Runoff vs Slope
Some real data on Slope Vs Runoff (source: Runoff and Soil Loss Under a Center Pivot)
Also see Table 2–8 in Irrigation Guide
Also see Table 2 in Center Pivot Management For Forage Production
If we assume those numbers correlate to infiltration then slope decreases effective infiltration by around 300%
Its conceptually & nicely increases surface storage (table 3) based on slope and residue cover which i think is missing in a lot of models.