Abstract:
Design point rainfall estimates assume a uniform distribution of rainfall over a catchment,
and hence are only representative for a limited area. For larger areas, areal reduction factors
(ARFs) are used to convert design point rainfall depths/intensities to an average areal design
rainfall depth/intensity for a catchment-specific critical storm duration and catchment area.
This paper presents a review of ARF estimation methods used nationally and internationally,
with comparisons of the South African methods in the C5 secondary drainage region using
standard input variables. The comparison of different ARF estimation methods confirmed that
the empirical methods adopted for general use in South Africa are based on a limited database
of observed rainfall data and are used without local correction factors beyond their original
developmental regions. This results in the characterisation of the actual rainfall process over a
catchment, and translation into questionable design peak discharge estimates. Therefore, the
ARFs in South Africa need to be re-investigated in the light of recent extreme flood events,
utilising the longer periods of record and denser rain-gauge networks which are now available
for analysis. The variation of ARFs with return period and with rainfall producing mechanisms
also needs to be investigated. Updated ARFs developed and verified using local rainfall data
will improve the accuracy of design hydrology for large catchments in South Africa when eventbased
rainfall-runoff deterministic methods are used.
