An important new collection of papers has just been published online in the Hydrogeology Journal:
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.
For full details read:
- Topical Collection: Determining groundwater sustainability from long-term piezometry in Sub-Saharan Africa
- Relationships between rainfall and groundwater recharge in seasonally humid Benin: a comparative analysis of long-term hydrographs in sedimentary and crystalline aquifers
- Assessment of the impacts of climate variability on total water storage across Africa: implications for groundwater resources management
- Characteristics of high-intensity groundwater abstractions from weathered crystalline bedrock aquifers in East Africa
- Characterization of recharge mechanisms in a Precambrian basement aquifer in semi-arid south-west Niger
Please note that all five papers are open until 30 April, after which only 3 of the papers will be Open Access.