AbstractA two-stage physically-based detention model has been developed for green roof systems. Vertical flow in the substrate is represented by the Richards equation, and horizontal flow in the underlying drainage layer is modeled by the Saint Venant equation. This two-stage physically-based model (2SPB) and the stormwater management model (SWMM) green roof low impact development control model (SWMM-GR) were validated using measured runoff profiles from two contrasting green roof systems: a conventional green roof system; and an innovative system intended to enhance detention. The substrate and drainage layer model parameters were identified from independent physical tests. The 2SPB model provides a reasonable estimation of runoff profiles from a conventional green roof system. However, the model is not capable of representing the flow conditions in the innovative green roof system’s detention layer, such that model results were less accurate for this system compared with the conventional green roof system. SWMM-GR model results for the conventional system were less representative of its overall detention performance due to the assumptions inherent in the substrate percolation model.