For an explantion go to:

http://pdhcenter.com/

See course H102

good luck

Here's where the hydrology purists and engineers who have to design things butt heads.

Rational does not give a volume of runoff (cf), only a peak rate (cfs).  The Modified Rational allows for construction of a volumetric hydrograph using the Rational peak rate, Tc and storm duration...rate as a function of time, volume as the integral.

Hydrology purists usually reject this method as a rough guess based on incorrect data, and technically, they are right.  Engineers, at least in my experience, largely accept this method as valid and conservative enough for small, largely homogenous (parking lots, grass fields) watersheds.  I have used Mod. Rational, I actually prefer it in those situations.  I have reviewed and approved plans based on calculations from it.

I hope that answer was not TOO evasive...

--Steve