Soil moisture dynamics near a gully head in relation to the trigger of collapse in granite red soil slope in southern China

Gully head collapse is caused by the combined action of water and gravity, resulting in the loss of soil and water in granite soil slope of southern China. However, the soil moisture dynamics near a gully head and the mechanism triggering collapse are still unclear. A five-year high-time-resolution soil water content (SWC) dataset was obtained from soil moisture sensors that were buried at four to seven depths at three positions near a typical gully head on a hillslope in Hubei Province. From 2016 to 2020, two large collapse events were recorded in June in 2016 and 2017. The results showed that the soil moisture regimes were relatively heterogeneous in terms of spatial variation near the gully head: the closer to the gully head, the lower the SWC; the closer to the surface, the greater the SWC variation. This was attributed to the different texture and hydraulic properties of the soil layers. Moreover, the SWC showed obvious periodic dynamics on a seasonal and annual basis, which was consistent with the seasonal precipitation pattern. Although single heavy rainfall events increased the SWC at the surface, high SWC at deep sandy soil layers (≥130 cm) occurred only late in the rainy season after a long period of infiltration during the wet years of 2016 and 2017. Only when the antecedent SWC reached a certain threshold at the deep soil (0.37 m3 m−3, 0.22 m3 m−3, and 0.30 m3 m−3 at 130 cm, 210 cm and 270 cm depth, respectively), the additional heavy rain (53.6 mm in 9 h) triggered the gully head collapse. The exact time of collapse was indicated by a specific instantaneous variation in SWC near the gully head, when the mean SWC in the soil profile increased to high level, whereas the standard deviation of SWC was low during the heavy rainfall. The results confirm that gully head collapse is driven by rainfall and a high SWC in the deep sandy soil layer, as well as provides a useful reference for soil and water conservation control.