**Guan-Wei Lin 1, Hsien-Li Kuo 1,\*, Chi-Wen Chen 2, Lun-Wei Wei <sup>3</sup> and Jia-Ming Zhang <sup>1</sup>**


Received: 3 December 2019; Accepted: 15 January 2020; Published: 16 January 2020

**Abstract:** Rainfall thresholds for slope failures are essential information for establishing early-warning systems and for disaster risk reduction. Studies on the thresholds for rainfall-induced landslides of different scales have been undertaken in recent decades. This study attempts to establish a warning threshold for large-scale landslides (LSLs), which are defined as landslides with a disturbed area more massive than 0.1 km2. The numerous landslides and extensive rainfall records make Taiwan an appropriate area to investigate the rainfall conditions that can result in LSLs. We used landslide information from multiple sources and rainfall data captured by 594 rain gauges to create a database of 83 rainfall events associated with LSLs in Taiwan between 2001 and 2016. The corresponding rainfall duration, cumulative event rainfall, and rainfall intensity for triggering LSLs were determined. This study adopted the tank model to estimate conceptual water depths (*S*1, *S*2, *S*3) in three-layer tanks and calculated the soil water index (SWI) by summing up the water depths in the three tanks. The empirical SWI and duration (SWI–*D*) threshold for triggering LSLs occurring during 2001–2013 in Taiwan is determined as SWI = 155.20 − 1.56*D* and *D* ≥ 24 h. The SWI–*D* threshold for LSLs is higher than that for small-scale landslides (SSLs), those with a disturbed area smaller than 0.1 km2. The LSLs that occurred during 2015–2016 support this finding. It is notable that when the SWI and *S*<sup>3</sup> reached high values, the potential of LSLs increased significantly. The rainfall conditions for triggering LSLs gradually descend with increases in antecedent SWI. Unlike the rainfall conditions for triggering SSLs, those for triggering LSLs are related to the long duration–high intensity type of rainfall event.

**Keywords:** soil water index; large-scale landslide; SWI–*D* threshold; early warning system
