Groundwater responses to decadal rainfall variability in semi-arid South Africa

• Groundwater responses to climate variability assessed in semi-arid South Africa. • Strong decadal variability in long-term rainfall and groundwater anomalies. • Highest SGI-SPI correlation occurs at 7- and 8-year accumulation periods. • Groundwater anomalies strongly correlate with decadal rainfall variability. • Rainfall patterns intensified, but long-term annual rainfall totals showed no trend. This study examines groundwater responses to climate trends and variability in semi-arid South Africa (SA). Groundwater is a vital water resource in the semi-arid regions of Africa, which are characterized by low, erratic rainfall and intermittent streamflow. In these regions, water scarcity is intensifying due to rising irrigation demands, urbanization, and climate change. Semi-arid SA experiences significant rainfall variability at inter-annual and decadal timescales. However, groundwater responses to SA’s climate remains underexplored, partly due to limited observational data. This study analyses long-term trends and variability in rainfall, temperature, and groundwater levels in the Hout/Sand River catchment from 1940 to 2022 using climate and groundwater indices. Groundwater response to rainfall variability is assessed through correlations between the Standardized Precipitation Index (SPI) and the Standardized Groundwater Index (SGI) across annual to decadal timescales. Results indicate no significant trends in total annual rainfall, but rainfall patterns intensified manifested as higher daily rainfall intensity, longer dry periods, and shorter wet periods. The highest SGI-SPI correlation occurs at 7- and 8-year accumulation periods (r = 0.80), which indicates a dependence of groundwater levels on antecedent rainfall consistent with the region’s decadal climate variability of dry and wet epochs. The study adds to our conceptual understanding of groundwater responses to large-scale climatic patterns, which is essential for assessing future water availability under climate change.