Mapping groundwater recharge in Africa from ground observations and implications for water security

Abstract

Groundwater forms the basis of water supplies across much of Africa and its development is rising as demand for secure water increases. Recharge rates are a key component for assessing groundwater development potential, but have not been mapped across Africa, other than from global models. Here we quantify long-term average (LTA) distributed groundwater recharge rates across Africa for the period 1970–2019 from 134 ground-based estimates and upscaled statistically. Natural diffuse and local focussed recharge, where this mechanism is widespread, are included but discrete leakage from large rivers, lakes or from irrigation are excluded. We find that measurable LTA recharge is found in most environments with average decadal recharge depths in arid and semi-arid areas of 60 mm (30–140 mm) and 200 mm (90–430 mm) respectively. A linear mixed model shows that at the scale of the African continent only LTA rainfall is related to LTA recharge—the inclusion of other climate and terrestrial factors do not improve the model. Kriging methods indicate spatial dependency to 900 km suggesting that factors other than LTA rainfall are important at local scales. We estimate that average decadal recharge in Africa is 15 000 km3 (4900–45 000 km3), approximately 2% of estimated groundwater storage across the continent, but is characterised by stark variability between high-storage/low-recharge sedimentary aquifers in North Africa, and low-storage/high-recharge weathered crystalline-rock aquifers across much of tropical Africa. African water security is greatly enhanced by this distribution, as many countries with low recharge possess substantial groundwater storage, whereas countries with low storage experience high, regular recharge. The dataset provides a first, ground-based approximation of the renewability of groundwater storage in Africa and can be used to refine and validate global and continental hydrological models while also providing a baseline against future change.

Recharge observations indicate strengthened groundwater connection to surface fluxes

“…However, the pattern is sufficiently monotonic to yield a highly significant correlation between climate aridity and the fraction of precipitation that recharges groundwaters (Spearman ρ = -0.674; p<0.001). This relationship is consistent with past work, which indicated that both precipitation and potential evapotranspiration can strongly affect Non-peer reviewed preprint submitted to EarthArXiv groundwater recharge (Gee & Hillel, 1988;Scanlon et al, 2006;Moeck et al, 2020;MacDonald et al, 2021).…”

An overview of multimethod imaging approaches in environmental geophysics

“…Many communities worldwide rely on groundwater for their water supply. Yet, climate change, pollution, and changing water demands are increasing the stress on these aquifers, and hence are changing groundwater security (MacDonald et al, 2021). Over-exploitation, e.g., in the Central Valley of California, ultimately leads to a decrease not only in communal water supply but also in crop production (Scanlon et al, 2012).…”

The influence of groundwater abstraction on interpreting climate controls and extreme recharge events from well hydrographs in semi-arid South Africa

“…Rainfall and aridity are a key control on groundwater hydrographs and recharge across the continent (Cuthbert et al 2019;MacDonald et al 2021). There is some evidence for a transition from either C1 or C2 to C3 or C4 down both catchments.…”

“…The rarity of such hydrographs has meant assessments of sub-Saharan African water security typically rely on datasets derived from large-scale models (e.g. Döll and Fiedler 2008), with limited validation using field observations (Sood and Smakhtin 2015), or large-scale data reviews (MacDonald et al 2021).…”