Event Mean Concentration-based Modelling Approach for a Large Scale Spatially Heterogenous Catchment using SWMM

Abstract This study evaluated the suitability of Storm Water Management Model (SWMM) for continuous rainfall-runoff modelling and investigated the Event Mean Concentration (EMC) approach in quantifying total nitrogen (TN) and total phosphorus (TP) wash off to the Maribyrnong river in Australia. The model demonstrated satisfactory performance for stream flow simulation during calibration (R 2 = 0.551, NSE = 0.522, KGE = 0.724, PBIAS = 3.96%) and validation (R 2 = 0.6695, NSE = 0.603, KGE = 0.503, PBIAS = 19.03%). For water quality, only TN concentrations in streamflow were satisfactorily validated (R 2 > 0.30 and PBIAS < ± 70%). Further, correlation analysis revealed the intricate role of land use in nutrient dynamics, highlighting the multi factor interdependency on runoff volume, wash off concentration and land use characteristics. Results suggest that SWMM can be effectively used for large scale, spatially heterogenous catchments, provided careful parametrization and calibration are undertaken. While correlation analysis supports the validity of EMC transferability, validation of TN concentration show less sensitivity to temporal pattern, highlighting the need for development of dynamic EMCs.