Variability in rainfall temporal patterns : a case study for New South Wales, Australia

In Australian Rainfall and Runoff (ARR) 1987, the recommended rainfall-based Design Event Approach (DEA) of design flood estimation has a number of limitations that are likely to introduce a probability bias in the final design flood estimates. Recently, considerable research has been undertaken on the development and application of the Joint Probability Approach (JPA)/Monte Carlo Simulation Technique (MCST) to design flood estimation to overcome the limitations associated with DEA. The applications of this method with Victoria and Queensland data have shown that MCST can overcome some of the limitations associated with the DEA and MCST can produce more accurate design flood estimates. However, the wider application of the MCST needs regionalisation of various input variables to the runoff routing model which include rainfall characteristics such as duration, intensity and temporal pattern. This paper focuses on the regionalisation of the rainfall temporal patterns in New South Wales (NSW), Australia using data from 86 pluviograph stations. The regionalised temporal patterns are then applied with the MCST to obtain design flood estimates for both gauged and ungauged catchments in NSW. Using the MCST, it has been found that the application of at-site and regional temporal patterns can give up to 10% differences in flood quantile estimates. In this study, rainfall inter-event duration (a new random variable) has been tested along with other random variables (i.e. rainfall duration, rainfall intensity, initial loss, continuing loss and runoff routing model storage delay parameter) with their probability distributions in the MCST, which has been shown to provide more accurate flood quantile estimates than the DEA. The findings of this research will assist to apply the MCST in practice in NSW. The method can be adapted to other parts of Australia and similar other countries.