Claps, P and Laio, F and Allamano, P (2011) Effects of disregarding seasonality on the distribution of hydrological extremes. Hydrology and Earth System Sciences.

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Abstract

This paper deals with the seasonality of hydro- climatic extremes and with the problem of accounting for their non-homogeneous character in determining the design value. To this aim we devise a simple stochastic experiment in which extremes are produced by a non-homogeneous ex- treme value generation process. The design values are esti- mated in closed analytical form both in a peak over thresh- old framework and by using the standard annual maxima ap- proach. In this completely controlled world of generated hy- drological extremes, a statistical measure of the error associ- ated to the adoption of a homogeneous model is introduced. The sensitivity of this measure, named return period ratio, to the typology and strength of seasonality is investigated. We find that neglecting 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 rainfall data from a 30 000 km2 region located in North-Western Italy is pre- sented to better clarify the effects of disregarding seasonality in a real case.

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