Frequency analysis of rainfall and streamflow extremes accounting for seasonal and climatic partitions

The extreme value distributions of hydrologic variables such as rainfall and streamflow play a key role in the design of water-related infrastructure. The relationship between the intensity, frequency and duration of extreme values is commonly evaluated using observed annual maximum values. This paper considers maximum values that occur within a given season and the relationship between seasonal maxima and annual maxima. Using a climatic index, the seasonal and annual maxima are also partitioned into multiple climatic states so that comparisons of seasonal maxima both within and between climatic states can be made. The distributions of seasonal and annual rainfall maxima are investigated for numerous locations around Australia and a detailed analysis, involving both seasonal and climatic partitions, is given for the daily streamflows of the Murray–Darling Basin, Australia. The results demonstrate significant differences between seasonal distributions within and between climatic states. The application of seasonal extremes to an urban design scenario is illustrated for Scott Creek in the Adelaide Hills, South Australia. The methodology presented can be used in a general setting to gain insight into the relationship between extremes from seasonal and climatic periods to their annual counterparts.