16 Pietersen JPJ, Gericke OJ, Smithers JC, Woyessa YE. Review of current methods for estimating areal reduction factors applied to South African design point rainfall and preliminary identifcation of new methods. J. S. Afr. Inst. Civ. Eng. 2015;57(1), Art. #1017, 15 pages. http://dx.doi.org/10.17159/2309-8775/2015/v57n1a2 TECHNICAL PAPER JOURNAL OF THE SOUTH AFRICAN INSTITUTION OF CIVIL ENGINEERING Vol 57 No 1, March 2015, Pages 16–30, Paper 1017 JACO PIETERSEN, who is a Civil Engineering Technologist and a member of SAICE, has lectured the practical components in Water Engineering and Geotechnical Engineering at the Central University of Technology, Free State (CUT FS), for the past three years. He has a special interest in food hydrology, especially the infuence of food peaks on hydraulic structures. After obtaining the BTech Eng (Civil) degree from the CUT FS, he worked for fve years at a civil engineering consulting frm in Bloemfontein, before joining the CUT FS. The results included in this paper will partially contribute towards his MTech Eng (Civil) degree from the CUT FS. Contact details: Faculty of Engineering and Information Technology, Department of Civil Engineering, Central University of Technology Free State (CUT), Private Bag X20539, Bloemfontein, 9300, South Africa T: +27 (0)51 507 3692, F: +27 (0)51 507 3254, E: jpietersen@cut.ac.za JACO GERICKE Pr Tech Eng has been lecturing in Hydrology, Irrigation and Water Engineering at the CUT FS for the past nine years. He has a special interest in food hydrology, water resources management and hydrological modelling. He was awarded the BTech Eng (Civil) and MTech Eng (Water) degrees from the CUT FS, the BSc (Hons) Applied Science: Water Resources Engineering degree from the University of Pretoria (UP) and the MSc Eng degree from the University of Stellenbosch. He worked for six years at the Department of Water Afairs before joining the CUT FS. He is currently registered for a PhD Eng at the University of KwaZulu-Natal. Contact details: Faculty of Engineering and Information Technology, Department of Civil Engineering, Central University of Technology Free State (CUT), Private Bag X20539, Bloemfontein, 9300, South Africa T: +27 (0)51 507 3516, F: +27 (0)51 507 3254, E: jgericke@cut.ac.za PROF JEFF SMITHERS Pr Eng graduated with a BSc Eng (Agric), MSc Eng and PhD degrees from the University of KwaZulu-Natal (UKZN). He has extensive academic and professional experience and has numerous publications in the design hydrology feld to his credit. He is currently Professor of Agricultural Engineering at the UKZN and has a part-time appointment at Jefares & Green Engineering and Environmental Consulting. He also serves as Adjunct Professor at the University of Southern Queensland, Australia, in the National Centre for Engineering in Agriculture. He is a Fellow of the South African Institute of Agricultural Engineers. Contact details: School of Engineering, University of KwaZulu-Natal, Pietermaritzburg Campus Pietermaritzburg, 3201, South Africa T: +27 (0)33 260 5490, F: +27 (0)33 260 5818, E: smithers@ukzn.ac.za DR YALI WOYESSA Pr Tech Eng has an MSc in Irrigation Engineering and a PhD in Soil Physics. He has been involved in teaching and research for more than 20 years at higher education institutions. His research interests include catchment management, water resources management and hydrological modelling. He currently leads a research group on Sustainable Water Resources and Environment, and is Head of the Department of Civil Engineering at the CUT FS. Contact details: Faculty of Engineering and Information Technology, Department of Civil Engineering, Central University of Technology Free State (CUT), Private Bag X20539, Bloemfontein, 9300, South Africa T: +27 (0)51 507 3452, F: +27 (0)51 507 3254, E: ywoyessa@cut.ac.za Keywords: areal reduction factor, areal design rainfall, critical storm duration, design point rainfall, return period INTRODUCTION In flood hydrology, the practising engineer or hydrologist frequently needs to estimate catchment design rainfall, i.e. rainfall information derived from observed rainfall data which comprises a depth or intensity, and duration associated with a given return period (T) or annual exceedance probability (AEP) (Gericke & Du Plessis 2011). However, design point rainfall estimates assume uniform spatial rainfall in 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 into an average areal design rainfall depth/intensity for a particular critical storm duration and catch- ment area (Alexander 2001). ARFs are estimated using either empirical and/or analytical methods. In many coun- tries, the current ARF approaches are mostly based on empirical methods, using either storm-centred or geographically-centred approaches. Studies into the large-scale estimation of ARFs have generally been limited to the United States of America (USA) (USWB 1957; 1958), the United Kingdom (UK) (NERC 1975) and Australia (Siriwardena & Weinmann 1996). Despite these studies, ARFs are still regarded as inconsistent in most cases, mainly as a consequence of the variation in predominant weather types, storm durations, seasonal factors and recurrence interval (Skaugen 1997; Asquith & Famiglietti 2000; Allen & DeGaetano 2005). Omolayo (1993) identified insufficient rain-gauge networks and a lack of short duration (sub-daily) rainfall data as the main reasons behind the limited research in this field and the inconsistent results. According to Asquith and Famiglietti (2000), the storm-centred approaches have not seen widespread application, due to the difficult inclusion of multi-centred storms. Omolayo (1993) indicated that storm-centred approaches are not suitable for estimating areal design rainfall from design point rainfalls, since extreme design point rainfall Review of current methods for estimating areal reduction factors applied to South African design point rainfall and preliminary identifcation of new methods J P J Pietersen, O J Gericke, J C Smithers, Y E Woyessa 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 event- based rainfall-runoff deterministic methods are used.