2.1 Experimental site and soil samples
The field experiment was carried out at the Yangling Soil and Water Conservation Experimental Station of the Chinese Academy of Sciences on the Loess Plateau (107°59′36.12″ E, 34°19′24.84″ N) (Fig. 1). The station is located in the warm temperate semihumid continental monsoon climate zone. The average annual precipitation is 637.6 mm, with more than 60% falling from July to October. The average annual temperature is 12.9 °C. An experimental slope was built on an artificial excavation surface in contact with the parent soil.
The tested soil was collected from abandoned soil produced by excavating slope engineering projects (Yang et al. 2019). The soil used in this experiment was silty loam with 11.02% sand (50 μm–2 mm), 61.42% silt (50–2 μm), and 27.56% clay (< 2 μm). To maintain the natural state of the soil, the soil was not passed through any sieve (Niu et al. 2020), but the organic litter layer, weeds, and gravel were removed from the soil.
2.2. Experimental design
The field scouring experimental setup included a water supply line, a constant barrel, a valve, a flow metre, a steady flow groove, and collecting barrels (Fig. 2). The experimental plot was 20 m in length, 1 m in width, and 0.5 m in depth. The inflow intensities could be adjusted through the valve opening sizes. The water was kept at a constant pressure to ensure uniform flow. A steady flow groove (1.0 m length, 0.5 m width, and 0.3 m depth) was located at the top of the plot. The experimental plot slope was 32°, a common slope for engineering accumulation bodies in the experimental region.
With the total amount of water held constant (900 L), four inflow rate patterns, including even (Fig. 3a), rising (Fig. 3b), falling (Fig. 3c) and rising-falling (Fig. 3d), were designed to indirectly reflect the time distribution characteristics of rainfall and the influence of underlying surface conditions on the confluence process (Zhang et al. 2016). The duration of the flow event was set to 45 min. The process of the flow event was divided into three stages: early period (0–15 min), intermediate period (15–30 min), and late period (30–45 min). According to previous tests and the unit discharge that occurred in the experimental region under the condition of a heavy rainstorm, the inflow rate was calculated and selected (Guo 2010). The four inflow rate patterns are shown in Fig. 3. Before the start of each test, the desired inflow rate was calibrated 3 times, and the test could be carried out only when the error was less than ±5%.
To simulate the engineering accumulation and ensure the consistency of the initial conditions of the slope, soil samples of 50 cm were placed into the runoff area by the tamping method. The soil bulk density was controlled at approximately 1.25 g cm-3, and the initial soil water content was controlled at approximately 22%. In the experiment, Bahraini grass (25%), Kentucky grass (25%), and Youmei grass (50%) were selected to form the grass strips. Field investigations found that these species are often used for engineering slope protection and greening. The sizes of the grass strips were 10 m×1 m and 1 m×1 m. Thirty days before the experiment began, the grass strips were transplanted to the experiment plot to grow naturally. Soil erosion can be effectively controlled when vegetation cover reaches 50% (Liu et al. 2008); therefore, the grass cover was set at 50% for this experiment. In this experiment, five spatial configurations of the grass strips included bare soil (Pattern Ⅰ), the upper slope (Pattern Ⅱ), the middle slope (Pattern Ⅲ), the lower slope (Pattern Ⅳ), and bands (Pattern Ⅴ), which are shown in Fig. 4.
For each experiment, the runoff production time was recorded with a stopwatch. Runoff samples were collected at 2 min (0-6 min) and 3 min (6-45 min) intervals, and the sampling times were recorded with a stopwatch. The experimental slope was divided into 5 observation sections with intervals of 4 m. The surface flow velocity of each section was measured across a distance of 2.5 m using a dye tracer (KMnO4) method. The mean velocity was obtained by multiplying the velocity by the correction factor of 0.75 (Luk and Merz 1992). After the experiment, runoff samples were weighed and left to stand for 24 hours, and the supernatant was poured off. Then, the remaining sediment was left to dry at 105 °C to calculate the sediment yield. This process was repeated once for each experiment.
2.4. Data calculation and analysis
The degree of runoff control (C) refers to the proportion of infiltration to rainfall or inflow. Under the condition of pure soil, C is one of the indexes to reflect the soil infiltration capacity, and when erosion-control measures are arranged on the slope, the index can describe the surface runoff control capacity of these conservation measures. C is an index to reflect the amount of runoff produced on the slope. The larger the runoff is, the smaller the water infiltration, which can lead to a decrease in the ability of conservation measures to regulate and control slope runoff (Wu et al. 2010). C is calculated using the following formula:
where C is the runoff control degree, I is the total rainfall or inflow (L) and R is the total runoff of the slope (L).
The process of sediment erosion driven by runoff is restricted by the amount of runoff and the flow-sediment relationship. Soil and water conservation measures mainly affect soil loss by adjusting the amount of runoff and the flow-sediment relationship (Zhang et al. 2016, Zhang et al 2017, Zhang et al 2019). The change in sediment yield on the slope before and after implementing the control measures can be calculated by the following formula:
where ΔW is the total sediment reduction (kg) after the measures take effect. W1, Q1, and S1 are the sediment yield (kg), runoff (L), and sediment concentration (kg/L) without measures, respectively. W2, Q2, and s2 are the sediment yield (kg), runoff (L), and sediment concentration (kg/L) with measures, respectively. ΔQ is the amount of runoff change (L) after the measures take effect. ΔS is the amount of sediment concentration (kg/L) after the measures take effect. S1ΔQ is defined as the amount of sediment reduction caused by decreasing runoff (SRR, kg). ΔSQ2 is defined as the amount of sediment reduction caused by flow-sediment relationship changes (SRS, kg).
All data analysis was performed using the SPSS16.0 software (IBM Corp., Armonk, NY, USA). ANOVA (P<0.05) was used to compare significant differences in the responses of inflow rate patterns and grass strip patterns to runoff and sediment under the same conditions. Pearson correlation analysis was used to examine the correlations among the inflow rate and grass strip patterns and the runoff and sediment. All figures were plotted using the Origin 8.5 software (OriginLab Corp., Northampton, MA, USA).