Controls on gully formation following forest clearing in a humid temperate environment

Abstract

We have constructed a chronology of gully initiation, forest clearing, and rainfall events for gullies eroded into pine plantations near Bombala, southeastern Australia, over the last 15 years. The chronology suggests that daily rainfall of 80-100 mm, which has a recurrence interval of 1.4-2 years, can initiate gully erosion on areas cleared of native forest within the previous year. Massive gully erosion was experienced from a daily rainfall of 200 mm with a recurrence of 10-15 years. Resistance to channel initiation effectively recovers within a year of disturbance, allowing only a limited opportunity for erosion. Analysis of the spatial pattern of gully erosion, using a digital elevation model, shows that gullies were initiated across all landscape positions. In contrast to previous studies, there is no clear topographic threshold that limits the extent of the gully network. We infer that the weak topographic threshold results from low resistance to scour, allowing local flow convergence to dominate over topographic accumulation of flow. Although resistance to scour is low relative to previous studies, a process threshold for gully initiation is still a useful simplification of the erosion processes. For the soils that we studied, the threshold for gully erosion relates to intense scour exposing erodible subsoils.