Curious about curve numbers

Dear Group:

While CNs are not a part of SWMM, many members have had a lot of experience with CNs - I would like to float a few questions regarding accounting for forest cover.

In the TR-55 manual(7/98) -2-8 curve numbers are assigned for two forest cover types: Aspen-oak and pinion-juniper, which are respectively deciduous and coniferous, but TR-55 shys from that distinction for some reason - maybe not enough data for generalization.

In winter Aspen and some oaks loose their leaves and interception drops off compared to summer foilage conditions. The pinion-juniper group of CNs range from 12 to 50% greater than the Aspen-oak group for same soils and "hydrologic condition".

With summer foliage conditions, one might expect the CNs for coniferous and deciduous to be fairly close. In winter, one would expect greater runoff from deciduous due to no foilage (larger CN) than in summer.

Are the large differences between the tree classes due to seasonal differences in interception? Or possibly seasonal variation in soil suction a la Green Ampt? Are these processes being reflected in the large difference in curve numbers?

The "Initial abstraction" Ia is supposed to account for interception and infiltration, among other processes. The footnotes to the table state that Ia is assumed to be average of 0.2S. It appears however that the CNs are accounting for differences due to interception and infiltration instead of Ia.

Does anyone have any comments on this, or know what foilage condition the published CN values are for?

Thanks,
Ron Kilmartin

Hello Ron;

There are some brief, though good, references on how to estimate the CN for forest and woodlands. They all seem to be based on the work done and published by the US Forest Service in the 1950's. Here are some which may help to explain the subject matter in more detail than what's depicted in TR-55 manual:

- USDA,SCS,1972:" NEH section 4- Hydrology ", GPO, Washington D.C. (see chapters 8 ,9 and 10).

- US Bureau of Reclamation,1977: "Design of Small Dams",GPO, Washington D.C. - 2nd edition,Revised Reprint (see App.A- Estimating Rainfall From Soil & Cover Data).

- Storey,H.C.,Hobba,R.L. & J.M.,Rosa,1964: " Hydrology of Forestlands and Rangelands ", Section 22, V.T.Chow(editor-in-chief),Handbook of Applied Hydrology ,Mc Graw Hill.

The refs. above may not give fully contenting answers to your question(s) but may help to clarify how they looked at it. The Initial Abstraction (which includes the interception, infiltration and surface storage occurring before runoff begins) is empirically determined to be (0.2 S) on the average. A simplistic description of the interrelations of the AMC,S and CN is given in chapter 10,SCS- NEH4,1972.

Regards,
Mostafa Bozorg-Zadeh