Athukoralalage et al. (2026) The impact of a mega-flood event on the water quality of the southern Murray-Darling Basin, Australia

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-03-03
• Authors: Dilanka Athukoralalage, Qingji Zhang, Richard W. McDowell, Luke Mosley
• DOI: 10.1016/j.jhydrol.2026.135236

Research Groups

• School of Agriculture, Food and Wine, Adelaide University, Adelaide, SA, Australia
• School of Biological Sciences, Adelaide University, Adelaide, SA, Australia
• AgResearch, Lincoln Science Centre, Lincoln, New Zealand
• Faculty of Agriculture and Life Sciences, Lincoln University, Lincoln, New Zealand

Short Summary

This study investigated the impact of a 2022–2023 mega-flood and five other major flow events on Total Nitrogen, Total Phosphorus, and Dissolved Organic Carbon dynamics in the southern Murray-Darling Basin, Australia. It found that the mega-flood significantly increased nutrient loads and prolonged water quality degradation, particularly in downstream areas due to extensive floodplain inundation and delayed nutrient release.

Objective

• To analyse water quality responses across several major flow events along the southern Murray-Darling Basin.
• To compare the nutrient concentrations and yields of the 2022–2023 mega-flood with other large events over the past decade.
• To examine nutrient behaviour across the pre-flow, high-flow, and post-flow phases.
• Hypothesis: Extensive floodplain reconnection during the mega-flood would generate proportionally greater nutrient yields and more pronounced hysteresis as floodwaters receded.

Study Configuration

• Spatial Scale: Southern Murray-Darling Basin (MDB), Australia. Three study sites along the River Murray: Jingellic (headwater, catchment area 11,839 km²), Torrumbarry (midstream, catchment area 56,173 km²), and Swan Hill (lowland, catchment area 83,069 km²). Floodplain inundation analysis covered river sections from Hume Dam to Torrumbarry (77 km) and Hume Dam to Swan Hill (80 km).
• Temporal Scale: 2011 to mid-2023. Six major flow events (FL-1 to FL-6), including the 2022–2023 mega-flood. Each event was analysed using consistent 5-month windows for pre-flow, high-flow, and post-flow phases.

Methodology and Data

• Models used:
  • LOADEST seven-parameter model (for estimating missing nutrient values).
  • River Murray Floodplain Inundation Model (Overton et al., 2006) (for estimating inundation extent).
• Data sources:
  • Continuous daily flow data from the Murray–Darling Basin Authority (MDBA).
  • Bi-weekly nutrient concentration data (Total Nitrogen, Total Phosphorus, Dissolved Organic Carbon) from MDBA.
  • 30 meter digital elevation models and MDBA catchment boundary shapefiles.
  • Land use data from the Department of Agriculture webpage.

Main Results

• The 2022–2023 mega-flood (FL-6) exhibited substantially greater total flow volume (e.g., 5.94 x 10⁹ m³ at Torrumbarry) and maximum water levels compared to other flow events.
• FL-6 contributed over 18% of the total Total Nitrogen (TN) yield and over 20% of the total Total Phosphorus (TP) yield over the entire 11-year study period within just five months at downstream sites (Torrumbarry and Swan Hill).
• Nutrient concentrations (TN, TP, DOC) generally increased from headwater to downstream sites, with peak concentrations during FL-6 at downstream sites (e.g., peak TP increased from 0.25 mg/L to 0.34 mg/L at Torrumbarry and from 0.2 mg/L to 0.5 mg/L at Swan Hill during FL-6 compared to FL-3).
• Downstream sites (Swan Hill, Torrumbarry) exhibited distinct counter-clockwise hysteresis during FL-6 (Hysteresis Index (HI) values from -0.09 to -0.27), indicating higher nutrient concentrations on the falling limb and delayed release from extensive floodplain inundation.
• The upstream headwater site (Jingellic) showed weak clockwise hysteresis during FL-6 (HI values from +0.03 to +0.11), reflecting rapid hillslope-driven mobilization on the rising limb.
• FL-6 resulted in significantly larger floodplain inundation (e.g., >105,000 hectares between Hume Dam and Swan Hill) compared to FL-3 (>36,000 hectares).
• FL-6 showed the lowest TN:TP ratios (<10–15 at peak) among all events, suggesting a shift towards phosphorus-enriched loads associated with soil and sediment mobilization.
• High Dissolved Organic Carbon (DOC) yields during FL-6 (exceeding 10–20 mg/L at downstream sites) indicate a potential for hypoxic blackwater events and require longer recovery times.

Contributions

• Provides a novel, multi-site assessment of nutrient dynamics during the largest flood in the southern Murray-Darling Basin since 1956.
• Integrates concentration-discharge patterns, event-based nutrient yields, and quantitative hysteresis metrics to characterize how large-scale overbank inundation alters nutrient transport pathways.
• Demonstrates that climate-driven mega-floods lead to proportionally larger nutrient loads and prolonged water-quality degradation, with distinct spatial differences in nutrient mobilization (rapid hillslope vs. delayed floodplain release).
• Highlights the importance of antecedent catchment conditions and the availability of transport-ready nutrient stores in determining the magnitude of nutrient export during extreme events.

Funding

The Murray–Darling Basin Authority (MDBA) is thanked for the provision of water quality and hydrological data.

Citation

@article{Athukoralalage2026impact,
  author = {Athukoralalage, Dilanka and Zhang, Qingji and McDowell, Richard W. and Mosley, Luke},
  title = {The impact of a mega-flood event on the water quality of the southern Murray-Darling Basin, Australia},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.135236},
  url = {https://doi.org/10.1016/j.jhydrol.2026.135236}
}