The El Niño event of 2015–2016: climate anomalies and their impact on groundwater resources in East and Southern Africa

Abstract

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.

Climate–groundwater dynamics inferred from GRACE and the role of hydraulic memory

“…Nearly all recharge events in dryland aquifer systems take place at a time of negative PCPA (likely SMD) with most coinciding with extreme rainfall as recently observed in a pan-African study by Cuthbert et al (2019a). Extreme recharge events also generally coincide with El Niño and La Niña events indicating an association with large-scale modes of climate variability identified previously in tropical Africa (Kolusu et al, 2019;Taylor et al, 2013a, b). In contrast, extreme recharge in humid aquifer systems is not consistently associated with either negative PCPA (likely SMD) or anomalous rainfall, though the latter is correlated with ENSO state.…”

Climate-groundwater dynamics inferred from GRACE and the role of hydraulic memory

“…Nearly all recharge events in dryland aquifer systems take place at a time of negative ∫PCPA (likely SMD), with most coinciding with extreme rainfall as recently observed in a pan-African study by Cuthbert et al, (2019b). Extreme recharge 465 events also generally coincide with El Niño/La Niña events indicating an association with large-scale modes of climate variability identified previously in tropical Africa (Kolusu et al, 2019;Taylor et al, 2013b dryland/slow GRT/long HM or humid/rapid GRT/short HM systems. These anomalous characteristics 495 may reflect groundwater storage decline through the escalation of groundwater abstraction referenced previously (Wada et al, 2014) and this hypothesis was tested through further analysis as follows below and in further detail in the SI.…”

Groundwater and resilience to drought in the Ethiopian highlands

“…Groundwater offers storage of a different magnitude than annual rainfall or river flow (MacDonald et al 2012) and across the African continent appears resilient to droughts of several years. No significant longterm regional decline in groundwater storage has been observed over the past 20 years (Bonsor et al 2018) despite shorter term variability (Kolusu et al 2019) and longer term decline (>1000 years) associated with increasing aridity from longer climate process across northern African aquifer basins (e.g. Gossel et al 2004, Edmunds 2009).…”