Groundwater holds potential for supplying Africa’s agricultural
water needs. It is more climate resilient than surface waters – this should not
be underestimated – and is more plentiful (estimated at more than 100 times
that of annual renewable freshwater resources) (MacDonald et al. 2012). However, it has a number of drawbacks, namely its
uneven distribution, the uncertainty of its quality and difficulty to manage.
It will not be suitable, based on current understanding and technology, to look
to it as a widespread supply of water for the majority of Africa’s agricultural
demands. However, where it can be used appropriately, it should be. I hope that
the future may start to deal with the drawbacks I have mentioned, such as
innovations in cheaper and more efficient transportation of water, and so I am
not writing off groundwater just yet.
Groundwater: Africa's farming future?
Friday 1 January 2016
Tuesday 1 December 2015
Over-abstraction
Despite there being large
amounts of groundwater (approximately 0.66 million km3 across Africa (MacDonald et al. 2012)), we still need to be cautious about how much we can
abstract sustainably, because over-abstraction of groundwater supplies can have
detrimental social, economic and environmental impacts (Pavelic et al. 2013). Not only can over
abstraction lead to depletion of groundwater reserves, it can also have other
consequences such as land subsidence, saline intrusion and increasing costs of
pumping, to name a few (Ogunba 2012, European Environment Agency 2016). Consequently,
management is very important.
Agricultural irrigation is a
major cause of groundwater over-abstraction (European
Environment Agency 2016) and managing this is very difficult with the current
trends in African farming leaning towards small-holder farming. Due to increasing access to low cost pumps and
climatic variability, “groundwater irrigation for small holder farmers in SSA
is growing in extent and importance” (Villholth 2013: 369). A higher number of
individual users makes it more difficult to impose and enforce groundwater
restrictions and/or monitoring to ensure sustainable abstraction (Knuppe 2011).
Small-scale, rural farmers are also more likely to have conflict with
management regulation than larger irrigation companies and projects, often
holding traditional attitudes and expertise (Knuppe 2011). Couple this with a
rather patchy and inadequate understanding of aquifer levels and behaviour, and
it makes for rather difficult management.
Sunday 15 November 2015
Groundwater for Poverty
"Eighty-five
percent of Africa’s poor live in rural areas and mostly depend on agriculture for their livelihoods" (You et al. 2010: v).
With such a high proportion of Africa's population depending upon agriculture for their income, it seems sensible that they should be provided with the opportunity to exploit and excel in this market. With rain-fed agriculture and irrigation proving climate-dependent (see previous blog post: 'Groundwater's climate resilience'), Groundwater holds the potential to not only increase the amount of land under irrigation, but to provide a more steady and stable income for Africa's poor.
Currently, only 6% of cultivated land in Africa is irrigated, meaning the majority depend on rain-fed agriculture (PAVE Irrigation Systems 2015). Groundwater could potentially increase the total land under irrigation in Africa by 120 times, improving livelihoods for approximately 40% of the continent's rural population (Windham-Wright 2015). And with groundwater supplies proving more stable than precipitation, farmers will find themselves less susceptible to the climatic fluctuations that plague Africa.
A Sub-Saharan African resident keeping the rewards of a groundwater-fed irrigation system
Source: PAVE Irrigation Systems 2015
Source: PAVE Irrigation Systems 2015
In a region so tortured by poverty, providing a steady income for many is an opportunity not to be missed. Groundwater holds the ability to do this by providing a stable and widely available resource for agricultural production.
As stressed earlier in this blog, effective monitoring and regulation must be implemented in order to ensure supplies are not over abstracted.
Thursday 12 November 2015
Groundwater quality
Much of the literature on groundwater in Africa is focused
on quantity rather than quality. However, we must remember that sufficient
water quality is needed for it to be used effectively in agricultural
irrigation, and therefore it is a worthy topic to be discussed. There is limited
research on this subject, so much of the impacts of climate change on
groundwater quality remain uncertain and should therefore be treated
cautiously. It is not the intention of this blog post to provide a
comprehensive audit of the effects of climate change on groundwater quality,
merely to highlight that it is something that must be considered when assessing
groundwater’s potential by discussing some of its impacts.
Water quality, affected by the chemical, physical and
biological changes of water, is value specific, in that its value relates to
the specific activity it is needed for (Green et al. 2011, FAO 2016). For example, for agricultural irrigation,
the FAO outlines a number of issues regarding water quality that can result in
decreased yields or lack of effectiveness (see my summary in Table 2).
Table 2: Water quality issues, information summarised from
FAO (2016)
Issue
|
Anticipated
Result
|
Salinity
|
If salts are present in soil or water, this can reduce the
water availability to the crop, potentially affecting yields.
|
Water Infiltration Rate
|
High sodium or low calcium levels in soil or water can
reduce rate of entry of irrigation water. This can result in insufficient
water quantities being infiltrated to supply crops between irrigations.
|
Specific Ion Toxicity
|
Ions (such as sodium, chloride or boron) can damage crops
and reduce yields if they appear in sufficient concentrations.
|
Miscellaneous
|
Too high levels of nutrients
can decrease yields and/or quality.
Unattractive deposits on crops
can reduce market potential.
“Excessive corrosion of
equipment increases maintenance and repairs.”
|
So we can see that changes in the make-up and quality of
groundwater could lead it to be unsuitable for irrigation. So how is it thought
that climate change will affect it? Green at al. (2011) provide a fairly
comprehensive list of the possible impacts, as summarized below:
1.
Recharge during prolonged dry and wet periods may
have greater and lower concentrations of salts and total dissolved solids
respectively.
2. High intensity precipitation events may cause high levels
of infiltration, potentially resulting in the mobilization of “large pore-water
chloride and nitrate reservoirs” in the unsaturated zone of aquifers in
semi-arid/arid regions. If these reach the water table, groundwater quality may
decline (544).
3.
Sea level rise, spatial and temporal variability in
precipitation and evapotranspiration and increased abstraction of groundwater
may lead to increased groundwater salinization in coastal regions.
4.
Increases in recharge rates could lead to growth of contaminant
transport, and thus groundwater exposure to contamination.
5. Temperature rises may alter subsurface biogeochemical
reactions, potentially altering groundwater properties and quality.
6. Increases in flood events may cause urban contaminant
levels to rise in groundwater such as oil, solvents and sewage.
7.
Sea level rise, and
subsequent seawater intrusion, may decrease the depth of the freshwater lens (a
layer of fresh groundwater that sits on top of the denser saltwater) in coastal
aquifers such as the Niger delta (Taylor et
al. 2009).
It is clear that adequate water quality is important to
agricultural irrigation and that the quality of groundwater is likely to change
in the face of anthropological climate change. However, studies are extremely
limited, and the magnitude of such predicted changes remains unclear. In
addition to this, the relationship between water quality and most climatic
variables in one that is non-linear, making it even more difficult to predict
(IPCC 2014a). My purpose in writing this blog post is to highlight that
groundwater quality should not be assumed as a constant or as suitable for
irrigation. Further research is needed, and is greatly encouraged on my part,
into how groundwater quality will be affected by climate change so that we are
able to ascertain whether its quality allows it to remain a suitable option for
agricultural irrigation.
Thursday 29 October 2015
Thursday 22 October 2015
Groundwater's Biggest Problem: Uneven Distribution Part 1
As shown in Figure 3, SSA has approximately three times the
per capita groundwater availability of China and nearly six times that of India,
the world's two biggest players in groundwater-fed irrigation systems (Giordano
2006). However, only 6% of cultivated
land in Africa is irrigated (PAVE Irrigation Systems 2015). One of the reasons
it has not been exploited to data is because Africa’s groundwater distribution
is highly variable, with the majority of groundwater storage in North Africa. An intermix of factors have caused this spatial
variability including geology and climate.
Geology
Supplies are dependent upon the ability of underlying rock to store and transfer water. A
large proportion groundwater is located in hard rock areas or at extremely deep
distances below the ground, making abstraction difficult or costly. Giordano
(2006) outlines four geological zones, of which Africa is made up of (see
Figure 5 for their distribution across the region), that alter the ability of
the ground to store and transfer water. A summary of these zones can be found
in Table 1.
Table 1: Four geological zones across SSA, information summarized
from Giordano (2006)
Hydrogeologic Zone
|
% of Region
|
Potential as a Groundwater Reserve
|
Crystalline Basement Rock
|
40
|
Poor supply source for groundwater due to its low
transmissivity.
|
Consolidated Sedimentary Rock
|
32
|
Can hold large amounts of water so high potential.
|
Unconsolidated Sediments
|
22
|
Groundwater is often held in hold in unconstrained
conditions within sands and gravels, and often found in river beds, so easily
accessible. However, “many African river systems are typified by fine and
very fine sediments, rather than coarse sand and gravel, reducing extraction
possibilities” (312).
|
Volcanic Rock
|
6
|
Can produce high groundwater yields and supply springs.
|
However, hard rock aquifers should not be totally written
off. Even those with low yield potential may be suitable for small-scale
irrigation. For a community water supply fitted with a hand pump, only a
sustained supply of >0.1 l s-1 is needed, in comparison to >51 l s-1 for
boreholes for commercial irrigation schemes (MacDonald et al. 2012). Supplies or recharge rates may mean that these larger
yields may be unreachable, but could hold potential for smaller scale, less
intensive activities.
Climate
Groundwater also depends on recharge potential, in order to
make it a sustainable resource. Unfortunately, due to past and current climatic
conditions, groundwater distribution is highly correlated with rainfall
patterns, meaning it tends to be highest in areas of high rainfall and lowest in
areas of low rainfall (Calow and MacDonald 2009). This depletes the
human value of groundwater.
Depth to
Groundwater
The depth of the groundwater below the surface is again,
unsurprisingly, highly variable. Depth affects both the practical accessibility
of the resource, and whether it is economically viable (MacDonald et al. 2012). Levels deeper than 50m are
not easily accessible by a hand pump and for those >100m, the cost of
extraction increases dramatically because more advanced drilling equipment is
required (MacDonald et al. 2012).
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