Comparison of annual maximum and peaks-over-threshold methods with automated threshold selection in flood frequency analysis: a case study for Australia

Flood frequency analysis (FFA) involves fitting of a probability distribution to observed flood data. Two main models, annual maximum (AM) and peaks over threshold (POT), are generally adopted in FFA. The POT model is underemployed due to its complexity and uncertainty associated with threshold selection and meeting independence criteria in selecting POT data series. This study evaluates the POT and AM models using data from 188 gauged stations in southeast Australia. The POT model adopted in this study applies different average numbers of events per year fitted with generalized Pareto (GP) distribution with an automated threshold detection method. For the AM model, the GP distribution is also adopted, and flood quantiles estimated by the two models are compared. It has been found that there are notable differences in design flood estimates between the AM and POT models. The study uses catchment characteristics data to understand the differences in quantile estimates between the AM and POT models. The percentage differences between the AM and POT models can be explained by mean annual rainfall (MAR) and mean annual evapotranspiration (MAE) (ARIs of 1.01–5 years), MAR (10-year ARI), stream density (SDEN) (for 20- and 50-year ARIs) and SDEN, main stream slope (S1085) and MAE (for 100-year ARI). Stations showing smaller % differences have relatively higher MAR and smaller MAE (indicating wetter condition); conversely, stations showing higher % differences have smaller MAR and higher MAE (drier condition).