Fallout radionuclide tracers identify a switch in sediment sources and transport-limited sediment yield following wildfire in a eucalypt forest

摘要

Fire can alter sediment sources and transport rates in river basins, changing landforms and aquatic habitats and degrading downstream water quality. Variability in the response between environments, between fires, and with time since fire makes predicting the catchment-scale effect of individual fires difficult. This study applies the fallout radionuclides ~(137)Cs and ~(210)Pb_(xs) to trace the sources and transport of fine sediment through a river network following a wildfire of moderate to extreme severity in the 629-km~2 eucalypt-forested Nattai River water-supply catchment near Sydney, Australia. The tracer analysis showed that post-fire erosion caused a switch in fine (<10 μm) sediment sources from 80% subsoil derived from gully and river bank erosion to 86% topsoil derived from hillslope surface erosion. The fine sediment phosphorus content increased 4-10 fold over pre-fire levels. Annual post-fire sediment yields estimated from suspended solids rating curves were 109-250 times higher than they would have been without fire. A large additional amount of sediment remained stored within the river network for at least four years, particularly in lower-gradient reaches. Analysis of a sediment core showed that surface erosion following a previous fire had supplied at least 29% of total catchment sediment yield over the past 36 years. It is concluded that wildfire can alter catchment sediment budgets in two ways. Firstly, a spatially-diffuse pulse of elevated erosion is associated with moderate or intense rainfall events in post-fire years. Secondly, pulses of elevated catchment sediment yield are driven by the timing and river sediment transport capacity of runoff events. Severe post-fire erosion and high interannual hydrologic variability can result in large sediment stores persisting within the river network for many years. Fallout radionuclide tracers are shown to be useful in quantifying fine sediment sources and transport dynamics following wildfire, and the contribution of wildfire to catchment sediment yield.