Determining the utility of a percentile-based wet-season start- and end-date metrics across South Africa

Across South Africa, the timing of wet-season rainfall within the summer-, winter- and year-round rainfall zones (SRZ, WRZ and YRZ) influences activities including crop production and water resource management. While numerous start- and end-date definitions exist to classify the wet season, most rely on threshold values specified for local-scale application. Focusing on large-scale application, we critically apply an objective and consistent methodology, defining wet-season start and end dates at 10% and 90% of cumulative annual rainfall, using daily rainfall data spanning 1987–2016 for 46 South African weather stations. SRZ, WRZ or YRZ conditions are successfully quantified across all locations. The longest mean SRZ wet season spanned 223 days from 8 September to 18 April at East London Wo (ELW), along the southeast coast. Robertson (ROB), within the south-western region, had the longest mean WRZ wet season of 244 days from 9 March to 7 November. Nearby, Riversdale (RIV) had the longest mean YRZ wet season of 270 days from 17 February to 13 November. Consistency with literature regarding rainfall drivers, start and end dates and the spatial distribution of rainfall seasonality, confirms utility of this metric across South Africa. Therefore, it facilitates comparable rainfall seasonality investigations across regions, including the arid western coast and the southern coast YRZ, where previous methods lack utility, and the calculation of trends in seasonality changes across the country. While for most locations trends are not statistically significant, this method enables further monitoring.