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 1

2.   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

via Blogger http://www.swmm5.net/2013/08/there-are-four-factors-in-rainfall.html