# Influence of Vegetation Filter Strip on Slope Runoff, Sediment Yield and Nutrient Loss
## 4 Discussion
### The Offset Effect of Influencing Factors on Erosion Control  
Multiple regression analysis illustrated the correlation between influencing factors and erosion indexes. However, the effects of influencing factors on soil erosion are complex, as they appear to be synergistic and antagonistic. In order to explore the mutual influence or offset effect among the different influencing factors, cluster analysis was further performed to study the dissimilarity of various influencing factors combinations on soil erosion. A total of 27 combinations which composed of different rainfall intensities (30 mm/h, 60 mm/h and 90 mm/h), slope gradients (5\({}^{\circ}\), 20\({}^{\circ}\) and 35\({}^{\circ}\)) and vegetation coverage type (bare slope, VFS1 and VFS2) were sorted (Table 5), and the runoff yield, sediment yield, TN and TP content in runoff and sediment under each combination were used as 6 evaluation indicators. By calculating the degree of dissimilarity and analyzing each index one by one, the treatment methods with relatively similar effects in preventing and controlling soil erosion in different combinations can be used to obtain the difference in the comprehensive effects of the three factors. According to the calculation results of the phase dissimilarity matrix, \(d(I,j)<0.05\) was selected as the standard for the classification of the corresponding soil erosion control measures. The classification results under each index are shown in Figure 8.  
From the dissimilarity of runoff yield (Figure 8a), under the combined conditions of different rainfall intensity, slope gradient and vegetation setting mode, the VFS may have the same effect in reducing the runoff yield on the slope. VFS can decrease flow volumes and velocity by increasing surface roughness and augmenting infiltration. This reduces the transport capacity of flow and the yield of runoff on the slope [11,52]. For example, the fact that No. 13 (60 mm/h, 5\({}^{\circ}\), VFS1) and No. 25 (90 mm/h, 5\({}^{\circ}\), VFS2) are in the same group indicates that VFS can effectively reduce the slope runoff during heavy rainfall, and it has been proved by previous studies [53,54]. Moreover, No. 2, 3 (30 mm/h, 20\({}^{\circ}\) and 35\({}^{\circ}\), bare slope) and No. 22 (60 mm/h, 5\({}^{\circ}\), VFS2) were also in the same group, indicating that the influence of rainfall intensity on runoff may be equal to the joint influence of VFS and slope gradient. In addition, the dissimilarity of No. 7 (90 mm/h, 5\({}^{\circ}\), and bare slope) was similar to that of No. 26 (90 mm/h, 20\({}^{\circ}\), VFS2). As well as the difference in runoff yield between the 20\({}^{\circ}\) exposed slope and the 35\({}^{\circ}\) slope with VFS with rainfall intensity of 90 mm/h, it can be proved that under the condition of constant rainfall intensity, the effect of VFS can be equal to the influence of slope increase on runoff. All of these conclusions can be based on the research which proved that the contributions of the independent variables on runoff were in the following order: rainfall intensity > soil protective practice > slope gradient > coverage of alfalfa [55]. From the perspective of sediment yield dissimilarity group (Figure 8a), the effects of rainfall intensity, slope gradient and vegetation setting mode will cancel each other out or have the same effect. According to the dissimilarity group, the sediment yield of No. 3 (30 mm/h, 35\({}^{\circ}\), bare land) was similar to that of No. 15 and 24 (60 mm/h, 35\({}^{\circ}\), VFS1 and VFS2), which indicating that under the condition of a certain slope gradient,  
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the sediment yield on the slope with increased rainfall intensity can be reduced by setting VFSs. The vegetation filter strip can effectively reduce the soil loss on the slope [56; 57; 58]. The combination represented by No. 3 was similar to No. 14 (60 mm/h, 20\({}^{\circ}\), VFS1) and No. 25 (90 mm/h, 5\({}^{\circ}\), VFS2). According to the research conclusion of [PERSON] et al.