Hydrological Monitoring of Ash-Fall Pyroclastic Soil Mantled Slopes in Campania (Southern Italy)

Ash-fall pyroclastic deposits that mantle mountain slopes around the Mount Somma-Vesuvius (Campania, southern Italy) are frequently involved in debris flows under high-intensity and prolonged rainfall, thus representing a principal geohazard for settlements located alongside the footslope areas. In such a geomorphological framework, to understand temporal and spatial hydrological dynamics occurring into ash-fall pyroclastic soil coverings is a key factor for assessing and modelling landslide hazard as well as for setting reliable early warning system. Along with this research focus, since 2011 field monitoring activities were carried out in a test area of the Sarno Mountains to assess hillslope hydrological processes that predispose and lead to slope instability. The analysis of pressure head time series, recorded along four hydrological years (Jan 2011–Dec 2014) in the whole thickness of the ash-fall pyroclastic soil cover, showed a composite temporal variability, from the daily to the seasonal time scales, related to rainfall patterns and evapotranspiration regime as well as to unsaturated flow dynamics. Unsaturated conditions were always observed with pressure head values ranging at the annual scale from about −0.6 m to, and beyond, −20 m. Such a marked hydrological dynamics of the ash-fall pyroclastic soil covers demonstrates the relevant role of antecedent hydrological conditions in predisposing landslide triggering under a single rainfall event. Therefore, results obtained by the proposed approach can be conceived as a fundamental basis to understand hydrological processes at slope scale. Moreover they can be used to set and calibrate hydrological numerical and slope stability models for estimating rainfall thresholds triggering slope instabilities as well as to assess inherent landslide hazard.