Characteristics of slope runoff and soil water content in benggang colluvium under simulated rainfall

The slope-produced runoff time (runoff time) of colluvium in a benggang (collapse hill and erosion gully) is related to rainfall characteristics and water infiltration as well as slope angle. Additionally, soil water content (SWC) directly correlates with rainfall intensity. This research analyzed the slope-runoff characteristics and the SWC distribution of the colluvium in a benggang catchment by simulated rainfall experiments in situ and the detection of the PR2/6 soil moisture profile probe. The simulated rainfall experiments were conducted at Liantanggang benggang in Wuhua county, Guangdong province of southern China. Four old and three new colluvial slopes were selected as the study cases, and the characteristics of their slope runoff and their SWC distribution were investigated using 28 simulated rainfall experiments. A negative power function existed between runoff time and rainfall intensity. Moreover, a negative logarithmic function existed between runoff time and initial SWC at a 10-cm depth in the old colluvium. A 3D model was established to predict the runoff time using rainfall intensity and initial SWC. When the rainfall intensity was >= 3.5 mm min(-1), the runoff time exhibited a linear negative correlation with the slope angle. The simulated rainfall caused variations in profile SWC. At the 10-cm depth, the old colluvium had an SWC over 20%, whereas the SWC of the new colluvium was below 20%. After 24 h passed, the infiltrated water was redistributed and deeper wetting front depths were observed in the new colluvium than in the old colluvium. The results demonstrate that the 'rainfall intensity-initial SWC-runoff time' model for the colluvial slopes of benggang catchments could be used to predict runoff time based on rainfall intensity and initial SWC. The wetting front depth of the profile SWC could imply that the unstable depth of the colluvium may be beyond the maximum depth of 100 cm.