# Influence of Vegetation Filter Strip on Slope Runoff, Sediment Yield and Nutrient Loss
## 1 Introduction  
Soil erosion is one of the major ecological and environmental problems worldwide, which has severely restricted the sustainable development of agriculture and disturbed the balance of terrestrial and aquatic ecosystems [1, 2, 3, 4]. For controlling the soil erosion, vegetative filter strips (VFSs) are often adopted as an effective management practice. From long-term practices, it has been proved that the VFSs are the best eco-friendly and economical way which has been widely welcomed in dealing with the problem of soil erosion [5, 6]. The VFS is a belt-shaped vegetation area often located at the lower edge of a field [7, 8]. It can reduce surface runoff and sediments through biological, chemical, physical and other means, relieving the pressure on aquatic environment [9, 10]. For example, the VFS can increase hydraulic resistance to surface flows and therefore reduce the surface flow velocity, promoting infiltration and decreasing sediment transport [11]. Moreover, VFS exerts effective influences on source pollution via removing suspended solids, nutrients and pesticides from runoff [12, 13, 14]. Many studies show that VFS has a good interception on sediments, nitrogen, phosphorus and other pollutants and plays an important role in protecting water bodies [15, 16]. The effect of VFS is related to a few influencing factors, including plant species, vegetation density, bandwidth, slope angle, slope length, surface roughness, inflow discharge, soilproperties and so on [7; 17; 18; 19; 20; 21; 22]. Essentially, the characteristics of VFS have determinative effects on runoff and sediment trapping. First of all, the species-specific performance is most important responsible for the VFS characteristics [7; 23]. Studies have measured that VFS of grass can reduce runoff and sediment up to 60% and 80%, respectively, by filtration, deposition, and infiltration [24; 25]. [PERSON] et al. [26] conducted experiments on the slopes of olive groves and found that the sand reduction rate of ryegrass strips was 71%, while that of mixed strips of oat grass and thyme was only 59%. By comparing different plants, it was found that the creeping plants with large leaves have a better sand blocking effect than the straight plants [27]. Generally, the biomass in VFS is linearly correlated with the interception effect of suspended particulate matter [28]. Another important characteristic of VFS is bandwidth. An experiment showed that 53% of soluble N and P can be removed from runoff with 0.7 m wide rescue filter strips [29]. In the experiment of [PERSON] et al. [30], they measured the effect of the VFS width: when the width was 9.1 m, the VFS could effectively intercept 91% of solid sediment, 69% of phosphorus and 74% of nitrogen in the wastewater; when the width was 4.6 m, 81% of solid sediment, 58% of phosphorus and 64% of phosphorus in wastewater could be effectively intercepted. In addition to the self-characteristics of the VFS, the effects of external factors such as inflow discharge, soil property, rainfall intensity and slope gradient are also quantitatively determined in numerous experiments. [PERSON] et al. [31] and [PERSON] et al. [32] studied how inflow discharge influences the purification effect of VFS on suspended particulate matter and nutrient salts in runoff. [PERSON] et al. [32] measured that when the inflow was 200, 400 and 600 L-h\({}^{-1}\), the interception rates of the simulated VFS for N were 74.9%, 62.0% and 58.3%, and the interception rates for P were 85.0%, 75.6% and 72.0%, respectively. The interception effect is significant when the incoming water flow is low. [PERSON] et al. [33] found the removal of nitrate through denitrification in VFS mainly depends on the physical and chemical properties of soil. VFS can remove larger sediment particles of sand and silt primarily, while clay particles may be partially removed depending on strip width. The factors of rainfall intensity and slope gradient affect the effectiveness of VFS through inflow velocity, hydraulic retention time and infiltration rate [34].