Spatiotemporal analysis of heavy summer rainfall events across the Free State Province, South Africa: 1981-2022

Heavy rainfall events negatively impact agricultural productivity (e.g. crop damage) and human health (e.g. waterborne disease outbreaks). Under anthropogenically induced warming, South Africa may experience more frequent and heavier rainfall events during the summer wet-season. Evidence from point-based rainfall records suggests that such changes may already be occurring across many summer rainfall zone regions; however, the understanding thereof is spatially unresolved. Therefore, focusing on the summer rainfall zone Free State Province, we used the CHIRPS rainfall dataset for 1981/82-2021/22 to explore trends and interannual variability in the magnitude and frequency of heavy rainfall events for summer months (October-April). Interannually, the El Nino-Southern Oscillation had a predictable influence, with positive (negative) anomalies for the frequencies and magnitudes of heavy rainfall events during La Nina (El Nino) years. Despite few statistically significant trends, the results broadly depict increases (decreases) by up to 0.4 days/decade (-0.2 days/decade) in the frequency of smaller (larger) heavy rainfall events. More consistent trend results were evident for heavy rainfall event magnitude indices, with declining (increasing) trends, up to -2 mm/decade (5 mm/decade), evident over central (northeastern and southwestern) regions. Our results highlight regions vulnerable to changes and interannual variations in heavy rainfall characteristics, which can inform adaptation strategies.