Operational soil moisture-based threshold for the assessment of landslide occurrence over Umbria region, central Italy

Landslides are one of the most dangerous natural hazards, causing every year fatalities, considerable damage and relevant economic losses. Early warning systems (EWS) for rainfall-induced landslides represent an useful tool for mitigating the impact of such hazard. The Umbria Regional Civil Protection Service developed a system able to take into account also the soil moisture conditions over the regional territory based on set of soil moisture-based thresholds. By identifying the soil saturation conditions before and after the rainfall event (obtained through a hydrological model), it has been seen that most of the activations occurred when the soil reached saturation. In this way, an alert can be issued when the amount of rainfall needed by the soil to reach saturation is observed. The amount of rainfall needed to reach saturation has been calculated through the definition of soil hydraulic parameters and the saturation degree at the start of the rainfall event. The obtained threshold is based on soil characteristics and it is independent by the input data (no need for recalibration or threshold adjustment). The proposed methodology is able to identify correctly most of the proposed events with a very limited amount of false alarms considering all the rainfall events occurred during the 1989-2022 period. Moreover, the use of high-resolution rainfall and soil moisture satellite-derived products has been tested for a limited time window to test whether these new sources of information can be used with benefit, even for operational purposes.