SWMM 5, Watersheds, Water Quality, Technology, Hydrology, Hydraulics - Watershed

CN Infiltration

Soil Conservation Service Curve Numbersti

One of the advantages of use of the curve number (CN) method for loss estimates (Section 3.3) is the highly structured method for CN estimates provided by the NRCS (SCS, 1972,1988; McCuen, 1998, Bedient and Huber, 2002 and virtually every hydrology text). Such estimates are embedded in engineering practice through Table 3-18, in which CN values are given as function of land use and soil Hydrologic Soil Group (A through D). Hydrologic Soil Group has the meaning indicated in Table 3-17, ranging from well-drained (type A) to poorly drained (type D). Every soil in the United States has an A-D classification, or sometimes a dual classification, such as B/D, meaning drained (artificially) and undrained (natural) condition.

Table 3-17. Hydrologic soil group meanings (SCS, 1972, Chapter 7).

Group	Meaning
A	Low runoff potential. Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep, well to excessively drained sands or gravels.
B	Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well to well-drained soils with moderately fine to moderately coarse textures. E.g., shallow loess, sandy loam.
C	Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine textures. E.g., clay loams, shallow sandy loam.
D	High runoff potential. Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a clay-pan or clay layer at or near the surface, and shallow soils over nearly impervious material.

Hydrologic Soil Group is provided on the Soil Survey Interpretation sheets discussed in Section 3.8.8.2. For instance, the Conestoga Silt Loam of Figure 3-37a is in hydrologic soil group B. A more concise listing is provided in the Soil Survey for each county, published by the NRCS or the local Soil and Water Conservation District.

Hence, parameter estimation for SWMM (and any use of curve numbers) is straightforward and relatively unambiguous. Curve numbers may also be used to back-calculate estimates of initial moisture deficit, as discussed earlier in conjunction with Equation 3-135, since the CN and storage, S, are related through

(3-136)

where it is essential to remember that units of S are in inches. And early discussion of Horton infiltration parameters has also been related to Hydrologic Soil Group.

Table 3-18. Runoff curve numbers for selected agricultural, suburban, and urban land use (SCS, 1986). (Antecedent moisture condition II; I_a = 0.2S)

Hydrologic Soil Group
Land Use Description		A	B	C	D
Cultivated land¹
Without conservation treatment		72	81	88	91
With conservation treatment		62	71	78	81
Pasture or range land
Poor condition		68	79	86	89
Good condition		39	61	74	80
Meadow
Good condition		30	58	71	78
Wood or forest land
Thin stand, poor cover, no mulch		45	66	77	83
Good cover²		25	55	70	77
Open spaces, lawns, parks, golf courses, cemeteries, etc.
Good condition: grass cover on 75% or more of the area		39	61	74	80
Fair condition: grass cover on 50 – 75% of the area		49	69	79	84
Commercial and business areas (85% impervious)		89	92	94	95
Industrial districts (72% impervious)		81	88	91	93
Residential³
Average lot size	Average % impervious⁴
1/8 ac or less	65	77	85	90	92
1/4 ac	38	61	75	83	87
1/3 ac	30	57	72	81	86
1/2 ac	25	54	70	80	85
1 ac	20	51	68	79	84
Paved parking lots, roofs, driveways, etc.⁵		98	98	98	98
Streets and roads
Paved with curbs and storm sewers⁵		98	98	98	98
Gravel		76	85	89	91
Dirt		72	82	87	89

1. For a more detailed description of agricultural land use curve numbers, refer to the SCS (1972) National Engineering Handbook, Section 4, “Hydrology.”

2. Good cover is protected from grazing and litter and brush cover soil.

3. Curve numbers are computed assuming that the runoff from the house and driveway is directed toward the street with a minimum of roof water directed to lawns where additional infiltration could occur.

4. The remaining pervious areas (lawn) are considered to be in good pasture condition for these curve numbers.

5. In some warmer climates of the country a curve number of 95 may be used.

Last updated by Robert E Dickinson Apr 21.

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