Climate change and groundwater
Groundwater is simply the water below the surface held in between the pores of soil and rock (USGS, 2001). Globally, groundwater is the largest accessible freshwater store and therefore, is typically regarded to be the go-to resource during times of freshwater shortages connected to low and variable river discharges (Goulden et al., 2009). Groundwater may be the only reliable water source in some arid and semi-arid areas where surface waters may be absent (Taylor et al., 2009; Taylor et al., 2013). Africa exhibits many groundwater stores (Figure 1) and Carter and Parker (2009) estimate that over 50 percent of Africa is dependent on groundwater resources, whilst Goulden et al. (2009) estimate that 75 percent of the population depend on the resource!
However, groundwater viability as an alternative water resource depends on recharge. Recharge results from excess precipitation infiltrating the subsurface, where the soil's hydraulic conductivity enables it to move downwards (Taylor et al., 2013). There are two types of recharge: diffuse and focused. Diffuse recharge results from direct recharge via the soil matrix in saturated soils and soil macropores and fractures that bypass the soil matrix. Focused recharge indirectly results in recharge via leakage from runoff and surface water sources. Focused recharge may be critical in semi-arid areas (Taylor et al., 2013).
So how might climate change affect this?
Climate change's impact on groundwater reserves are expected to be a lot smaller than climate change's impact on surface water stores (Niang et al., 2014). Again, the predicted impact of climate change on groundwater resources will vary across the continent, specifically between climatic zones; areas receiving between 200mm and 500mm of annual rainfall are expected to experience a decline in rainfall (McDonald et al., 2009). These areas include The Sahel, the Horn of Africa, and southern Africa.
Significantly, due to the high potential evapotranspiration across much of Africa, groundwater reserves are commonly replenished during intense rainfall events, especially shallow aquifers (Taylor et al., 2013). As mentioned early, climate change is expected to increase rainfall intensity, causing more variable river discharge and flooding: interestingly this shift may actually enhance groundwater recharge. Taylor et al. (2013) found that increases in intensive monthly rainfall (under GCM projections for the 21st century in the region of East Africa) sustains groundwater reserves, significantly in areas where communities are completely dependent on the resource for freshwater. Consequently, groundwater has the ability to play a key role in climate change adaptation and resilience and in turn, can enhance food production and security via small-scale irrigation schemes (Carter and Parker, 2009; MacDonald et al. 2009).
But... Africa is experiencing huge rates of growth, with the region's population expected to more than double by 2100 (!) (UN, 2015). Population growth and urbanisation are expected to increase the domestic and agricultural demand for freshwater and even has the potential to degrade water quality (Taylor et al., 2009); water resources obtained from groundwater resources are vulnerable to contamination from inadequate hygiene in rapidly urbanising centres (Howard et al., 2003). These non-climatic drivers are expected to impact groundwater resources more considerably than climate change (Niang et al., 2014)
So unlike surface water resources, groundwater reserves may potentially be positively affected by climate change and instead may offer an adaptive strategy to the problem, especially shallow aquifers. However, there are noticeable concerns over the sustainability of groundwater reserves from the increasing demand for freshwater from population growth and high rates of urbanisation: ultimately, if groundwater reserves are going to be used as an adaptive strategy to the impacts of climate change, they need to be managed accordingly to ensure that the quantity and quality of groundwater resources exhibit sustainable levels.
Of course, this is easier said than done... I'm sure this is an issue I will be returning to later on!
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| Groundwater storage map of Africa (Macdonald et al., 2012) |
However, groundwater viability as an alternative water resource depends on recharge. Recharge results from excess precipitation infiltrating the subsurface, where the soil's hydraulic conductivity enables it to move downwards (Taylor et al., 2013). There are two types of recharge: diffuse and focused. Diffuse recharge results from direct recharge via the soil matrix in saturated soils and soil macropores and fractures that bypass the soil matrix. Focused recharge indirectly results in recharge via leakage from runoff and surface water sources. Focused recharge may be critical in semi-arid areas (Taylor et al., 2013).
So how might climate change affect this?
Climate change's impact on groundwater reserves are expected to be a lot smaller than climate change's impact on surface water stores (Niang et al., 2014). Again, the predicted impact of climate change on groundwater resources will vary across the continent, specifically between climatic zones; areas receiving between 200mm and 500mm of annual rainfall are expected to experience a decline in rainfall (McDonald et al., 2009). These areas include The Sahel, the Horn of Africa, and southern Africa.
Significantly, due to the high potential evapotranspiration across much of Africa, groundwater reserves are commonly replenished during intense rainfall events, especially shallow aquifers (Taylor et al., 2013). As mentioned early, climate change is expected to increase rainfall intensity, causing more variable river discharge and flooding: interestingly this shift may actually enhance groundwater recharge. Taylor et al. (2013) found that increases in intensive monthly rainfall (under GCM projections for the 21st century in the region of East Africa) sustains groundwater reserves, significantly in areas where communities are completely dependent on the resource for freshwater. Consequently, groundwater has the ability to play a key role in climate change adaptation and resilience and in turn, can enhance food production and security via small-scale irrigation schemes (Carter and Parker, 2009; MacDonald et al. 2009).
But... Africa is experiencing huge rates of growth, with the region's population expected to more than double by 2100 (!) (UN, 2015). Population growth and urbanisation are expected to increase the domestic and agricultural demand for freshwater and even has the potential to degrade water quality (Taylor et al., 2009); water resources obtained from groundwater resources are vulnerable to contamination from inadequate hygiene in rapidly urbanising centres (Howard et al., 2003). These non-climatic drivers are expected to impact groundwater resources more considerably than climate change (Niang et al., 2014)
So unlike surface water resources, groundwater reserves may potentially be positively affected by climate change and instead may offer an adaptive strategy to the problem, especially shallow aquifers. However, there are noticeable concerns over the sustainability of groundwater reserves from the increasing demand for freshwater from population growth and high rates of urbanisation: ultimately, if groundwater reserves are going to be used as an adaptive strategy to the impacts of climate change, they need to be managed accordingly to ensure that the quantity and quality of groundwater resources exhibit sustainable levels.
Of course, this is easier said than done... I'm sure this is an issue I will be returning to later on!
Hi! What an interesting article! I just wrote a post about the Nubian Sandstone aquifer and so this is a really interesting supplement! It might also be good to check, in how far hot weather extrems (induced by climate change) also increase the pressure on the groundwater resources (alongside population growth, which you mentioned), which might offset the effects of increased replenishment. I don't have any sources for that - it's just a guess.
ReplyDeleteHi Louisa - thanks for the comment! When you say 'hot weather extremes' are you meaning drought or increased temperature anomalies? But yes completely! As you probably know, it's also very dependent on the type of aquifer as well. I plan to look more specifically at the ways groundwater can be an effective adaptive strategy - so I am sure this is something I will delve into deeper!
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