Spatial analysis of trend in extreme daily rainfall in southern France

Abstract

This paper makes a regional evaluation of trend in yearly maxima of daily rainfall in southern France, both at point and spatial scales on a regular grid of 8 × 8 km2. In order to filter out the high variability of rainfall maxima, the current analysis is based on a non-stationary GEV modeling in which the location parameter is allowed to vary with time. Three non-stationary models are considered for each series of maxima by constraining the location parameter to vary either linearly, linearly after a given date or linearly up to a final date. Statistical criteria are used to compare these models and select the best starting or final point of putative trends. The analysis shows that, at regional scale, the best distribution of maxima involves a linear trend starting in year 1985 and that this trend is significant in half the region, including most of the mountain ranges and part of the Rhone valley. Increases in yearly maxima are considerable since they reach up more than 60 mm/day in 20 years, which is more than 40 % of the average maximum in this area.