A stormwater treatment assessment considers how runoff from roofs, driveways, paving and other impervious areas is managed by the treatment measures connected to them. The presence of a tank, raingarden or permeable surface does not by itself establish that the required treatment outcome will be achieved.

Performance depends on the relationship between catchment area, treatment capacity, rainwater reuse, physical connections and any impervious surfaces that remain untreated. A measure that is appropriately sized and connected can contribute strongly to the assessment. The same measure may contribute much less where it receives only a small catchment or cannot operate as represented in the model.

This guide focuses specifically on how common WSUD measures can support quantitative stormwater treatment performance. For the wider difference between site strategy and modelling, see STORM vs WSUD.