Groundwater recharge: an upside to floods?

Groundwater: turns out there's considerably more to it than making an enormous fluorescent mess.

A few weeks ago I attended a lecture on groundwater, something I'd never thought much about before (although chucking dye down potholes and seeing where it comes out always looks fun). One topic that came up was a consequence of flooding that was entirely new to me: contributing to the renewal of groundwater resources.

Groundwater comprises a massive 98% of the world's freshwater resources, and as populations rise and surface water resources become less reliable due to climate change, it's likely to become increasingly important resource in many parts of the world. In southern Africa most of the of the population is reliant on groundwater as a primary source for domestic and agricultural use, but there's limited knowledge about the capacities and resilience of many aquifers in the region.

A map of the groundwater resources in Africa, and a chart showing the total groundwater storage and annual renewable freshwater for each country. Adapted from Macdonald et al., 2012.

In arid regions of eastern and southern Africa the recharge of aquifers is highly episodic and closely associated with extreme rainfall and flooding that are often linked to El Niño events.

A flood at the site of the Makutapora aquifer in Tanzania. Nearly 20% of the recharge of the aquifer over 56 years was linked to the heavy rainfall and floods associated with the 1998 El Niño. Source: GroFutures.org

The Makutapora aquifer is the primary water source for Dodoma, the Tanzanian capital. Since the 1980s water levels in the aquifer have dropped significantly, and a historical analysis of the levels of groundwater recharge in the aquifer show that unless the rainfall is unusually high, very limited recharge occurs. Over the 56 years of the study, 60% of the recharge came from the top 7 wettest years. The biggest recharge event occurred in 1998, when severe rainfall and floods were linked to one of the strongest El Niño events on record. There was also significant recharge from floods during the strong El Niño of 2015-2016.

The good news is that this suggests that resilience of groundwater supplies in these arid areas may be greater than previously thought: more extreme rainfall and flooding events linked to climate change may mean that the associated recharge will compensate for increased demands on groundwater. So what can be done to ensure that as much floodwater as possible makes it into aquifers?

Diagram showing potential approaches to MAR. Source: National Water Commission, Canberra

Managed aquifer recharge (MAR) refers to the artificial recharge of aquifers, widespread in many parts of the world. There are few large scale projects in Africa, although small-scale MAR methods such as sand dams are common development interventions. Given the forecastability of phenomena such as El Niño, MAR may be a way to extract an upside from the associated extreme floods by ensuring that freshwater resources become more climate resilient.