Peaks-Over-Threshold Flood Analysis

Peaks-over-threshold (POT) flood analysis models every independent flood peak that exceeds a chosen high threshold, rather than only the single largest peak in each year. The number of exceedances in time is treated as a Poisson process and the amounts by which peaks exceed the threshold are modeled with the Generalized Pareto distribution — the extreme-value limit for threshold exceedances given by the Pickands-Balkema-de Haan theorem. Because a wet year may contain several damaging floods and a dry year none, POT (also called the partial duration series) uses the data more efficiently than the annual-maximum approach, which is why Lang, Ouarda, and Bobée's 1999 operational guidelines and USGS Bulletin 17C both treat it as a key complement to annual-maximum frequency analysis. The method delivers the same design-flood quantiles for chosen return periods, often with lower variance at short return periods.