Recent Changes in the Statistics of Extreme Daily Precipitation in Mainland Portugal

Changes in global and land precipitation have been observed over the last decades across different time scales. Particularly, studies conducted at the global scale have usually been consistently reporting increasing trends in extreme precipitation; but at the regional level the findings may differ, which require special attention because of the expected impact of extreme precipitation on society and ecosystems. Despite general trends, the local precipitation structure, relevant generating mechanisms and other specificities can dictate sometimes unexpected differences in precipitation characteristics and change over short distances that cannot be disregarded.

In general, no significant annual precipitation trends have been reported for mainland Portugal (South-western Europe); in this region there is strong precipitation spatial variability and gradients and very marked seasonality. In this study we analyzed ground-based point data to get insight into the properties of local and regional land precipitation in the recent past. Daily precipitation data recorded over more than 70 years were tested over different multi-decadal periods. We explored the statistics and frequency of extreme precipitation events and their intensity. Results show that after the mid-seventies the proportion of the total precipitation attributed to heavy and very heavy precipitation events increased and, consequently, daily precipitation events show a tendency to become more intense in this region. Moreover, analysis shows that the most extreme events could be changing at a faster absolute rate in relation to the mean than more moderate events. Whereas many engineering studies are supported by precipitation statistics derived from data recorded several decades ago, it is therefore important to re-evaluate design procedures and the operation of many hydraulic structures.