Gully internal erosion triggered by a prolonged heavy rainfall event in the tableland region of China's Loess Plateau

Gully erosion is a severe form of soil erosion, but gully internal erosion processes are poorly understood, especially at the event scale. To investigate gully internal erosion intensity and understand the related gully development mechanism in an agricultural environment with gully head stabilization and vegetation restoration efforts, two successive field investigations were carried out just before and after a prolonged rainfall event in 2021 in the tableland region of China's Loess Plateau. Thirteen gullies were investigated and all experienced gully internal erosion, while most gully boundaries were stable during the heavy rainfall event based on the comparison of the UAV digital orthograph maps (DOMs acquired with Unmanned Aerial Vehicle) before and after the rainfall event. The proportion of gully internal erosion area to gully internal area of the 13 investigated gullies ranged from 3 to 55%, with average areal erosion proportion of the gully sidewall and gully bed of 21% and 36%, respectively. The erosion area of subdrainage units (SDUs) on the gully sidewall was positively correlated to the SDU area, average SDU slope gradient and vegetation type, while the erosion area on the gully bed was positively correlated to the gully area, gully depth and gully bed slope gradient. Gully internal erosion was not significantly correlated with gully drainage area because the connectivity between the upslope and gully areas was interrupted and the effective drainage area of the gully was obviously reduced by soil erosion conservation measures, including terraces on the upslope drainage area, shrub belts, and water barriers. Thus, gully internal erosion is still active under the heavy rainfall storm against the background of the ‘Grain for Green’ and ‘Gully Stabilization and Tableland Protection’ programs, and integrated measures for preventing both gully expansion and gully internal erosion must be further enhanced in the context of climate change.