Hillslope gully erosion in savanna rangelands tributary to the Great Barrier Reef: Catchment hydrogeomorphic processes, sediment and nutrient fluxes
Gully erosion is a globally significant land degradation phenomenon and a particularly important issue in the savanna rangelands tributary to the Great Barrier Reef (GBR) World Heritage Area, northeast Queensland, Australia. In this region, gullies provide the primary sources of sediment and particulate nutrients (e.g., nitrogen and phosphorus) to the coast, which have detrimental impacts on the health of the reef ecosystem. Effective management of gully erosion in the GBR catchments, and tropical savannas more broadly, is constrained by a limited understanding of the key hydrological and geomorphological processes driving gully development. The effect of changing land management on sediment and nutrient fluxes within gullied catchments, also remains poorly understood.
The aim of this thesis is to improve understanding of hillslope gully erosion processes and fluxes of sediment and nutrients, in savanna rangelands tributary to the GBR. A novel, low-cost topographic survey technique, known as structure-from-motion with multi-view stereo photogrammetry (SfM), was first tested in the Upper Burdekin catchment. SfM was found suitable to quantify gully density, temporal gully geomorphic change, and catchment characteristics that influence gully development. High resolution mapping of hydrological flow pathways enabled spatially explicit prediction of where gully extension is likely to occur as a result of overland flow. Application of an index of hydrological connectivity clearly demarcated parts of the hillslope most connected to the gully network. Bare and scalded areas, roads, and cattle trails were identified as important runoff source areas and hydrological conduits driving gully extension. Analysis of three-dimensional SfM topographic models enabled examination of key within-channel erosion processes, including mass wasting, rilling and bed scour. Finally, 17yr trends in rainfall, runoff, ground cover and water quality, in three headwaters of the Upper Burdekin catchment exposed to different grazing land management strategies, were evaluated. Reductions in grazing pressure in one catchment (Weany Creek) at the onset of monitoring, did not reduce runoff or decrease sediment and nutrient loads, with annual and event loads similar in magnitude to a heavily grazed catchment (Wheel Creek). Exclusion of cattle from another catchment (Main Creek) only led to improvements in recent years following a period of above-average rainfall, suggesting recovery trajectories in these degraded systems are slow and strongly influenced by climate.
The thesis contributes valuable new knowledge and understanding of gully erosion processes, as well as landscape and water quality recovery trajectories following land management change. Ultimately, the research will aid in the development of improved process-based models of gully initiation and evolution, more accurate estimation of sediment and nutrient delivery to receiving water bodies, and more informed prioritisation of catchment remediation strategies.
Jack Koci is a passionate environmental scientist with expertise in tropical hydrology, geomorphology and water quality. He is currently completing a PhD within the Sustainability Research Centre at the University of the Sunshine Coast. Prior to commencing his PhD in 2016, Jack worked as a Research Officer at the Australian Centre for International Agricultural Research (ACIAR), where he developed and managed agricultural research for development projects across the Asia-Pacific region. Jack completed a Bachelor of Science majoring in Hydrology and Water Resources with Honours (Class I) at James Cook University in 2012.