Groundwater: a hidden solution?

With surface water supplies becoming more unreliable in a warming world, how will Africa's increasing population and water demands be met? Perhaps we should instead look below the surface...could groundwater be the answer to Africa's current and future water problems?

In many parts of semi-arid and arid Africa, groundwater is the only reliable source of freshwater, as surface waters are ephemeral seasonally or year-round (Taylor et al 2013). A huge advantage of groundwater is that it is usually of potable quality, and is more resilient to climate variations than surface water due to its slow movement in the subsurface (Taylor et al 2009). Future climate projections highlight the disproportionate impact on Africa, whereby warming is predicted to be 1.5 times the global mean (Taylor et al 2009; Niang et al 2014). It is therefore vitally important to understand how preferential warming in Africa will impact freshwater availability. Today's post focuses on a recent study by Taylor et al (2013), which looks at evidence for groundwater dependence on heavy rainfall in the Makutapora Wellfield, Tanzania, East Africa.

This area in central Tanzania has one of the longest published records of groundwater levels in the tropics, spanning 55 years from 1955-2010. Using this record, Taylor et al (2013) discover that groundwater levels have been declining in the Makutapora Wellfield, due to increased abstraction to provide potable water to the capital, Dodoma. The removal of water from the deeply weathered granite aquifer has risen from 0.1 million m3 in the period 1955-1979, to 0.9 million m3 per month since 1990 - an increase of ~900% in half a century! How is the aquifer not depleted yet? It appears episodic recharge events are the key to sustaining intense abstraction rates...

Figure from Taylor et al (2013) demonstrating the non-linear relationship between rainfall and groundwater recharge. Shaded grey is months with "extreme" (95th percentile) rainfall, solid vertical line is the median (50th percentile), dashed line is third quartile (75th percentile).

For almost two thirds of the record, zero recharge occurs. When recharge does occur in the rest of the record, it is only under highly intense seasonal rainfall and ENSO (El Nino Southern Oscillation) events. This suggests a very non-linear relationship between rainfall and recharge events in East Africa. In a warmer world, where intensification of the hydrological cycle causes more frequent, intense precipitation events (Niang et al 2014), could replenished groundwater supplies be a viable solution for Africa's water problems? In many parts of the continent, groundwater may play a significant strategic role in the future as a way of adapting to changing surface freshwater resources (Taylor et al 2009).

The seven largest groundwater recharge events are indicated by arrows. Clear trend that El Nino years produce the largest recharge events (Taylor et al 2013).

From the figure above, we can see that 5 of the 7 major recharge events occurred during an ENSO year. For Tanzania, this extreme climate oscillation event brings additional intense rainfall and replenishes aquifers. As I'm sure you've heard in recent news stories across the globe, this Christmas our planet is set to experience one of the biggest ENSO events of the century! To date, the 1997/1998 ENSO (see graph above) was the largest event, accounting for 25% of the replenishment over the last 55 years in Taylor et al's (2013) study. With an even bigger El Nino event predicted this winter, imagine the groundwater recharge potential for the Makutapora Wellfield...

If we can improve monitoring and data collection in regions like this, it could be possible to anticipate when these huge recharge events, that sustain groundwater abstraction, will occur. Adaptive strategies may be enforced, and Africa's population might be able to enhance the water being replenished. All this only scratches the surface with groundwater in the future - what if soil infiltration capacities cannot transmit heavier rainfall events (and thus permit recharge)? What if the quality of groundwater is contaminated by pathogens from faecal runoff into open springs with intense precipitation, leaving supplies unusable? One thing seems fairly certain: in a warming world, not all of Africa's wells will run dry.