Background Lanzhou city has carried out many land construction projects due to urban expansion and formed many landfill loess sites. Its structure is loose and lacks necessary protective measures. Under the action of extreme rainfall, soil erosion is serious. Therefore, it is of great significance to reveal the mechanism of soil erosion under soil-water coupling by exploring the influence of water content change caused by early rainfall on soil erosion.

Methods Based on the field investigation, this study selected the typical landfill loess site in Lanzhou city to sample, and used the indoor straight channel generalized model test to carry out the simulated rainfall erosion test of the landfill loess gully bed, combined with the soil erosion phenomenon and the volumetric water content, pore water pressure analysis and study.

Results 1) The erosion process of landfilled loess gully bed is dominated by the interaction of hydraulic erosion and gravity erosion. With the increase of soil water content, loess landfill is prone to collapsibility and liquefaction, and hydraulic erosion is enhanced. 2) The variation rule of soil water content and pore water pressure during the development of gully bed erosion are obviously consistent, whose significant fluctuations during the soil instability and large erosion damage processes are the key factors for soil structure damage and erosion collapse. 3) With the increase in water content of the soil body caused by the previous rainfall, the shear strength of the soil body decreases, the water flow scouring the gully bed is prone to erosion and damage, the erosion rate of the gully bed increases, and the evolution of retroactive erosion is accelerated.

Conclusions This study shows that the change of water content caused by pre-rainfall has a significant effect on soil erosion. Pre-rainfall will destroy the surface soil erosion and cause the increase of water content, which can effectively weaken the soil strength and aggravate the soil erosion damage. The results are expected to provide a reference in soil erosion controlling in arid loess areas, while putting further insights into considering the effects of integrated soil-water interactions of loess landfill and the importance of soil pore structure on gully bed erosion characteristics under rain scour conditions in future study.