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

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Abstract

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.

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Acknowledgements

The authors acknowledge SAWS for providing meteorological data and Marike Kluyts for editorial assistance during the preparation of this paper. We also appreciate the insightful reviewer comments and recommendations which have aided in improving this paper.

Funding

This work was supported by the National Research Foundation (NRF), South Africa under Grant number 107742 and the University of the Witwatersrand Faculty of Science Research Committee grant both awarded to SJR. JMF is funded by the DST-NRF Centre of Excellence for Palaeoscience.

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Authors and Affiliations

  1. School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg, South Africa

    Sarah J. Roffe, Jennifer M. Fitchett & Christopher J. Curtis

  2. Department of Geography, Environmental Management and Energy Studies, University of Johannesburg, Johannesburg, South Africa

    Christopher J. Curtis

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  1. Sarah J. Roffe
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  2. Jennifer M. Fitchett
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Correspondence to Jennifer M. Fitchett.

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Roffe, S.J., Fitchett, J.M. & Curtis, C.J. Determining the utility of a percentile-based wet-season start- and end-date metrics across South Africa. Theor Appl Climatol 140, 1331–1347 (2020). https://doi.org/10.1007/s00704-020-03162-y

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