Changes in DOM and alum doses for two rivers of contrasting catchments after intense wildfires

Abstract

The dynamics of dissolved organic matter (DOM) in two river waters were investigated after their catchments had been severely burnt in the 2019/20 Australian wildfires. Shortly after these wildfires, dissolved organic carbon (DOC) concentrations were recorded at high levels (∼ 19 mg/L & 30 mg/L) and these became much lower (up to ∼80% less) in the following winter when river flows had increased. Satellite imagery-based data indicated up to 95% of catchment areas burnt and up to ∼50% subsequent vegetation recoveries after two years. Shifts in burn index values for the burnt areas coincided with DOC concentration variations. Specific colour of waters increased up to 40% as daily river flows increased, indicating higher input of humic content from the burnt catchments. Chlorophyll-a was detected at highest levels in waters soon after the fires when river flows were lowest. Enhanced alum doses were predicted using two feed-forward models; one based on DOC and turbidity data and the other based on UV@254nm, colour and turbidity. Doses predicted using the two models showed high correlations (r > 0.9) and were highest for waters directly after the fires. These models were developed for diverse source waters including those impacted by extreme climate events.