Factors related to gully erosion in woody encroachment in south-eastern Australia

Abstract

Gullies initiate when the equilibrium within a drainage line is upset by increased discharge or decreased soil resistance to detachment and transport of soil particles, and is triggered and accelerated by human-induced changes to vegetation cover. Woody encroachment is the increase in density, cover, extent and biomass of woody plants, and can result in low ground cover, which increases the potential of soil erosion. This paper reports the relationships between gully erosion and site characteristics such as topography, vegetation cover and road infrastructure in a 120-km² study area of semi-arid woodland subject to woody encroachment in south-eastern Australia. This study characterised gully morphology and activity, identified the factors related to gully volume, and estimated the topographic thresholds for sub-catchments subject to woody encroachment. Firstly, relationships between gully volume, activity, topography, vegetation cover and road infrastructure were explored. Secondly, site characteristics of 32 sub-catchments with and without gullies were compared. Finally, the topographic thresholds for unstable and stable sub-catchments with and without gullies, respectively, were estimated using the slope – drainage area relationship (S = aA⁻ᵇ). All gullies were active and high gully volume was related to large sub-catchments, long gullies and short distances to roads. Stable sub-catchments (without gullies) had gentler slopes, higher foliage projective cover and ground cover than unstable sub-catchments (with gullies). When data for sub-catchments with and without gullies were combined, the main factors related to gully volume were drainage area at the gully or stream heads and their local slope, and ground cover. The mean topographic threshold for gully development provided insights into the drainage area and slope conditions needed for gully development. Gully erosion in the study area is most likely the result of interactions among topography, vegetation and human-made structures (roads) in space and time. The results of the study advance our understanding of gully erosion dynamics in areas of woody encroachment, and can be used to identify areas susceptible to gullying.