convert Rational C to SCS (NRCS) CN? 1

[JohnnyS]

I saw something on this a long time ago. Wondering if anyone has a resource to find equivalent values for the same land use between Rational and SCS runoff coefficients?

 

[francesca]

The methods are completely different; there isn't a way to convert one to the other.

 

[JohnnyS]

Sure there is! I know they are different methods, but the coefficients have a similar purpose. As such there should be some kind of relationship.

Actually, after I posted, I found what I was looking for in my old textbook. One conversion may be found in, "Introduction to Hydrology" third ed., Viessman, Lewis & Knapp.

 

[TerryScan]

JohnnyS,

You might find it usefull to use the forum's search featrures. Using it, I found this thread which pretty much covers your topic:

http://www.eng-tips.com/viewthread.cfm?qid=70645

 

[JohnnyS]

Thanks TerryScan. I did search before I posted. I guess sometimes the search works better than at other times depending on what words you choose.

The thread you referred me to contains the same conversion in my old text.

 

[blueoak]

Why would you want to convert though? When you are using curve number you are also typically looking at infiltration rates and hydraulic reaction of the shed. If you are just using the EFH2 method I understand, but how hard is it to look up a soils book, look at the watershed and make a guess of CN.

A lot of times the C or CN is based on engineering judgement. Some sheds requires a lot of different weighting and averaging to get an appropriate "average" C or CN that is consistent with the intent of the study.