Dominant hydro-climatic drivers of nutrient dynamics: Evidence from multiple catchments in the southern Murray–Darling Basin

Hydroclimatic variability and catchment characteristics, including slope, land use, and agricultural intensity, are key drivers of nutrient export and retention, and river water quality. Climate change is increasing the frequency and severity of extreme events, such as droughts and floods, further influencing these dynamics. The southern Murray–Darling Basin, Australia’s largest river system, provides a case study to examine how these factors shape long-term nutrient dynamics in under-studied arid and semi-arid regions. We applied a Random Forest model to assess drivers of water quality across nine catchments, including hydroclimatic, soil, topographic, land use, and cumulative Standardised Drought Index (SDI) variables. A 40-year dataset enabled robust analysis of spatial and temporal patterns, with models showing moderate to strong performance (NSE>0.5; R² = 0.41–0.80). Rainfall, cumulative SDI, and low soil moisture each contributed 15–20% relative importance in predicting Total Nitrogen (TN), Total Phosphorus (TP), and oxidised nitrogen (NOx). Evapotranspiration (∼15%) was also influential and associated with lower rainfall (p < 0.05). Post-drought nutrient loads (TN, TP, SRP) increased with drought severity, reflecting accumulation during dry periods and mobilisation during rewetting. These results highlight the dominant role of hydroclimatic controls on water quality and inform adaptive management under increasing climate variability.