# Influence of Vegetation Filter Strip on Slope Runoff, Sediment Yield and Nutrient Loss
## 2 Materials and Methods
### Experimental Design and Measurement  
The experiment was based on three independent variables that included the setting modes of VFS, the slope gradient and the rainfall intensity. Through the analysis of the terrain slope of the study area, three gradient variables of 5\({}^{\circ}\), 20\({}^{\circ}\), 35\({}^{\circ}\) were selected; according to the rainfall data of meteorological station set in the study area, three rainfall intensities of 30, 60, and 90 mm/h were selected. In order to study the effects of vegetative filter strips with different settings, the two VFSs were set up as shown in Figure 2 and were named VFS1 and VFS2, respectively. Based on a field survey on the entire area of the eroded gully, the representative herb that grows well in the area, _Stipa benthamiana_, was selected as the vegetation for the experiment. _Stipa benthamiana_ is a grassy herb widely distributed in arid regions, sensitive to water conditions and holding strong tilering ability. In the experiment, after the tanks were filled with soil, the trench was opened according to the setting modes of the vegetation filter strip. Then, _Stipa benthamiana_ was excavated from the waste slope of the study plot, and the bottom root system was retained and transplanted with soil into the tank. The distances of plant stems in the VFS were the same as the original growth conditions surveyed in the field. After the vegetation was transplanted, enough water was sprinkled on the slope to ensure that the basic experimental conditions of the slope were consistent. Figure 1: The location of the study gully area which is located in Xuanhua district, Zhangjiakou City, Hebei Province, China. The red dot represents the experimental station in the gully area. The soil tank (Figure 3) used in the experiment was a two-in-one runoff tank with variable slope and the tank size was length \(\times\) width \(\times\) depth 2 m \(\times\) 1 m \(\times\) 0.4 m. This time, the combination of the two tanks was selected, that is, the actual width of each simulated micro-runoff plot was 0.5 m. The settings of the two sub-tanks in a radial flow tank were exactly the same. A confluence is provided at the lower end of every runoff tank and every exit is equipped with a bucket for collecting surface runoff generated by rainfall. The nozzle height of artificial simulated rainfall system can be adjusted within 3-6 m, and the terminal velocity of raindrop is close to that of natural rainfall. It can control the rainfall intensity of 20-200 mm/h, and the rainfall uniformity is 65-95%, which can basically meet the test requirements. The soil used in the experiment was taken from the gully slope of the study area. The soil was stratified with 10 cm as the standard and kept consistent with the bulk density of the original soil. Considering the influence of wind and natural rainfall, the tests were conducted in the morning without wind or rain in 2019-2020. The rain intensit  
Figure 3: The soil tank used in the experiment. Figure 2: Two different vegetation cover modes: (**a**) VFS with whole piece named VFS1; (**b**) VFS with two pieces named VFS2. Note: the green stands for vegetation filter; and the blue stands for bare ground. the formal test. After the calibration, the slope of the flume was adjusted from left to right in a sequence of 5\({}^{\circ}\), 20\({}^{\circ}\) and 35\({}^{\circ}\). When the water flow on the slope was laminar, it was regarded as the beginning of runoff production. After recording the time required for runoff production, a bucket was placed at the outlet to collect runoff and sediment samples. After the production flow stabilized, the time for each test began and lasted 30 min (excluding the production flow time). The discharge outlet was directly connected with the 100 mL sampling bottle to take the discharge sediment mixture for 1, 7, 13, 19 and 25 min after retiming, and there were buckets placed at the outlet to receive the runoff in the rest of the time. The end flow time and water level in the bucket were recorded at the end of rainfall.