There are Four factors in Rainfall Dependent Infiltration and Inflow or RDII in SWMM 5:
1. The fractional response to Rainfall or R from 0 to 12. The Time Base of the Unit Hydrograph or T in hours * Dimensionless K Shape Factor
3. The Sewershed Contributing Area in acres or hectares and
4. The Maximum, Initial Abstraction and Recovery Rate for R on a Monthly Basis in units of inches, mm or mm/day,
5. The fifth and probably the most important factor is the Rainfall
