AREAL REDUCTION FACTORS FOR DESIGN RAINFALL ESTIMATION IN THE C5 SECONDARY DRAINAGE REGION OF SOUTH AFRICA

Abstract

Design point rainfall estimates assume a uniform distribution of rainfall over a catchment and hence are only representative of a limited area. For larger areas, Areal Reduction Factors (ARFs) are used to convert design point rainfall depths or intensities to an average areal design rainfall depth or intensity for a catchmentspecific critical storm duration and catchment area. The overall purpose of this study is to develop an enhanced methodology to express the spatial and temporal rainfall variability at a Quaternary Catchment (QC) level by means of geographically-centred and probabilistically correct ARFs. The ARF values presented in this study are based on observed daily rainfall data as extracted from 223 rainfall stations situated in the C5 secondary drainage region. The methodology adopted is based on a modified version of Bell’s (1976) geographically-centred approach. Individual sets of ARF values were derived for each of the 23 QCs present in the C5 secondary drainage region by considering various storm durations (1, 8, 16, 24, 72 and 168 hours) and corresponding recurrence intervals (2, 5, 10, 20, 50, 100 and 200 years). The climatological variability in the two tertiary drainage regions (C51 and C52) of the C5 was also recognised by conducting separate regression analyses in each region. The statistical differences in the regional ARF values highlight the presence of dominant weather types in each region. The statistical differences also confirm that ARFs are influenced by different rainfall-producing mechanisms while not being constant for various storm durations and exceedance probabilities or recurrence intervals such as geographically-centred probabilistically correct ARFs. It is recommended that the findings from this study be expanded to other regions in South Africa, ultimately to devise both improved design rainfall and flood estimates.