## 2 Materials and Methods
### Data Collection and Analysis Methods
#### 2.4.2 Measurement of Canopy Interception Capacity for Rainwater  
Random 1 m \(\times\) 1 m quadrats were selected within the designated study area. All plants within the quadrats were harvested, and their branch and leaf weights were individually measured to determine the average weight per quadrat. Simultaneously, branch and leaf samples were collected, weighed in situ, immersed in water for 20-30 min, and reweighed once no further water dripping occurred. This procedure was repeated three times for different samples. The branches' and leaves' maximum water absorption rate (\(P\)) was initially calculated and subsequently used to determine the canopy interception capacity (H). The calculation formula is as follows [30]:  
\[ \begin{array}{c}p=\frac{g1-g2}{g1}\times 100\%\\ h=\frac{G\times P}{W}\end{array}  \tag{1}\]  
In the formula, \(P\) represents the maximum water absorption rate of branches and leaves (%); \(g1\) stands for the weight of branch and leaf samples before water absorption (kg); \(g2\) corresponds to the weight of branch and leaf samples after water absorption (kg); h signifies the canopy interception of precipitation (mm); \(G\) denotes the average weight of branches and leaves per quadrat (kg); and \(W\) represents the area of the quadrat (m\({}^{2}\)). Figure 2: The four runoff plots. The four runoff plots of the experimental design (**a**), The central experimental site layout conditions (**b**). 2.4.3. Determination Method for Unit Area Litter Water Holding Capacity  
Diagonal sampling was employed to ensure the experiment's precision [30]. Three 1 m \(\times\) 1 m quadrats were selected using a sampling frame along the diagonals of the study area. Within these quadrats, the thickness of the litter was measured, and the weight after air-drying was recorded. Subsequently, the desiccated litter samples were immersed in water for 24 h, then retrieved and reweighed after no further water dripping [30]. The water-holding capacity of the litter was calculated by subtracting the dry weight from the wet weight and dividing the result by the quadrat area. 2.4.4. Determination Method for Water Absorption Characteristics of Litter  
The litter's water absorption capacity was assessed using an indoor soaking method, with subsequent calculations of water absorption levels and rates at various time intervals: 0.25, 0.5, 1, 2, 4, 8, 10, 24, and 48 h.  
2.4.5. Measurement of Surface Runoff Reduction Capacity  
Surface runoff-induced soil erosion constitutes the primary form of hydraulic erosion [31]. This study focused on observing and analyzing total runoff and sediment yield resulting from natural rainfall in the standard runoff plots planted with perennial sweet peas in the slate region of low mountainous hills. 2.4.6. Method for Determining Soil Water Storage and Retention Capacity  
The analysis of soil water storage capacity primarily involves evaluating soil bulk density, porosity, and mechanical composition in areas where perennial sweet peas are planted. As a vital layer in forest hydrology and ecology, soil influences various attributes such as water-holding capacity, retention, erosion resistance, and permeability [32; 33]. Bulk density reflects soil physical characteristics influenced by texture, structure, porosity, and organic content [34]. Lower bulk density indicates improved soil structure and enhanced water retention and infiltration due to organic aggregates fostering well-aggregated soil [35]. Soil pores comprise capillary and non-capillary types, with capillary pores playing a significant role in water movement and evaporation, ensuring a balance between plant root uptake and moisture preservation [36]. More extensive non-capillary pores store water, minimizing evaporation and facilitating the movement of water to lower soil layers, forming groundwater through gravitational influence [37]. 2.4.7. Method for Determining Soil Infiltration  
The Modified Double-Ring Infiltrometer Method was employed to measure soil infiltration.