Keywords: Episodic recharge, WTF-method, 1D-modelling, climate, Limpopo basin
Determining the long-term sustainability of groundwater use in arid and semi-arid regions with high climate variability requires an understanding of the recharge processes that replenish the aquifers as well as their major drivers significantly affecting their temporal and spatial variability. Four hydrographs that range from 42 to 46 years in duration, each within deeply weathered gneiss, were chosen on the basis of their proximity to rain gauges, interpretability throughout, and visible evidence of recharge events in the records. Recharge is quantified using: (1) the Water Table Fluctuation method, applying an inverse approach to estimate recharge from the event-based rises in groundwater level at daily and monthly time-steps; and (2) the HYDRUS 1D model: computing recharge using a dynamic soil-moisture balance model incorporating rainfall, evapotranspiration, soil properties and vegetation characteristics. The hydrographs show that groundwater levels after each rainy season, Oct-Apr, are highly variable in terms of recovery from the previous dry season levels though large episodic recharge events disproportionately contribute to groundwater replenishment. Hence, characterising these recharge events further and their relationship to climate and land use is pivotal to understanding and predicting long-term aquifer sustainability within the wider framework of climate resilience. Correlations from our records show a tendency for episodic recharge to occur during La Niña years, but not strongly enough to act as a predictive tool for significant recharge years. Finally, we provide evidence of focused recharge from surface water bodies, likely in cases further promoted by preferential flow pathways along geological structures like fractures and dikes typical of the region.