Development and implementation of a multi-scale city-wide urban stormwater quality model

Background Urban stormwater runoff is a major source of diffuse contamination impacting the ecological health of receiving waterways. Traditional contaminant load models often rely on aggregated land-use assumptions, limiting their ability to identify contaminant hotspots or predict instream concentrations and with few attempts to implement city-wide modelling. To address these gaps, we developed the Instream Contaminant Concentration Model (ICCM) for Christchurch, New Zealand—a cloud-based decision-support system that integrates surface-level contaminant generation with instream water quality outcomes at multiple scales.Methods ICCM combines the MEDUSA 2.0 contaminant load model with treatment simulation and scenario management modules, enabling source-to-discharge modelling for a city of over 400,000 people. Over 225,000 individual impervious surfaces are simulated for TSS, zinc and copper loads for individual events across multiple years.Results Results are aggregated at treatment system, sub-catchment and waterway reporting nodes, supporting compliance with regulatory thresholds. ICCM simulations demonstrate contaminant reductions under progressive implementation of ~500 treatment systems, achieving up to 77% TSS and 71% zinc reductions in some catchments by 2043. The platform’s scenario manager enables rapid evaluation of “what-if” options for treatment strategies and urban intensification.Conclusions This work illustrates how multi-scale and cloud-based modelling can transform stormwater management, providing robust evidence for planning and investment decisions.