As options are selected from the drop down lists, the appropriate runoff coefficient is selected.
The runoff coefficient is calculated by looking up the values in table 1 in document Hydraulics-07-F.pdf, authored by ???.
//Curve Numbers are calculated by looking up the values in the tables 2-2a to 2-2d in document TR-55.pdf, authored by Natural Resources Conservation Service (NRCS). TR-55 is a widely used approach to hydrology in the US.