Changes in sediment loads and discharge from small mountain catchments following wildfire in south eastern Australia

A severe wildfire burnt over 1 million ha of forested land in southeastern Australia in early 2003. Included in the burnt area were two 136 and 244 ha hydrologic research catchments in the East Kiewa valley that had been decommissioned following a 10 year study, and an adjacent larger (10,050 ha) gauged catchment, the West Kiewa River. The three catchments were re-instrumented to measure discharge and suspended sediment fluxes at 15 min timesteps, with additional measurement of bedload at the East Kiewa sites. Analysis of annual rainfall-discharge relationships and of flow duration curves for the first 2 post-fire water years indicated annual flow increases of around 65-75% for Slippery Rock Creek in both post-fire water years and 40% and 76-94% for Springs Creek in 2003-2004 and 2004-2005, respectively. The flow impacts on the less severely burnt West Kiewa River were far more subdued, and did not exceed pre-fire variability in annual flows. Flow duration curve analysis did not indicate changed runoff generating processes at the daily timescale, with the curves shifting upward uniformly over the flow regime. Total sediment exports increased by factors of 8-9 (up to 2.96 t ha−1 yr−1) from the East Kiewa catchments in year 1 after the fire, diminishing to a 2-4-fold increase in year 2. Flow increases accounted for much of the year 2 exports as suspended sediment concentrations decreased toward pre-fire levels. Post-fire suspended load exports from the larger West Kiewa catchment were much lower per unit area, probably reflecting the smaller area of high intensity burn, less intensive burning of riparian areas and possible scale effects. The results were observed under near-average rainfall conditions, with the exception of one short duration high intensity storm that produced almost 50% of the combined sediment loads from the East Kiewa catchments.