# Influence of Vegetation Filter Strip on Slope Runoff, Sediment Yield and Nutrient Loss
## 3 Results
### The Effects of VFS on Runoff Production and Sediment Yield  
#### 3.1.1 The Effects of the VFS on Runoff Production  
As shown in Figure 4a, under the condition of 30 mm/h rainfall intensity and of 5\({}^{\circ}\) slope, runoff yields for VFS1 and VFS2 were 3.82 L and 3.38 L, respectively, reducing by 31% and 39% in comparison with the control. In such case, it can be seen that the effect of VFS2 on runoff reduction was better than that of VFS1. However, under the same rainfall intensity but the slope gradient increased to 20\({}^{\circ}\), the runoff reduction of VFS1 turned to be higher than that of VFS2. Compared with the control, the reduction rates of runoff production under both VFSs were 52% and 47%, respectively. When the slope gradient increased from 20\({}^{\circ}\) to 35\({}^{\circ}\), a similar result was reached for the comparison of VFSs effect. Under 60 mm/h rainfall intensity (Figure 4b), VFS1 reduced 33%, 3% and 10% runoff production on three slope gradients compared with the control, and VFS2 showed a 14% higher effect than that of VFS1 on 20\({}^{\circ}\) slope and a weaker effect on 5\({}^{\circ}\) and 35\({}^{\circ}\) slope. Under 90 mm/h rainfall intensity (Figure 4c), the comparison of VFS1 and VFS2 demonstrated opposite results to that of 60 mm/h rainfall intensity for the three slope gradients (Figure 4c). The reduction of runoff production under VFS2 was 13% less on 20\({}^{\circ}\) slope and 8% more on 5\({}^{\circ}\) and 35\({}^{\circ}\) slope than that under VFS1. Figure 4: Effect of rainfall intensity, slope gradient on runoff in two different vegetation cover modes and the control plots: (**a**) Rain intensity is 30 mm/h; (**b**) Rain intensity is 60 mm/h; (**c**) Rain intensity is 90 mm/h. Figure 5: Effect of rainfall intensity, slope gradient on sediment yield in two different vegetation cover modes and the control plots: (**a**) Rain intensity is 30 mm/h. (**b**) Rain intensity is 60 mm/h. (**c**) Rain intensity is 90 mm/h. Figure 6: Effect of rainfall intensity, slope gradient on TP loss in runoff and sediment under different vegetation coverage types: (**a**) Rain intensity is 30 mm/h; (**b**) Rain intensity is 60 mm/h; (**c**) Rain intensity is 90 mm/h. Figure 7: Effect of rainfall intensity, slope gradient on TN loss in runoff and sediment under different vegetation coverage types: (**a**) Rain intensity is 30 mm/h; (**b**) Rain intensity is 60 mm/h; (**c**) Rain intensity is 90 mm/h. Based on the analysis of the experimental results, we did not reach a consistent conclusion on the interception effect of VFS1 and VFS2 on runoff. According to the results of runoff yield, we calculated that when other factors were fixed and rainfall intensity increased from 30 mm/h to 60 mm/h and 90 mm/h, the average interception effect of VFS1 on runoff decreased from 43% to 15% and 36%. When the slope increased from 5\({}^{\circ}\) to 20\({}^{\circ}\)and 35\({}^{\circ}\), the average interception effect of VFS1 on runoff decreased from 37% to 33% and 24%. A previous study have also shown that VFS can effectively intercept runoff by reducing runoff velocity, and increases the infiltration rate [36]. However, the increase of rainfall intensity and slope gradient affects the runoff interception benefit of VFS [37]. [PERSON] et al. [38] also proved that compared with the bare land control group, the flow reduction effect of grassland filtration zone was significantly reduced after rainfall intensity and slope were increased, respectively. This was close to the result of our research. However, when the VFS was adjusted from VFS1 to VFS2, its interception effect on runoff was different. When the rainfall intensity was small, the closure effect of VFS2 was not as good as that of VFS1 with the increase of slope, which may be due to the small infiltration amount of exposed surface existing in the middle of VFS2, thus increasing the production flow rate. In addition, the closure effect of VFS2 became worse and worse with the increase of slope in the case of heavy rain.