NRCS Lag

[rlondeen]

What are the limitations of the NRCS Lag method for determining Tc? I know there is a range for the CNs used (50 to 95) but are there any risks to using this method with very small watersheds or using this Tc in the Rational Method?

 

[psmart]

I suggest that you review NEH Chapter 15, which is the authoritative reference on SCS Tc procedures. It states in part:

Applications and limitations
The watershed lag equation was developed using data from 24 watersheds ranging in size from 1.3 acres to 9.2 square miles, with
the majority of the watersheds being less than 2,000 acres in size (Mockus 1961). Folmar and Miller (2000) revisited the development of this equation using additional watershed data and found that a reasonable upper limit may be as much as 19 square miles.

Peter Smart
HydroCAD Software

http://www.hydrocad.net