Tree-ring based, regional-scale reconstruction of flash floods in Mediterranean mountain torrents

In small mountain catchments of the Mediterranean basin, heavy and intense rainfalls produce frequent flash floods that can cause economic damage and even fatalities in the often densely populated valley areas. To reduce future damage of such events, the application of mitigation strategies aimed at hydrological control and disaster risk reduction is essential. However, to reach this goal, flash flood frequency and magnitude need to be characterized in space and time. This task is highly challenging in areas where existing records are either lacking, short or highly incomplete. In poorly or completely ungauged catchments, tree-ring based flash flood reconstruction has proven useful to overcome this limitation and is now considered a valuable, alternative approach to extend flow records back in time. Tree-ring based reconstructions of past flash floods have been employed widely in many different geographic contexts worldwide, but were less frequently used in Mediterranean torrents, and not at all in Southern Italy so far. Here, we present a regional flash flood reconstruction from five poorly gauged mountain catchments in Calabria (southern Italy) to reconstruct both the timing and the magnitude of five extreme flash flood events of the last 60 years. We show that the return periods of these events ranged between 5 and 30 years based on hydraulic (Manning's equation) and hydrological (rational formula) approaches. Validation of the reconstructed discharge values was done by using direct water depth measurements (obtained from a gauging station). Our results suggest that the combination of different methods allows a reliable reconstruction of extreme hydrological events by easy-to-survey parameters in ungauged torrents of Mediterranean environments. We also demonstrate that tree-ring approaches have the potential to be applied in ungauged (or poorly gauged) Mediterranean mountain catchments as well so as to improve records of the frequency and magnitude of past flash flood activity.