Moisture and Species Effect on Water Colour Generation in an Australian Temperate Forested Catchment

South-eastern Australian forested catchments supply most of Melbourne's drinking water, and discoloured water poses significant challenges for authorities because of the need for complex and costly treatment procedures. Identifying the sources and mechanisms of colour production is therefore critical. We investigated the role of leaf litter from five common catchment tree species in generating water colour under three moisture conditions (Saturated, Moist and Air-dry) by conducting a laboratory leaching experiment. Results showed that leaf litter under Moist conditions (2871.2 ± 383.24 PCU) produced significantly higher cumulative colour compared with Saturated (496 ± 98.34 PCU) and Air-dry (452.2 ± 62.67 PCU) conditions. This is likely due to optimal microbial decomposition under Moist conditions, where both oxygen and water availability are sufficient. In contrast, Saturated samples exhibited a faster initial rate of colour production that peaked earlier (Week 1) than Moist (Week 2) and Air-dry (Week 4) samples, likely driven by a combination of leaching and decomposition processes. In Moist and Saturated samples, decomposition was strongly correlated with the colour generation process. When species effects are considered, Eucalyptus radiata had the highest cumulative colour production (1711.33 ± 892.61 PCU), whereas Eucalyptus regnans had the lowest (730 ± 320.44 PCU). Our findings highlight the critical role of litter moisture in driving colour generation in drinking water catchments. Future research should focus on understanding how changes in rainfall patterns and subsequent litter moisture levels may influence colour production. Such insights could inform management strategies to mitigate water discolouration and reduce treatment costs.