Linking soil macropores, subsurface flow and its hydrodynamic characteristics to the development of Benggang erosion

•The distribution of macropore leaded to the uneven distribution of preferential flow.•The preferential flow was one of the reasons for the collapse of Benggang.•The maximum Reynolds number of subsurface flow was observed near the Benggang wall.•A Reynolds number greater than 3 was a criterion for the occurrence of PF. The increasing population in south China is exposed to Benggang erosion that deteriorates slope stability and agricultural production. However, the influence of subsurface flow, especially macropore flow, on the development of Benggang erosion and the collapse of Benggang remains inadequately studied. To address this issue, we measured the spatial distribution of soil macropores on an active Benggang-eroded slope in a subtropical region in China. During the experimental period of 20151221 to 20180116, two collapse events were observed on the study slope. In addition, the soil water content at the depths of 20, 40, 60, and 80 cm was monitored at a time interval of five minutes at six sites along the slope. The velocities of vertical subsurface flow traveling between measured soil depths were determined based on the difference in the response time of soil water content to rainfall along the depth. Further, the Reynolds number (Re) of subsurface flow in different water patterns, e.g., Darcy flow (DF) and preferential flow (PF) was calculated based on soil pore size and flow velocity. Results indicate that the number of macropores near the Benggang wall (i.e., the most active area of erosion on a Benggang slope) was almost twice that on the upper slope. The presence of abundant macropores near the Benggang wall facilitated faster subsurface flow and a greater variation in soil water content in time and space, which significantly influenced Benggang erosion and collapse. For both the DF and PF, the maximum Re was detected near the Benggang wall. The Re of PF was ten times higher than that of DF. The Re being greater than 3 was the threshold to initiate PF. Moreover, the larger Re of PF likely enhanced the leaching of fine soil particles and promoted the occurrence of Benggang erosion. This study confirms the key role of macropore flow in Benggang erosion, which helps better understanding, modeling and controlling of Benggang erosion.