CN Method & issues for obtaining a more correct model

[snowhund]

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&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?

Erik

 

[psmart]

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:

http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/docs/H&H_papers/curve_number/CN_init_abstr.doc

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[RWF7437]

snowhund,

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.