Multi-temporal drought rarity curves—A yearly classification of meteorological drought severity in France

Abstract

Droughts are recurrent phenomena that present a large variety of space and time patterns making rather difficult the assessment of their rarity and the comparison between events.

Our study focuses on the “memory effect” of meteorological drought over France using the daily raingauge network maintained by Météo-France over 1950–2022. The proposed easy tool of rarity curves analyses how drought events build and persist across time. The approach is purely statistic, assuming that drought consequences depend on the probability of non exceedance (“rarity”) of antecedent rainfall accumulations. In order to cover a large spectrum of “memory effects”, we consider a continuum of accumulation periods ranging from a few weeks to several years. The rarity curve of a given year displays the most severe rarity values encountered during the year as a function of the various accumulation periods (256 values ranging from 4 to 260 weeks in our case).

Over the study period of 1950–2022 we show how the shape of rarity curves discriminate short- and long-term historical droughts. A k-means algorithm proves to be useful to classify the different years into typical drought situations from continuous drying to continuous wetting over the antecedent five years. Looking at the chronology of the found clusters also allows studying how droughts build and persist across time.