Forging ahead: incorporating seasonality into extreme rainfall estimation for Western Australia

Abstract

Seasonal extreme rainfall estimates were derived for Western Australia based on the CRC-FORGE approach developed by the Cooperative Research Centre for Catchment Hydrology. The method derives estimates of large to rare design rainfalls up to an annual exceedance probability (AEP) of 1 in 2000. Current practice for estimating these rainfalls is through interpolation of a design rainfall frequency curve between rainfalls of 1 in 100 AEP and the Probable Maximum Precipitation. The application of CRC-FORGE is the recommended practice in Book VI of Australian Rainfall and Runoff (Nathan & Weinmann 1998) aimed to reduce uncertainty and increase reliability of rainfall estimates. The method has previously been applied on an annual basis in all other States. For Western Australia, the CRC-FORGE methodology was applied annually and seasonally, for winter and summer. For the other States, at-site rainfalls were found to be generalised extreme value (GEV) distributed and design rainfalls were calculated for each State as one homogeneous region, except Tasmania which used two homogeneous regions. The application of CRC-FORGE to Western Australia found that a number of homogenous regions were required and that a combination of the GEV and Pearson type 3 (P3) statistical distributions was needed to characterise the at-site rainfall across some of the regions. The result of the study is a seasonal design rainfall database for five durations (24,48, 72,96 and 120 hours) and six AEPs (1 in 50,1 in 100,1 in 200,1 in 500,1 in 1000 and 1 in 2000) for Western Australia, which will assist with dam safety studies, bridge design, and other flood based studies.