Abstract
Different ways to estimate future return levels for extreme rainfall are described and applied to the Iberian Peninsula (IP), based on Extreme Value Theory (EVT). This study is made for an ensemble of high quality rainfall time series observed in the Iberian Peninsula over the period 1961-2010. Both, peaks-over-threshold (POT) approach and block maxima with the Generalized Extreme Value (GEV) distribution will be used and their results compared when linear trends are assumed in the parameters: threshold and scale parameter for POT and location and scale parameter for GEV. Both all-days and rainy-days-only data sets were considered, because rainfall over the IP is a special variable in that a large number of the values are 0. Another methodology is then tested, for rainy days only, considering the role of how the mean, variance, and number of rainy days evolve. The 20-year return levels (RLs) expected in 2020 were estimated using these methodologies for three seasons: fall, spring and winter. GEV is less reliable than POT because fixed blocks lead to the selection of non-extreme values. Future RLs obtained with POT are higher than those estimated with GEV, mainly for some observatories showing significant positive trend for the number of rainy days. Fall becomes the season with heaviest rainfall, rather than winter nowadays, for some regions.
Highlights
The precipitation regime in the Iberian Peninsula (IP) is highly variable due to its complex topography leaving no wide areas without coverage given that the spatial variability of IP rainfall is such that certain places receive more than 3000 mm/yr, while others, for example, in the southeast, receive on average less than 200 mm/yr, the lowest values in Europe
A study of the Extreme Value Theory (EVT) to estimate non-stationary return levels of extreme rainfall for present and near future has been developed for the Iberian Peninsula
The two widely approaches in EVT, block maxima and peaks-overthreshold were used in order estimate the 20-year return level (RL) in present time and the expected in 2020
Summary
The precipitation regime in the Iberian Peninsula (IP) is highly variable due to its complex topography leaving no wide areas without coverage given that the spatial variability of IP rainfall is such that certain places receive more than 3000 mm/yr, while others, for example, in the southeast, receive on average less than 200 mm/yr, the lowest values in Europe. The aim in this work is not to propose a methodology for the RLs estimation but to compare two ways of dealing with possible trends in rainfall to estimate near future extremes This objective is tackled for a set of complete daily rainfall time series from 76 gauges for the period 1961-2010 for the whole Iberian Peninsula. Data homogeneity was evaluated using the R-based program RHTestV3, developed at the Climate Research Branch of the Meteorological Service of Canada, and available from the ETCCDMI Web site (http://etccdi.pacificclimate.org/) This program is capable of identifying multiple step changes at documented or undocumented change-points. For the POT approach, the threshold used for the definition of extreme rainfall, for each season separately, was the 98th percentile of all the daily rainfall values for the whole period, and for the rainy days only, the 95th percentile of the rainfall values
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