Preliminary establishment of a mass movement warning system for Taiwan using the soil water index

The soil water index (SWI) represents the estimated soil moisture content and is an important indicator as a reference for issuing mass movement warnings nationwide in Japan. Many studies recently have pointed out that the occurrence of mass movements is highly related to whether the value of SWI is within the top historical rankings for the same location. Some studies have proposed a quantitative method, which is to calculate the ratio of the SWI at a certain location to the maximum value of the past decade (i.e., the normalized soil water index; NSWI). This study used 279 mass movements that occurred in Taiwan during 2006–2017 to analyze SWI and NSWI during the rainfall events that caused those mass movements. The results showed that the SWI at the times of mass movements was between 61.10 and 700.93, while the NSWI was between 0.14 and 1.87. On average, mass movements occurred within 1.71 h after the maximum SWI and NSWI appeared. The NSWI can effectively reflect the historical ranking of the current SWI value in a location, so different NSWI ranges could be used to define different warning levels for mass movement occurrence. This study also used an integrated system to link historical rainfall data and establish an interface that allows easy exploration of NSWI during rainfall events. In the future, further connecting this system with real-time rainfall data and even the estimated rainfall data should provide a major breakthrough for the mass movement warning system in Taiwan.