Territorial early warning systems for rainfall-induced landslides

Among the many 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. Two categories of landslide early warning systems (LEWSs) can be defined as a function of the scale of analysis. Systems addressing single landslides at slope scale can be named local LEWSs (Lo-LEWSs), systems operating over wide areas at regional scale are herein referred to as territorial systems (Te-LEWSs). In the literature there are several proposals schematizing the structure of LEWSs. They highlight the importance of the interconnection among different know-how and system components, as well as the key role played by the actors involved in the design and deployment of these systems. This worldwide review is organized describing and discussing the main components of 24 Te-LEWSs, following an original conceptual model based on four main tiles: setting, modelling, warning and response. Te-LEWSs are predominantly managed by governmental institutions, thus information is often difficult to find in the literature and, when available, it is not always complete and thorough. The information considered herein has been retrieved from different sources: articles published in the scientific literature, grey literature, personal contacts with system managers, and web pages. Te-LEWSs mainly deal with rainfall-induced landslides, thus pluviometers are the main monitoring instruments. Intensity duration thresholds are typically employed and meteorological modelling is often used to forecast the expected amount of rainfall in order to issue a warning with a given lead time. Public or internal statements are disseminated for increasing the preparedness of both the public and institutions or agencies. Since the beginning of the 21st century, Te-LEWSs are slowly becoming a commonly used risk mitigation option, employed worldwide, for landslide risk management over wide areas. Considerations and insights on key-points for the success or the failure of Te-LEWSs are presented, differentiating among issues related to the efficiency and the effectiveness of the system. Among them, the important role played by performance analyses of the warning model for increasing the system efficiency is thoroughly discussed.