3.2.2. Vegetation Root

According to structural stability and vegetation ecology theory, if the root systems are more complicated and profound, the soil is more stable, and infiltration is higher [90]. Therefore, well-developed root systems can help reduce runoff by directing rainfall into groundwater storage more quickly, helping to reduce the slope strain and stabilizing the slope. The vegetation root architecture was determined using Yen's [91] pull-out tests. Root samples from each of the four species were manually uprooted from the field, washed, and the root diameters were measured using a measuring tape.

#### 3.2.3. Estimation of Surface Runoff

Rainfall surface runoff is an indicator for determining the water loss of a slope [92]. It occurs when the intensity of rainfall is greater than the intensity of infiltration, leading to the failure of excess water to infiltrate [93]. To determine the rainfall rate, surface runoff, rainfall duration, and rainfall intensity were recorded. The rainfall events were based on

the following criteria: (i) Erosive rainfall with daily rainfall amount greater than 12 mm (ii) Similar rainfall intervals (iii) Rainfall duration of no less than 60 min. These factors proved to be effective in rainfall interception for surface runoff [94]. Duration is the extent of the rainfall, and intensity is the rate at which it rains, mathematically expressed by the height of the rainfall layer per minute (mm/min) [95]. Rainfall intensity is the ratio of the total rainfall amount in a given period to the duration of the period [96]. The present study focuses on the relationship among runoff, rainfall, soil type, vegetation, slope, with the primary focus of finding an effective means of reducing the rate of surface runoff, which will practically reduce the effect of landslides. Different methodologies exist for computing and estimating surface runoff [97]. However, this research applies the rational method to determine the surface runoff of the site experiment.
