Estimation of Areal Reduction Factors in South Africa, Part 1: Development and evaluation of a software interface

Areal Reduction Factors (ARFs) are used to convert average design point rainfall depths to an areal (catchment) design rainfall depth to serve as primary input to estimate the design flood associated with a specific storm duration and return period. This paper presents the development and critical evaluation of a stand-alone software interface to estimate regional, geographically-centred ARFs using the methodology developed by Pietersen (2023). By considering standard input variables in a range of catchment areas, the application of the ARF software demonstrated that it is an easy-to-use software tool for the rapid estimation of ARFs in both gauged and ungauged catchments, and with the aid of the default and combined algorithm, geographically-centred ARFs which vary with return period can be estimated. Given that the other geographically-centred ARF methods reviewed in this paper do not take return periods into account and have not been validated using local and up-to-date rainfall data applicable throughout South Africa (SA), it is recommended that the ARF software be incorporated as the standard estimation procedure for long duration (> 24-hour), geographically-centred ARFs in SA. In addition, the proposed methodology and ARF software should also be further validated for short (< 24-hour) and long (> 24-hour) durations by applying it to actual catchments distributed throughout SA and subjected to different rainfall-producing mechanisms.