Rainfall Criterion for Early Warning

This chapter reviews the practice of landslide early warning on the basis of rainfall observation. Because rainfall is a good proxy of the landslide likelihood, many early warning practices are based on rainfall observation and data interpretation. For reliable early warning, a regional rainfall observation has to be implemented so that real-time and continuous rain data may be provided to the early warning officers. The practice employs two rainfall indices that represent the short-term and the long-term influences of rain on slope instability. It is desired therein that the temporal decay of the long-term effect is taken into account. The other approach to the long-term effect is the use of Sugawara’s tank model, by which the amount of water stored in subsoil (soil water index, SWI ) is assessed. Whatever approach is employed, the rainfall-based early warning is a regional warning and does not suit the risk evaluation of a particular slope.