Effect of varying wheatgrass density on resistance to overland flow

Vegetation cover plays an essential role in control of soil erosion in the Loess Plateau of China. The hydraulic resistance to overland flow under disturbed vegetation is critical in the study of water and soil conservation. A series of experiments on runoff hydraulics was conducted on exposed slopes and surfaces with grass cover. Wheatgrass patches with vegetation density ranging from 0.58% to 5.76% and inflows ranging from 5 to 40 L/ min, incorporating slopes ranging from 4 degrees to 12 degrees were used in this study. A total of 264 groups of hydraulic runoff tests were conducted. The test results showed that, with increasing vegetation density, the flow pattern evolved from transient to turbulent flow. Both vegetation density and slope affected the Froude number (Fr). Vegetation density is an essential factor affecting flow resistance. On the premise of variation in vegetation density, the Kendall coefficient of Re and flow resistance exceeded 0.727, and the Spearman correlation coefficient exceeded 0.872. The total resistance of overland flow under vegetation cover can be divided into form resistance and grain resistance. With increased vegetation density, the proportion of form resistance of the total resistance increased from 95% to 99%. Vegetation was the main factor that constitutes flow resistance and controls soil erosion. These results are helpful for understanding the influence of vegetation community distribution on mechanisms of slope runoff and soil erosion.