Preface: Landslide early warning systems: monitoring systems, rainfall thresholds, warning models, performance evaluation and risk perception

Among the diverse mitigation measures available for reducing the risk to life related to landslides, early warning systems certainly constitute a significant option available to the authorities in charge of risk management and governance. Landslide early warning systems (LEWSs) are nonstructural risk mitigation measures usable at different scales of analysis. Basically, they are used to monitor one or more variables responsible for triggering landslides and to generate and disseminate timely and meaningful warning information to enable individuals, communities and organizations threatened by a hazard to act appropriately and in sufficient time to reduce the possibility of harm or loss (UNISDR, 2009). The installation of a LEWS is often a cost-effective risk mitigation measure and, in some instances, the only suitable option for sustainable management of disaster risks (Glade and Nadim, 2014). The increasing trend shown in the last decade in the employment of LEWSs, in particular at a regional scale, in developing countries confirms the previous statement. Several general schemes of LEWS were proposed in the literature, among which are those recently presented by Intrieri (2013), Fathani et al. (2016), Sättele et al. (2016), Calvello (2017) and Piciullo et al. (2018). Even if a general scheme to describe the structure of a LEWS can be provided, the choice of variables to be measured and monitored varies with the type of landslide that is being forecast and the system's objectives (Lacasse and Nadim, 2009).