Abstract. The impact of climate variability on groundwater storage has received
limited attention despite widespread dependence on groundwater as a resource
for drinking water, agriculture and industry. Here, we assess the climate
anomalies that occurred over Southern Africa (SA) and East Africa, south of
the Equator (EASE), during the major El Niño event of 2015–2016, and their
associated impacts on groundwater storage, across scales, through analysis
of in situ groundwater piezometry and Gravity Recovery and Climate Experiment (GRACE)
satellite data. At the continental scale, the El Niño of 2015–2016 was associated with a
pronounced dipole of opposing rainfall anomalies over EASE and Southern
Africa, north–south of ∼12∘ S, a characteristic pattern
of the El Niño–Southern Oscillation (ENSO). Over Southern Africa the most intense drought event in the
historical record occurred, based on an analysis of the cross-scale areal
intensity of surface water balance anomalies (as represented by the
standardised precipitation evapotranspiration index – SPEI), with an
estimated return period of at least 200 years and a best estimate of
260 years. Climate risks are changing, and we estimate that anthropogenic warming
only (ignoring changes to other climate variables, e.g. precipitation) has
approximately doubled the risk of such an extreme SPEI drought event. These
surface water balance deficits suppressed groundwater recharge, leading to a
substantial groundwater storage decline indicated by both GRACE satellite
and piezometric data in the Limpopo basin. Conversely, over EASE during the
2015–2016 El Niño event, anomalously wet conditions were observed with an
estimated return period of ∼10 years, likely moderated by the
absence of a strongly positive Indian Ocean zonal mode phase. The strong but
not extreme rainy season increased groundwater storage, as shown by satellite
GRACE data and rising groundwater levels observed at a site in central
Tanzania. We note substantial uncertainties in separating groundwater from
total water storage in GRACE data and show that consistency between GRACE
and piezometric estimates of groundwater storage is apparent when spatial
averaging scales are comparable. These results have implications for
sustainable and climate-resilient groundwater resource management, including
the potential for adaptive strategies, such as managed aquifer recharge
during episodic recharge events.
The El Niño event of 2015–16: Climate anomalies and their impact on groundwater resources in East and Southern Africa