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Abstract
Different ways to estimate future return levels (RLs) for extreme rainfall, based on extreme value theory (EVT), are described and applied to the Iberian Peninsula (IP). The study was done for an ensemble of high quality rainfall time series observed in the IP during the period 1961–2010. Two approaches, peaks-over-threshold (POT) and block maxima (BM) with the generalized extreme value (GEV) distribution, were compared in order to identify which is the more appropriate for the estimation of RLs. For the first approach, which identifies trends in the parameters of the asymptotic distributions of extremes, both all-days and rainy-days-only datasets were considered because a major fraction of values of daily rainfall over the IP is zero. For the second approach, rainy-days-only data were considered showing how the mean, variance and number of rainy days evolve. The 20-year RLs expected for 2020 were estimated using these methods for three seasons: autumn, spring and winter. The GEV is less reliable than the POT because fixed blocks lead to the selection of non-extreme values. Future RLs obtained with the POT are greater than those estimated with the GEV, mainly because some gauges show significant positive trends for the number of rainy days. Autumn, rather than winter, is currently the season with the heaviest rainfall for some regions.
Highlights
The precipitation regime in the Iberian Peninsula (IP) is highly variable due to its complex topography, for which reason no wide areas are left without coverage
The POT approach is based on the asymptotic convergence of the exceedances of a high threshold u to a general Pareto distribution (GPD) when u tends to infinity
Considering all days, the 20-year return levels (RLs) for the two approaches were estimated for the present day by using the daily rainfall time series
Summary
The precipitation regime in the Iberian Peninsula (IP) is highly variable due to its complex topography, for which reason no wide areas are left without coverage. Precipitation is generated by different physical processes during the different times of the year [2], the intensification of rainfall during the rainy seasons is due to frontal systems coming from the Atlantic Ocean [3,4], which cause persistent rainfall. For the eastern IP, rainfall is produced by easterly flows leading to heavy convective precipitation over the Mediterranean area, especially when there is colder air at high levels. Over extensive regions of the IP, a few rainy days concentrate much of the annual precipitation [5]. This variability leads to a study of extreme rainfall over the IP being interesting
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