Estimation of future extreme rainfall in Barcelona (Spain) under monofractal hypothesis

Climate change effects on subdaily rainfall (from 5 min to a few hours) can hardly be measured in mid-latitude climates due to the high natural variability of the precipitation patterns and their effects on local topography. The goal of this study was to obtain change projections of intensity-duration-frequency (IDF) curves, for up to 2-h precipitation events, comparing two approaches that use the daily outputs of the downscaled Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model projections: (a) direct scaling of the expected probable precipitation, from 2-year to 500-year return periods of daily rainfall and (b) a new semi-stochastic approach, built by combining the physically forced outputs of climate models (on a daily scale) and stochastic simulation given by the probability distribution of a concentration index (n-index) for individual rainfall events (on a subdaily scale). The approaches were applied to a set of 27 stations located around Barcelona, Spain, including a long reference series (with 5-min rainfall records since 1927), representing the highly variable Mediterranean climate. The validation process showed a systematic error (bias) generally smaller than 10%, especially for rainfall extreme events with durations of less than 2 h. The concentration n-index and IDF curves were projected by 10 downscaled CMIP5 climate models under 2 emission scenarios (RCP4.5 and RCP8.5), obtaining a consensual increase in both relative concentration and absolute intensities in Barcelona. Ensemble projection of rainfall concentration (n-index) showed an increase up to 10% by 2071-2100 and about 20% (15%-30% range) for maximum intensities of 2-year to 500-year return periods. Results provide robustness in decision-making regarding the design of stormwater management infrastructure at a local scale.