Combining elemental fractions as novel tracers with hysteresis analysis to improve the quantification of sediment sources during large storm events in (sub)tropical catchments

Sediment pollution in (sub)tropical rivers and lakes of Queensland and East Africa is rapidly increasing, largely driven by subsurface erosion of deep alluvial and volcanic soils. These regions experience strong rainfall variability and flooding linked to climate and topographic controls, resulting in highly episodic soil loss and sediment transport. However, monitoring sediment sources during extreme events in remote (sub)tropical catchments remains challenging, meaning current understanding is often based on low temporal resolution data or visually dominant erosion features. In addition, the current set of sediment tracing approaches struggle to discriminate sources in deep tropical and alluvial soils or behave non-conservative in these environments.

This study addresses these methodological limitations to improve quantification of dominant sediment sources and soil loss processes in (sub)tropical catchments. We combine multiple water and suspended sediment monitoring tools, including low-cost automatic samplers, to capture the fluxes and variability of suspended sediment. We subsequently developed a novel sediment fingerprinting approach based on sequential extraction of elemental soil fractions. This tracer framework enables discrimination not only between catchment zones but also among multiple subsurface soil layers in deep alluvial and volcanic profiles. The tracer data are integrated into mixing models and event-scale sediment hysteresis analyses to construct dynamic sediment budgets and capture non-linear sediment responses to extreme rainfall.

Our results reveal the critical role of downwearing and chemical dissolution processes in large alluvial gullies of northern Queensland. These processes are largely neglected in current catchment models and gully analyses because they are not evident from repeat imagery assessments of gullies that demonstrate headcut retreat and bank collapse. In the Albert River (Southeast Queensland), we show that flooding associated with tropical Cyclone Alfred contributed approximately 60% of annual sediment export, dominated by erosion of subsurface soils from recent urban developments. This contrasts with earlier assessments in which radionuclide tracers provided only a binary subsurface signal, which together with visually evident bank collapse from aerial imagery led to attribution of sediment sources to alluvial bank erosion. Overall, our approach demonstrates how sediment source contributions and gully erosion processes shift dynamically during storm events, offering improved process understanding and more targeted management options under increasing climate extremes.