Short Duration Rainfall Extremes in Ireland: Influence of Climatic Variability

A widely-noted change in the North Atlantic circulation in the 1970s affected the spatial distribution and seasonal pattern of rainfall over Ireland. To examine if this was accompanied by a change on short duration precipitation extremes, multi-decadal time series from the second half of the twentieth century of thirteen hourly precipitation stations in Ireland have been analysed for the occurrence of extreme values over several durations of up to 24 h. Strong evidence was found for a change since the late 1970s in short duration rainfall depths, particularly in the west of the country. Precipitation depth-duration-frequency analyses over two sub-periods showed that at several locations, storm event magnitudes which corresponded to a 30 year return period before 1975 had a return period close to 10 years in the post-1975 period. The widespread increase in spring and autumn rainfall and the local increases in the frequencies and magnitudes of severe rainfalls have implications for engineering hydrology, flood risk analysis and water resources management. The necessity of using up-to-date data to derive design storm magnitudes is stressed, due to the possible influence of underlying climatic shifts. Furthermore, as non-stationarity has been demonstrated, the use of long timeseries extending beyond thirty years into the past will result in underestimation of storm intensities in many areas.