The U.S. Environmental Protection Agency (EPA) Hydrological Simulation Program-Fortran (HSPF) model simulates watershed hydrology and the movement of pollutants over pervious and impervious land, as well as instream channels. The accuracy of modeling channel hydraulics is important for proper total maximum daily load estimation. Therefore, EPA created best management practice tools for the model, which helps users model green and gray infrastructure practices. The online tools enable users to create hydraulic function tables, which establish depth–area–volume–discharge relationships for engineered systems. To model infiltration via green infrastructure in small urban watersheds, try the green infrastructure tool.