Effects of disregarding seasonality on the distribution of hydrological extremes

摘要

This paper deals with the seasonality of hydroclimatic extremes and with theproblem of accounting for their non-homogeneous character in determining thedesign value. To this aim we devise a simple stochastic experiment in whichextremes are produced by a non-homogeneous extreme value generation process.The design values are estimated in closed analytical form both in a peak overthreshold framework and by using the standard annual maxima approach. In thiscompletely controlled world of generated hydrological extremes, a statisticalmeasure of the error associated to the adoption of a homogeneous model isintroduced. The sensitivity of this measure, named return period ratio, tothe typology and strength of seasonality is investigated. We find thatneglecting seasonality induces a downward bias in design value estimators.The magnitude of the bias may be large when the peak over threshold approachis adopted, while the return period distortion is limited when the annualmaxima are considered. An application to rainfall data from a30 000 km2 region located in North-Western Italy is presented tobetter clarify the effects of disregarding seasonality in a real case.