How does land use/cover influence gully head retreat rates? An in-situ simulation experiment of rainfall and upstream inflow in the gullied loess region, China

Land use/cover plays a crucial role in gully head retreat (GHR). However, little is known about how land use/cover influences GHR rates. An in situ simulation experiment of rainfall and upstream inflow was conducted in a gullied loess region to investigate hydraulic erosion, mass movements, and linear GHR processes under three types of land use/cover (bare land, grassland, and shrub-grass land). The results showed that the average linear GHR rates on grassland and shrub-grass land were 87-89% and 72-81% lower than that on bare land (0.99-2.06 cm min(-1)), respectively. Gully heads retreat by hydraulic and gravitational erosion. In the case of hydraulic erosion, upstream runoff incision is dominant on bare land, while undercutting by on-wall and jet flow dominates on grassland and shrub-grass land. In the case of mass movement, collapse dominates with a frequency of 62-100%, of which gully sidewall collapse is most common, especially on bare land and it acts to widen the gully. Gully headwall collapse dominates on grassland and shrub-grass land to cause retreat of the gully head. Overall, on bare land, upstream runoff incision dominates GHR. However, on grassland/shrub-grass land GHR is mainly driven by the undercutting of on-wall and jet flow and subsequent gully headwall collapses. As a consequence, the GHR length on bare land exponentially increases over time, while on grassland or shrub-grass land, it discretely increases as an analogous step function. Moreover, the average linear GHR rate on grassland was considerably lower than that on shrub-grass land, implying that an optimized vegetation restoration pattern enhances GHR control.