Substantial increases in groundwater withdrawals are expected
across Sub-Saharan Africa to help nations increase access to safe water and to
amplify agricultural production in pursuit of UN SDG 2 and SDG 6.
Long-term groundwater-level records or chronicles play an important role
in developing an improved understanding of the hydrogeological and climatic
conditions that control access and sustain well yields, informing where, when
and how groundwater withdrawals can sustainably contribute to building
resilience and alleviating poverty.
Evidence from chronicles in seasonally humid Benin and Uganda
show annual cycles of replenishment from direct, diffuse recharge generated
preferentially by heavy rainfalls. Kotchoni et al. show how chronicles
from different geological environments in Benin can be modelled very
effectively on a daily timestep with an improved watertable fluctuation model.
In semi-arid southwestern Niger, chronicles show that
recharge to weathered crystalline rock aquifer systems occurs directly from
rainfall but is restricted by a thick clayey aquitard developed from
schist. However, greater recharge is shown to occur indirectly via riverbeds of
ephemeral streams which provide preferential pathways through the saprolite.
Evidence from the Makutapora Wellfield of semi-arid central Tanzania
that groundwater, abstracted at rates exceeding 30,000 m3/day, is
sustained by episodic recharge associated with El Niño Southern Oscillation
(ENSO). Further, abstracted groundwater is partially modern, derived from
rainfall within the last 10–60 years.
Studies from Benin and Niger highlight the low
storage of weathered crystalline rock aquifers and the importance of modern
recharge in sustaining groundwater use. The low storage and low but highly
variable hydraulic conductivity of weathered and fractured crystalline rock
aquifers found over more than 40% of Sub-Saharan Africa may, however, have a
potential advantage. Such aquifer systems restrict opportunities for intensive
and competitive abstraction and are thus potentially self-regulating.
Low-intensity groundwater abstraction distributed across the landscape also
complements existing land-tenure systems in many areas of Sub-Saharan Africa
dominated by smallholder agriculturalists.
The chronicles provide invaluable datasets to help direct
assessments of past impacts of climate variability—e.g. ENSO, Atlantic
Multi-decadal Oscillation (AMO)— and abstraction on groundwater storage. Such
records, when continuously updated, can also provide key input to water
resources management by tracking emerging risks to water security from
groundwater storage decline or groundwater flooding (e.g. Murray et al. 2018).
Regional-scale (>50,000 km2) networks of long-term
piezometric records can also be used to test the reliability of largescale,
satellite observations from the Gravity Anomaly and Climate Experiment (GRACE).
Indeed, the emergence of GRACE measurements of changes in total terrestrial
water storage adds a potential tool, albeit at a much larger scale (>200,000
km2), to estimate changes in groundwater storage where records do
not exist. However, there are substantial uncertainties from such estimates.
New field experiments assessing groundwater storage in the Iullemmeden Basin have recently been completed under GroFutures by a team of early career researchers. From December 2018 to January 2019, Fabrice Lawson from the Université d’Abomey Calavi and IRD in Benin (UAC/IRD), and Jean-Baptist Gnonhoue (IRD, Benin) worked with colleagues, Rabilou Mahaman and Boukari Issoufou from the Université Abdou Moumouni of Niamey (UAMN) in Niger to conduct field experiments applying Magnetic Resonance Sounding (MRS) and Time Domain Electromagnetics (TDEM), to generate in-situ measurements of groundwater storage. Such measurements of groundwater storage are rare despite their fundamental importance to understanding quantitatively the availability and sustainability of groundwater resources.
In total, the team conducted experiments at 26 sites, including 13 in the Dallol-Maouri region in central Niger and 13 in the Goulbi-Maradi region in southeastern Niger, just north of the border with Nigeria. Fabrice and Jean-Baptist led this field research sharing their expertise with their counterparts in Niger. Fabrice and Jean-Baptist (an early career scientist and engineer, respectively) developed their expertise to apply these sophisticated instruments under the tutelage of Jean-Michel Vouillamoz of IRD based at the University of Grenoble, France. The team was also supported by the lead GroFuturesresearchers in the Iullemmeden Basin Observatory, Professor Yahaya Nazoumou at UAMN and Dr. Guillaume Favreau of IRD, Niamey.
Results from field experiments and the subsequent processing and analyses of data will inform not only evaluations of available groundwater resources in these regions of the Iullemmeden Basin but also stress-testing of groundwater development pathways, developed by stakeholders, through numerical modelling. Well done to Fabrice, Jean-Baptist, Rabilou and Boukari!
Combined Benin-Niger GroFutures field team with supporters working in Goulbi-Maradi (left); Early Career scientists under GroFutures, Fabrice Lawson (UAC/IRD, Benin), Jean-Baptist Gnonhoue (IRD, Benin), Rabilou Mahaman (UAMN, Niger), and Boukari Issoufou (UAMN, Niger) running MRS experiments in Goulbi-Maradi (right).
Crystalline Bedrock aquifers underlie about 40% of Sub-Saharan Africa and can generally sustain low-intensity abstraction. However, pumping rates and dependency is increasing in many areas, particularly for cities like Addis Ababa, Dakar, Nairobi and Dodoma. Projected growth in population and water demand for agriculture, plus the effects of climate change, mean that it is essential to develop a better understanding of the sustainable yields from these types of aquifers.
The study focuses on five groundwater abstraction boreholes, 3 in Uganda, 2 in Tanzania.
Long term groundwater records are only available for one of the boreholes and it shows that recharge happens more when the rainfall is more intense, which is often associated with periodic El Niño Southern Oscillation (ENSO) events.
Chemical analysis of the water was used to determine the residence times of the groundwater (how long the water has been in the aquifer since it fell as rain). Overall, that most pumped water comes from modern recharge (within the last 10-60 years), so while abstractions are not mining pre-modern groundwater, there may be a component of older water that is coming out.
Groundwater abstraction appears to be supported by recharge from across multiple years, rather than just the most recent wet season.
The investigation of the five sites shows that long term, high intensity groundwater abstraction is possible from East African weathered crystalline basement aquifers, but the sustainability is constrained, in part, by the high inter-annual variability in recharge. Therefore operation of such pumping stations needs to include sustained monitoring of groundwater levels, pumping rates and rainfall as a minimum.
Key Note Presentation by Prof. Alan MacDonald @ 1st SADC Groundwater Conference
Keywords: (Drought, Climate, Change, Infrastructure, Groundwater Resources, Resilience).
“Recent droughts have highlighted the need to understand and forecast the resilience of water supplies to climate variability. Resilience of groundwater supplies is determined by several factors: groundwater storage; long term recharge; permeability; and the infrastructure put in place to abstract groundwater.
“Drawing on recent research from across Africa, mainly funded through the UPGro programme, this talk examines the relative importance of each of these factors for rural drinking water supplies, and attempts to distinguish between the behaviour of the groundwater resource and the water infrastructure.
“A variety of data are presented and evaluated: detailed groundwater level monitoring of springs, wells and boreholes; national survey data of borehole functionality; groundwater residence time indicators; and also information from GRACE and global Land Surface Model.”
UPGro has a strong presence at the event as part of the build-up to Africa Water Week next month, in Gabon:
Opening Keynote: Karen G. Villholth (GroFutures) and Jude Cobbing.
“Adapting to Climate Change in the SADC Region – A Focus on Groundwater.”
Presentation: Theresa Mkandawire (Hidden Crisis) presented “An analysis of hand pump boreholes functionality in Malawi.”
Keynote speaker: Alan McDonald. (Hidden Crisis/GroFutures) “Resilience of rural
groundwater supplies to climate change”
Presentation: Thokozani Mtewa,Evance Mwathunga and Wapulumuka Mulwafu.
(Hidden Crisis) “They gave us breakfast and a good meal’: Roles, perceptions and
motivations of water point area mechanics in the maintenance of borehole handpumps in Balaka district, Malawi.”
Keynote speaker: Dr Callist Tindimugaya “Groundwater and African National Development Strategies”
You can follow the conference on Twitter: #gwconference2018
photo : Prof Theresa Makandawire presenting UPGro Hidden Crisis work in Malawi (credit BGS via Twitter)
Groundwater Recharge – the set of processes that govern how rainwater seeps through soils and rocks to replenish aquifers – is not well understood across much of Africa. It is important to understand because it is central to determine the sustainable use of groundwater resources;
The authors analyse three rare sets of long-term (19-25 years) groundwater-level observations from three different, but common, geological settings in Benin;
The year-to-year changes in groundwater storage correlate well with rainfall patterns, but there were big differences the relate to the type of geology:
In the shallow, sand aquifer as much as 40% of the rainfall becomes groundwater
In the deeper sandstone and weathered crystalline rocks, a much lower proportion of rainfall becomes groundwater recharge (13% and 4% respectively)
Recharge was found to occur on a seasonal basis; however on a daily basis the groundwater fluctuations are best explained with a threshold of 5-15 mm per day – meaning that only more intense rainfall events lead to recharge.
These results are consistent with the growing body of evidence that, in Sub-Saharan Africa, intensification of rainfall associated with climate change may increase groundwater recharge.
Because the groundwater recharge is so strongly influenced by geology, it is essential for water resource planning that good geological maps are available and used, and that investment is made into long-term groundwater monitoring of strategic aquifers.
GRACE (Gravity Recovery and Climate Experiment) satellite can be used to estimate changes in water storage on time resolution of 1 month and a spatial resolution of about 450 x 450 km.
GRACE can be used to estimate groundwater storage changes where it is the dominant water mass. It is therefore useful in many areas of Sub-Saharan Africa where there are relatively few direct groundwater level measurements.
The paper focuses on the major sedimentary aquifers basins, where the majority of Africa’s groundwater resources are to be found. Away from these basins, groundwater storage is 1-2 orders of magnitude less.
There is no evidence of continuous long-term declining trends of Total Water Storage (mostly groundwater) in any of the major sedimentary aquifers, which indicates that none are stressed by current abstraction rates – howeverit is important to stress that local scale depletion may be occurring but is beyond the resolution of GRACE to detect.
There are also some interesting findings in regard to the combination of GRACE and Land Surface Modelling and how well (or not) they represent groundwater recharge processes in the different basins.
Read the full paper here:
Bonsor, H.C.; Shamsudduha, M.; Marchant, B.P.; MacDonald, A.M.; Taylor, R.G. Seasonal and Decadal Groundwater Changes in African Sedimentary Aquifers Estimated Using GRACE Products and LSMs. Remote Sens.2018, 10, 904. http://www.mdpi.com/2072-4292/10/6/904
The GroFutures Team, working with the Tanzanian Ministry of Water and Irrigation, expanded monitoring infrastructure in the Upper Great Ruaha Observatory (UGRO) to include interactions between groundwater and surface water.
An outstanding question regarding the sustainability of groundwater withdrawals for irrigation and drinking-water supplies is whether groundwater in the agriculturally intensive lowlands is replenished by river flow, sustains river flow, or both depending upon the season.