Why do I get the warning message "The catchment is currently set to modified rational method. The rational method will be used during calculations"?
You most likely have the GVF Rational (StormCAD) solver selected in the calculation options (Analysis > Calculation Options). This solver assumes the rational method for the catchment even if the catchment is set to "modified rational".
The catchments for the rational method and the modified rational method have the same input: Tc, C and the area, and both methods use Q=CIA. However, the modified rational method creates a hydrograph, while the rational method does not since it is just steady state. If you need to see a hydrograph based on the modified rational method, you will need to select another solver that will calculate an unsteady simulation (flow over time).
Understanding the Modified Rational Method