Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

CN Method & issues for obtaining a more correct model

CN Method & issues for obtaining a more correct model

CN Method & issues for obtaining a more correct model

Over the past decade the NRCS CN Method (particularly the selection of a composite CN) has been shown to be fairly inaccurate when modeling smaller storm events (< 2 inches).  The problem is an underestimation of runoff depth and this affects designs for water quality and stream channel protection among other issues.  

D. Woodward and R. Hawkins wrote an interesting article titled, "Runoff CN Method: Examination of the Initial Abstraction Ratio" that advocates revising the Ia/S ratio to 0.05 AND changing the formula for S.  The article may be found on NRCS web site under Hydraulics & Hydrology – Technical References  http://www.wsi.nrcs.usda.gov/products/W2Q/H&amp;H/tech_refs/CNarchive.html  

Implementing the recommendations in the paper combined with using  a distributed CN when setting up models would help fix some of the problems.  However, there is no model model capable of being adjusted at this time to implement the paper's suggestion (one can't just modify the Ia/S ratio).

Prof. Richard Hawkins told me that the likelyhood of the NRCS revising the TR-20 code anytime soon is remote due to initia of various forms.  Peter, are you aware of the paper, the issues with the CN method and do you have any thoughts on providing an alternate runoff methodology modifying the current CN Method or a model using the Green-Ampt infiltration equation?


RE: CN Method & issues for obtaining a more correct model

As you may be aware, the Ia/S ratio CAN be changed in HydroCAD.  You will find this on the Advanced tab of the calculation settings screen.  This capability was added a number of years ago in order to provide more flexibility in modeling small rainfall events.

If you need to use different CN values, these can be entered directly.  Or you can customize the CN lookup table as required.

As for alternate runoff equations, we can certainly consider this for a future release.

BTW: The paper you referred to is listed under a slightly different title.  It's the seventh item on the page:

Peter Smart
HydroCAD Software

RE: CN Method & issues for obtaining a more correct model


I read with interest two of the articles you provided the link to. Thank you.

But, I note that the NRCS(SCS) viewpoint is, as always, focused on agricultural lands and concerns. Most practicing engineers, in the USA I know are usually called upon to analyze urban or urbanizing watersheds where soil type, cover crop, grading practices and similar farming practices are pretty much irrelevant.

I also note that the articles pay scant attention to the question of calibrating hydrology models. They "hint" that his might be a good thing to do but offer no real help.

This is probably the wrong forum to discuss these things so I apologize to Mt. Smart for horning in on his territory. If this interests you, as it does me, there are some good links in the Storm Flood forum describing calibration and urban watershed hydrology.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close