Groundwater as beneficiary of anthropogenic climate change?

The latest posts have outlined that the greatest challenge to be faced by Africa is not necessarily the decrease in the total mean annual rainfall under climate change, but rather the increased variability in rainfall events and thus the distribution within the year. Having established that the general trend of precipitation is towards fewer, more intense rainfall events, I want to dedicate this post to explaining how under certain conditions this can improve the water resources available in Africa.
The dependence on groundwater across Africa varies but serves as the only reliable perennial source of water for many rural communities. The overall dependence is estimated to be as high as 50%.

Groundwater recharge is strongly determined by the effective precipitation in a given year – however, as evidence from East Africa suggests, it is a non-linear relationship, in which recharge is more strongly related to extreme rainfall events than to the totals (Owor et al. 2009 and Taylor et. al., 2013). While these recent studies examine a past data records, there are important lessons to be learnt for future adaptation to an intensified hydrological cycle.

I want to provide an overview of the findings first:

1. The Upper Nile basin is located within the tropics of Africa, a region where climate change effects are expected to be especially pronounced. A data record of 1999-2008, pairing groundwater to rainfall data, is used to show that the magnitude of water level fluctuation is better correlated to the number of heavy rainfall events (>10mm/day) than to the total sum of rainfall. (Owor et. al. 2009)
2. A 55-year long record from a different basin in East Africa, in central Tanzania, provides further coherent evidence for this phenomenon. The observed recharge events interrupt long periods of groundwater level decline, and are highly correlated with extreme rainfall events occurring during the summer monsoon. In fact, the highest recharge events occur during ENSO years which brings intense rainfall to Tanzania (and thus result from regionally dominant modes of tropical climate variability) as shown in Figure 1. The evidence further shows that 80% of cumulative recharge over the whole observed timespan is derived from the highest 13 recharge events. (Taylor et. al., 2013)

Figure 1. Showing El Nino and La Nina events alongside the main recharge events (Source)

My thoughts regarding future climate change:

The intensification of the hydrological cycle through climate change (last 2 posts) will provide an increase in the occurrence of extreme events, which are those that episodic recharge events depend on in these two areas. Groundwater reserves are already seen to become the freshwater source of choice in the future due to their resilience in quality and quantity relative to surface waters (for a great review see Kundzewicz and Doell, 2009). 

The findings in the studies above suggest that ground water resource replenishment can in fact benefit from this new turn in the global climate. While there is significant uncertainty in the prediction of ENSO events through GCMs, the probability of positive IOD modes is suggested to increase under anthropogenic global warming, bringing higher monsoonal rainfall to East Africa. The enhanced groundwater reserves have the potential to act as a buffer against the increased variability in surface water resources, and could be the solution to water security in part of Africa that experience a similar recharge response to extreme events. When looking at the geographical distribution, using the brilliant maps of MacDonald et al (2012) for example, we can see that most parts of Africa have the potential to develop groundwater, with much being at low depths from the surface. This means that in these low-lands even poorest farmers will be able to use this relatively stable water supply with limited technology (and economic input!) to support their livelihoods by elevating the insecurity of using rain-fed agriculture  in future climate change scenarios.

We cannot however consider this as a solution that fits all – it is strongly dependent on geological properties of the soil surface and subsurface, and the way other effects of the expected climate change (e.g. sea level rise) influence in situ.