Conceptual hydrological modeling of the soil-bedrock interface at the bottom of the pyroclastic cover of Cervinara (Italy)

On the basis of the data collected by a monitoring station, a simplified mathematical model of the hydrological behavior of the layered pyroclastic cover of the slope of Cervinara (southern Apennines) has been developed. The model considers a single homogeneous soil layer, for which effective hydraulic characteristic curves have been defined. The top boundary condition accounts for the effects of evapotranspiration. The bottom boundary condition conceptually simulates, by means of a linear reservoir model, the hypothesized effects, on the soil water potential at the soil-bedrock interface, of the fluctuations of the water table of an ephemeral aquifer stored in the underlying fractured limestone. Despite its simplifying assumptions, the model satisfactorily reproduces the observed soil water potential at all the monitored depths. The obtained results indicate that even the highest rainfall intensity can pass through the highly conductive unsaturated soil cover, and leak through the fractured bedrock. Only when the water level in the underlying aquifer is high, as it happens after long lasting periods of rainfall, the establishment of the vertical water potential gradients, needed for the leakage of high infiltration peaks, leads to soil saturation at the bottom of the cover. Such a picture provides a possible interpretation of the triggering mechanism of the landside occurred in 1999 along the slope.