Erosion and nutrient loss resulting from Ash Wednesday (February 1983) wildfires a case study

Summary

A wildfire on Ash Wednesday (16 February 1983) near Warburton in the Victorian Central Highlands left large areas of burnt forest in a highly erodible state and, in one particular locality, an intense thunderstorm of short duration six days after the fire caused gross erosion of catchment slopes and gullies. Limited sampling and measurement were undertaken in a 35 ha study area to indicate the causes and extent of erosion and to estimate quantities of nutrients lost during and following the wildfire. Recognising the imprecision of the data due to restricted sampling, it was estimated that in the order of 800 t of ash plus loose soil were washed from the study area (equivalent to about 22 t ha⁻¹), and that this eroded material contained about 2 900 kg of nitrogen and 220 kg of phosphorus. Losses of nitrogen and phosphorus through erosion together with volatilisation and convective transfer of ash were each estimated at approximately one-third of the total quantity of these nutrients held in the above-ground biomass. Soils in the study area were observed to be hydrophobic for more than three months following the wildlife. It is concluded that burnt forests can remain in a highly erosive state for protracted periods until the soils lose their hydrophobicity and revegetation commences.