*2.4. Calculation of Landslide Volume*

The Chenyulan watershed has suffered by several severe natural disasters, such as Typhoon Herb (in 1996), 921 earthquake (in 1999), and Typhoon Morakot (in 2009), resulting in significant landslides, debris flows, change in landslide characteristics of central Taiwan [4]. Many studies have conducted the survey of landslide characteristic changed after the 921 earthquake [26,27,54–57]. Due to the lack of landslide calculation in the SWAT model, we integrated the landslide estimation equation developed

by [28] into the SWAT model. The landslide volume was calculated by using the correlation between the landslide area and volume (R2 = 0.79) (Equation (19)).

$$
\ln(\text{V}) = 0.687 \ln(\text{A}) + 2.326 \tag{19}
$$

where V is estimated landslide volume (m3); A is landslide area (m2).

**Figure 5.** Comparison of L factor and S factor.

It was reported that the percentage of landslide and debris flow in total sediment load would be greater than 60% when the daily cumulative precipitation is higher than 350 mm [58]. Therefore, the landslide volume estimation is triggered only when the daily precipitation exceeds 350 mm. The landslide area is read into the transformed landslide volume estimate equation as follows (Equation (20)).

$$\mathbf{V} = \mathbf{e}^{2.326} \cdot \mathbf{A}^{0.687} \tag{20}$$

In this study, the landslide area was identified by the land use survey map. The landslide volume is calculated by Equation (19), and then added into the sediment yields of HRUs.
