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

[...]a search of the keywords “landslide”, “warning” and “system” in the abstracts of all articles that have ever been published in the Division's NHESS journal produced 698 results. Within this framework, this special issue was initially conceived to collect the most relevant works presented to the session SSS9.5/NH3.13 on “Landslide early warning systems: monitoring systems, rainfall thresholds, warning models, performance evaluation and risk perception” within the 2017 General Assembly of the European Geosciences Union. A broad range of conditions is covered, ranging from case studies characterized by scarcity (Shi et al., 2018) to the abundance of data (Devoli et al., 2018; Vaz et al., 2018) and, in some cases, also the measurement of hydrologic (Segoni et al., 2018a) or geotechnical variables (Canli et al., 2018; Salvatici et al., 2018) to strengthen the forecasting models. [...]the works included in the special issue describe early warning systems at very different stages of employment. The most debated unresolved issues in rainfall threshold research include the following the definition of objective and automatic procedures to define the thresholds (Staley et al., 2013; Segoni et al., 2014; Iadanza et al., 2016; Vessia et al., 2016; Rossi et al., 2017; Melillo et al., 2018); the need for taking into account the hydrological conditions of the hillslope system with more complex approaches (Posner et al., 2015; Bogaard and Greco, 2018); the evaluation and quantification of diverse uncertainties (Nikolopoulos et al., 2015; Destro et al., 2017; Marra et al., 2017; Rossi et al., 2017; Marra, 2018; Peres et al., 2018); the importance of validation procedures (Staley et al., 2013; Gariano et al., 2015; Lagomarsino et al., 2015); the use of rainfall data gathered from ground-based radars or satellites (Robbins, 2016; Destro et al., 2017; Rossi et al., 2017; Brunetti et al., 2018); the implementation of the thresholds into hazard management procedures and early warning systems (Kirshbaum et al. 2015; Rosi et al., 2015; Piciullo et al., 2017, 2018; Krøgli et al., 2018; Wei et al., 2018; Pecoraro et al., 2018).