**5. Conclusions**

Due to the limited number of large-scale landslides (LSLs) in a specific region and the difficulty of obtaining their occurrence times, it is still not easy to establish effective rainfall thresholds for LSLs. Exact time and location information on the LSLs in Taiwan provides the opportunity to develop regional rainfall thresholds for triggering LSLs. In addition to the rainfall thresholds determined by general rainfall factors (intensity, duration, and cumulative rainfall), the conceptual water depth estimated by the three-layer tank model can be used as one of the hydro-meteorological conditions that cause landslides. This study analyzed rainfall factors and evaluated conceptual water depths, including *S*1, *S*2, *S*3, and the Soil Water Index (SWI), in three-layer tanks to assess hydro-meteorological thresholds for LSLs from 2001 to 2016 in Taiwan. The analysis of rainfall duration (*D*), cumulative event rainfall (*E*), and hourly rainfall (*I*) indicated that the hourly rainfall at the occurrence time is not a crucial factor for triggering an LSL. In fact, the cumulative event rainfall and duration may be the determining factors for triggering an LSL. The average antecedent SWI is 16.3 mm, and the average SWI when LSLs occurred is 311.9 mm. Sixty-Two events occurred when *S*<sup>3</sup> was higher than *S*1, which indicated that the deeper water might have a higher relationship with the initiation of LSLs. The triggering rainfall for 75 LSLs from 2001 to 2013 was used to make an SWI–*D* threshold (SWI = 155.20 − 1.56*D* and *D* ≥ 24 h) and was tested with 8 LSLs from 2015 to 2016. A substantial amount of water content within deeper materials is required for initiation of an LSL. The result verified that the SWI can be treated as an indicator of possible LSL initiation. This study also compared the rainfall conditions for LSLs with those for small-scale landslides (SSLs) and revealed that the antecedent rainfall for triggering LSLs is higher than that for triggering SSLs. Unlike the rainfall conditions for triggering SSLs, which are associated with the shorter duration–high intensity type, the rainfall conditions for triggering LSLs are related to long duration–high intensity rainfall events. Through understanding the different hydro-meteorological conditions for LSLs and SSLs using the tank model, the results of this study provide the potential to develop an enhanced landslide early warning model considering disasters of various scales.

**Author Contributions:** G.-W.L. devised the research, the main conceptual ideas, and the proof outline. H.-L.K. and G.-W.L. performed the analytic calculations. G.-W.L., H.-L.K., C.-W.C., L.-W.W., and J.-M.Z. contributed to the interpretation of the results. G.-W.L. took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis, and manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors gratefully acknowledged the financial support of the Ministry of Science and Technology of Taiwan and the Soil and Water Conservation Bureau, Council of Agriculture, Executive Yuan of Taiwan.

**Acknowledgments:** The source of all seismic and rainfall information included in this paper was the Institute of Earth Sciences, Academia Sinica of Taiwan, and the Seismology Center, Central Weather Bureau (CWB), Taiwan.

**Conflicts of Interest:** The authors declare no conflict of interest
