Effects of Vegetation on Runoff Hydrodynamics and Erosion Morphologies in Headcut Erosion Processes in the Loess Tableland Region

Abstract Vegetation significantly affects the soil properties and runoff processes of gully head systems, thereby affecting their development. However, the mechanisms underlying the effects of vegetation on gully headcut erosion remain unclear. To explore these mechanisms, a series of simulation experiments were carried out on plots with four types of vegetation and bare land (BL). The results revealed that vegetation reduces the runoff velocity in the upstream area (Vup), gully head brink (Vbrink), and gully bed (Vbed) areas by 15%–70%, 3%–54%, and 1%–30%, respectively, and that vegetation type impacts Vup, with no obvious impacts on Vbrink, the jet flow velocity (Vjet) or Vbed. Vegetation reduced the jet flow shear stress (τ jet ) under low inflow discharge, but under high inflow discharge, it increased τ jet . Different vegetation types exhibited different effects on the increase in the Darcy–Weisbach friction factor ( f ) and Manning roughness coefficient ( n ) in the upstream area, whereas the effect of vegetation on the f and n value of the gully bed was not obvious. Vegetation reduced the gully head retreat length. Compared with BL, vegetation reduced the rate of soil loss by 31%–95%. Vegetation significantly and directly affects soil characteristics, hydrodynamic parameters, and gully head morphology. The gully head morphology significantly and directly influences the soil loss rate, which ultimately affected the length of gully head retreat. These findings contribute to a deeper understanding of the role of vegetation in gully headcut erosion, offering a scientific foundation for the implementation of preventive measures against such erosion.