Climate corrected Intensity-Duration-Frequency (IDF) curves: a case study in Rome, Italy

Intensity-Duration-Frequency (IDF) curves are commonly used in hydraulic engineering to estimate design storms, which in turn are employed for designing many hydraulic infrastructures. IDF are generally developed by using the annual maxima (AM) for fixed durations, which are extracted from rain gauge time series, under the major assumption that observed values are representative for future conditions as well. In the last decades, however, the increase of greenhouse gas emissions caused an alteration in the hydrological cycle, inducing an increase in sub-daily rainfall in many regions of the world, such as the Mediterranean area. The direct consequence of this phenomenon is the increasingly frequent failure of hydraulic structures, especially in urban areas, often causing urban floods. It appears evident, in this context, the need for a methodology aimed at correcting and updating IDF curves based on the possible future scenarios of greenhouse gas emissions. In the last decades, many global and regional circulation models (GCM and RCM) have been developed, which are able to simulate climatic variables in both present and future conditions, based on the greenhouse gas emission scenario. In this study, we propose a method to correct historical IDF for the city of Rome by using the precipitation output of a regional model, provided by the EURO-CORDEX project. Rainfall AM at fixed durations, recorded by rain gauges in the historical period, are used to calibrate a relationship that links AM together with daily annual maxima extracted from the RCM. Once the relationship is calibrated and validated, then, it is used to derive AM at the fixed durations for the future conditions, adopting as input the RCM output in future scenarios. Finally, the AM at fixed durations are employed to derive IDF curves with a statistical approach. Results show that rainfall intensity increases in the future scenarios and that this increase grows with growing return periods.