Effects of seasonality on the distribution of hydrological extremes

Abstract

Abstract. This paper focuses on the seasonality of hydroclimatic extremes and on the problem of accounting for their non-homogeneous character in determining the design value. To this aim we devise a simple stochastic world in which extremes are produced by a non-homogeneous extreme value generation process. The design values are estimated in closed analytical form both in a peak over threshold framework and by using the standard annual maxima approach. In this completely controlled world of generated hydrological extremes, a statistical measure of the error associated to the adoption of a homogeneous model is introduced. The sensitivity of this measure, assumed in terms of return period ratio, to the typology and strength of seasonality is investigated. We find that seasonality induces a downward bias in design value estimators. The magnitude of the bias may be large when the peak over threshold approach is adopted, while the return period distortion is limited when the annual maxima are considered. An application to a prealpine to alpine transition region located in North-Western Italy is presented to better clarify the effects of disregarding seasonality in a real case.