*4.1. Rainfall Conditions and SWI for Triggering Large-Scale Landslides*

To define the rainfall threshold for landslide initiation, a detailed analysis of the rainfall conditions for the 83 landslides considered in the study was performed. Approximately half (41) of the LSLs collapsed when the cumulative event rainfall (*E*) exceeded 1000 mm (Figure 5). Moreover, 12 of these events happened after rainfall accumulations of more than 1500 mm in total. The amounts of hourly rainfall at the occurrence time points ranged from 0 to 91.8 mm, which included 31 events with values lower than 20 mm and 18 zero-value events (hourly rainfall was equal to zero at the occurrence time of the landslide). The duration (*D*) analysis showed that 63 of the LSLs occurred when the rainfall duration was longer than 48 h, and only one case had a duration time of less than 24 h. The results of the analysis of cumulative event rainfall, hourly rainfall, and duration indicated that hourly rainfall at the occurrence time was not a compelling factor in triggering large-scale landslides. Accordingly, average rainfall intensity (*I*) is usually adopted in the analysis of rainfall thresholds instead of hourly rainfall at the occurrence time of the landslide. On the other hand, the cumulative event rainfall and duration may have more remarkable effects on the conditions for triggering LSLs.

**Figure 5.** Statistics of triggered LSL number for each rainfall factor.

The SWI results contained the precipitation during the event as well as the antecedent precipitation of 30 days preceding the event. The average antecedent SWI was 16.3 mm, ranging from 1.2 mm to 56.2 mm. The SWI at the occurrence times of the LSLs was 311.9 mm on average, and the maximum and minimum values were 706.3 mm and 70.7 mm, respectively (Figure 6). The average *S*<sup>3</sup> value was 131.7 mm, which is higher than the average *S*<sup>1</sup> value of 78.1 mm. The values of *S*1, *S*2, and *S*<sup>3</sup> represent the depths of water content in the three simulated layers. SWI may not directly express the real water content in the deep rock formation. However, we need to note that the cumulative rainfall or rainfall intensity used in traditional rainfall analysis does not indicate the real water content in the rock formation, either. The statistics of the above two rainfall factors even neglect hydrological factors such as infiltration and drainage. In SWI calculations, infiltration and drainage are considered, and we can infer that the water entering the deep part of the soil is more closely related to the water content inside the rock formation than the cumulative rainfall is. Therefore, this study holds that replacing the cumulative rainfall factor with the SWI (including the values of *S*1, *S*2, and *S*3) to construct the rainfall threshold for landslides is an enhanced approach [21,44].

The SWI analysis also revealed that 62 LSLs had *S*<sup>3</sup> values higher than the *S*<sup>1</sup> values. Under the condition that SWI is high, the situation *S*<sup>3</sup> > *S*<sup>1</sup> means more water remaining in the deeper layer but not in the shallow layer. Also, the average value of the ratios of *S*<sup>3</sup> to SWI at the occurrence times of LSLs was 0.46. In contrast, the average of the ratios of S1 to SWI at the occurrence times of LSLs was only 0.22. Because *S*<sup>3</sup> represents the water content of the deeper soil layer, the results indicated that the deeper water might have a higher relationship with the occurrence of LSLs. The hydrographs of SWI demonstrated that most of the LSLs collapsed when the rising trend became smooth and even started to fall (Figure 7). According to the SWI model, the falling and smooth trend of the SWI hydrograph indicates the declination of rainfall events and the lack of recharging of the water in upper slope materials. This phenomenon responded well to the characteristic of cumulative rainfall and rainfall duration for triggering LSLs.

**Figure 6.** Statistics of triggered LSL number with *S*1, *S*2, *S*3, and SWI.

**Figure 7.** Hourly changes in the SWI from the beginning of rainfall to landslide occurrence. The two general curves for small-scale landslides (SSL) were adopted from Chen, et al. [27].
