tropics and sub-tropics. This, therefore, suggests that climate change may enhance recharge even where total rainfall is reduced.

These findings are built on observational studies and differ from hydrological modelling projections, which have previously projected that groundwater resources may reduce in the dry subtropics. This discrepancy is a function of most modelling studies ignoring preferential flows and focused recharge, which are closely associated with rainfall intensity. This means that studies assuming diffuse recharge alone will underestimate recharge and future groundwater resources in response to more intense rainfall, risking sub-optimal or damaging management interventions.

The Intergovernmental Panel on Climate Change (IPCC) has acknowledged these concerns, stating in their Sixth Assessment Report (AR6) that no confident assessment of groundwater can be made due to limitations with our current numerical representations of preferential flow and focused recharge. Such an admission is concerning given the importance (which is only likely to grow) of groundwater across sub-Saharan Africa.

If focused recharge and preferential flow represent key recharge pathways, why are numerical representations of these processes so inadequate? It partly relates to historical denial within hydrology and the investment pumped into our current generation of models, but it also reflects limited observational data.

A lack of robust data frustrates the development of a cohesive conceptual framework to integrate the major space-time factors that dictate their occurrence and the ability for any hydrological models to be robustly validated. Although progress has been made in establishing in-situ techniques capable of identifying and quantifying preferential flow and focused recharge, research is limited within sub-Saharan Africa. In fact, a recent review paper found that just one out of 150 in-situ studies have been conducted in sub-Saharan Africa.

This limited data availability hampers our understanding of recharge processes, prevents robust modelling, and often necessitates the use of more simplistic models that are unsuitable for water assessments under climate change. Until data coverage massively increases, we cannot expect to make substantial progress in modelling capabilities.

‘Limited data availability hampers our understanding of recharge processes.’

It is also important to note that climate change is not the sole anthropogenic pressure impacting groundwater resources; agricultural, industrial, and mining activities may also adversely affect groundwater quantity and quality.

All of which highlights the pressing need for an expanded and integrated monitoring network of groundwater levels, rainfall, soil characteristics, and land use (plus many more) that considers the impact of a range of anthropogenic pressures. If not, key management decisions may be taken on the back of questionable conceptual understanding.

Featured Image: Nadine Venter / Unsplash

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