Modelling seasonal non-point source phosphorus loads from a mixed urban and agricultural land use watershed

Non-point source phosphorus (P) loads from urban watersheds can impair downstream water quality. Factors influencing P loads including land use and seasonality are currently not well understood. Further, the suitability of available modelling tools to predict these loads is unclear. In this study, the build-up wash-off model in the Storm Water Management Model (SWMM) was used to simulate total suspended solids (TSS), total phosphorus (TP), and soluble reactive phosphorus (SRP) loads from a mixed land use watershed. The model performed well for simulating event and seasonal loads but was less able to capture intra-event concentration variability, specifically for SRP. In warm-weather months, the model indicated different seasonal trends for urban and agricultural land uses between TSS, TP, and SRP loads. SRP loads were highest in the fall season, particularly from old low-density residential areas, and are predicted to increase with urbanization. The study findings can help inform future strategies to manage P loads from mixed land use watersheds.