Spatial and temporal drivers of water colour variability in temperate Eucalyptus forested catchments under a changing climate

Increasing water colour concentrations in freshwater ecosystems worldwide pose significant challenges for treatment and the sustainable supply of drinking water. While this global trend has driven extensive research, studies remain heavily concentrated in boreal peatlands and lowland forests, leaving critical knowledge gaps regarding colour generation mechanisms in other ecosystems. Well-drained eucalyptus-dominated temperate forests of the Southern Hemisphere represent one such understudied system, yet produce water colour concentrations parallel to those from northern catchments. Understanding colour dynamics in these systems is essential for predicting climate change impacts on drinking water supplies across large regions of Australia and similar temperate forests globally. Therefore, we investigated the spatial and temporal drivers of water colour generation across temperate, eucalyptus forest-dominated drinking water catchments in south-eastern Australia to understand current variability and predict responses to future climate scenarios. We combined spatial surveys across diverse topographic gradients spanning 260 km² of catchments, with a year-long event-based monitoring using automatic samplers at five contrasting study sites, collecting and measuring a total of approximately 650 samples, to capture colour at baseflow and stormflow conditions. Controlled laboratory incubation experiments were conducted for three months, using leaf litter collected across a climate gradient, enabling us to isolate mechanisms of colour generation under different environmental scenarios. Dissolved Organic Carbon concentrations were highly correlated with true colour (r² = 0.75), indicating that colour originates from organic matter decomposition. Our findings reveal a productivity-moisture paradigm: high-productivity systems receiving high precipitation maintain consistently high colour concentrations year-round (108.5 ± 41.1 PCU: mg Pt-Co Equiv.), due to favourable conditions for microbial decomposition of abundant litter. In contrast, lower-productivity areas show pronounced seasonality, where low colour during dry periods (55 ± 14 PCU: mg Pt-Co Equiv.) due to limited microbial activity, but colour concentrations were more than double during wet seasons (123 ± 39 PCU: mg Pt-Co Equiv.) when accumulated dry litter rapidly decomposes. Supporting laboratory experiments also confirmed this mechanism, where litter stored under prolonged dry conditions generated equivalent colour concentrations (785 ± 193 PCU: mg Pt-Co Equiv.) upon rewetting as continuously moist litter (831 ± 161 PCU: mg Pt-Co Equiv.), regardless of initial field conditions. Event-based monitoring revealed that colour peaks, sometimes reaching 177 PCU: mg Pt-Co Equiv., coincide with hydrograph peaks, with predominantly anticlockwise hysteresis loops (67%) indicating distant catchment sources. The asymptotic discharge-colour relationships, which accounted for 50% of the variability in event-flow colour, suggest a finite pool of soluble organic compounds available for leaching during events. We integrated these data with hydrological models, including PERSiST and INCA-C, to investigate catchment-scale processes and climate projections. Our initial results indicate that predicted climate shifts toward prolonged droughts punctuated by intense rainfall will create a boom-bust dynamic, with extended low-colour periods followed by pronounced colour pulses with subsequent storms, amplifying challenges for drinking water management in a changing climate.