Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa

Groundwater in sub-Saharan Africa supports livelihoods and poverty alleviation1,2, maintains vital ecosystems, and strongly influences terrestrial water and energy budgets. Yet the hydrological processes that govern groundwater recharge and sustainability—and their sensitivity to climatic variability—are poorly constrained4. Given the absence of firm observational constraints, it remains to be seen whether model-based projections of decreased water resources in dry parts of the region4 are justified. Here we show, through analysis of multidecadal groundwater hydrographs across sub-Saharan Africa, that levels of aridity dictate the predominant recharge processes, whereas local hydrogeology influences the type and sensitivity of precipitation–recharge relationships. Recharge in some humid locations varies by as little as five per cent (by coefficient of variation) across a wide range of annual precipitation values. Other regions, by contrast, show roughly linear precipitation–recharge relationships, with precipitation thresholds (of roughly ten millimetres or less per day) governing the initiation of recharge. These thresholds tend to rise as aridity increases, and recharge in drylands is more episodic and increasingly dominated by focused recharge through losses from ephemeral overland flows. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by large-scale climate controls. Intense precipitation, even during years of lower overall precipitation, produces some of the largest years of recharge in some dry subtropical locations. Our results therefore challenge the ‘high certainty’ consensus regarding decreasing water resources in such regions of sub-Saharan Africa. The potential resilience of groundwater to climate variability in many areas that is revealed by these precipitation–recharge relationships is essential for informing reliable predictions of climate-change impacts and adaptation strategies

Use of atmospheric tides to estimate the hydraulic conductivity of confined and semi-confined aquifers

International audienceCharacterizing groundwater responses to natural drivers is cost effective and offers great potential in hydrogeological investigations. However, there is a lack of method development and evaluation, for example by comparing results with those derived from using conventional methods. This paper presents a modified method to calculate the hydraulic conductivity (K) of confined aquifers using the well water response to atmospheric tides. The approach separates the Earth and atmospheric tide influences on filtered well water-level records in the time domain. The resulting ill-posed regression deconvolution problem can be overcome by constraining the well water response to atmospheric tides in order to follow a physically realistic semi-diurnal barometric response function (S2-BRF), or to follow directly a modified hydraulic model (BE-Hvorslev) similar to a slug test evaluation. An analysis with synthetic data shows that K up to 10-4 m/s can be estimated when pressure records with short sampling intervals are available. Application to a field dataset from Cambodia and Benin, with 20-minute to 60-minute sampling intervals, respectively, results in K values of 5.82∙10-7 m/s and 2.9.10-7 m/s. This agrees with results independently derived from pumping tests for both confined sediments and semi-confined hard-rock conditions. This method offers a promising and low-cost approach to derive K solely from monitoring datasets in confined aquifers. This is especially advantageous for low-conductivity formations where hydraulic testing takes time