# Application of Compound Terrain Factor LSW in Vegetation Cover Evaluation
## 3 Materials and Methods
### Vegetation Cover Evaluation Based on Certainty Factor
#### 3.4.2 Control Group Factor Selection  
In order to verify the validity of the new slope factor for vegetation cover evaluation, it is informative to introduce other types of topographic factors to perform a controlled trial. The control group factors were selected not only to consider the degree to which they affect vegetation growth, but were also focused on reflecting the topographic conditions. After referring to relevant studies of scholars both domestically and abroad, and fully considering the feasibility of acquisition, data scale and scope of the study area, the potential evaluation factors were finally determined, as follows:  
1. LS factor  
Since the Universal Soil Loss Equation (USLE) was proposed by [PERSON] and [PERSON] [64], the LS factor has been widely used to calculate soil erosion prediction in multiple regions and landscapes around the world, and has gradually led to the derivation of the Modified Universal Soil Loss Equation (MUSLE) [65], the Revised Universal Soil Loss Equation (RUSLE) [66], the Chinese Soil Loss Equation (CSLE) [67], and other serialized models suitable for different geographical environments. In view of the fact that the LS factor represents the combined effect of slope span and steepness [68] and that the LSW factor is an improved optimization based on the LS factor, it is necessary to include the LS factor in the control group.  
2. Topographic wetness index (TWI)  
Topography will act profoundly on rainfall redistribution [69], which consequently affects the spatial distribution of soil water content. This factor is intuitively indicative of thespatial heterogeneity of hydrological conditions that characterize surface watersheds [70]. In turn, changes in hydrological conditions are necessarily important instructions for vegetation growth.  
3. Aspect  
The aspect stands for the direction of the slope surface, so it will affect the difference of hydrothermal conditions in the local slope area. In general, gullies on sunny slopes are at a later stage of development than gullies on shady slopes [70; 71], with steep and short slope morphology and high weathering fragmentation of the slope body. These factors will affect water evaporation, slope erosion, etc., changing the distribution of pore pressure on groundwater and the physical and mechanical properties of the rock soil mass of the slope. They will also have a significant impact on vegetation growth due to the strong water retention of shady slopes, which is conducive to soil deposition and improving permeability [72; 73].  
3.4.3. Evaluation Process  
To comprehensively explore the relationship between LSW factors and vegetation cover, the evaluation method flow is shown in Figure 4, which is divided into four steps overall.  
Figure 4: Process of vegetation cover evaluation based on deterministic coefficients.  
1. Obtain the corresponding topographic factors.
2. Based on the tools of reclassification, spatial analysis, and superposition analysis in ArcGIS 10.7, the CF value of each factor level is calculated and then combined with the numerical distribution law to carry out multi-factor impact analysis. After the topographic impacts of the site environment were analyzed, the evaluation of vegetation susceptibility was examined with the help of the Receiver Operating Characteristic (ROC) curve.
3. Combined with the distribution characteristics of soil erodibility K values in the study area, qualitative description and correlation analysis were carried out for K and LSW to support the rationality for vegetation cover evaluation by LSW.
4. Obtain the vegetation cover susceptibility zoning map of the study area, and also use the indicators A (ratio of the area of this susceptibility class to the total area of the study area), B (ratio of the vegetation within this susceptibility class to total vegetation), and the actual vegetation occurrence ratio B/A (the ratio of vegetation density within a certain susceptibility class to that of the total study area) for reliability testing.