Q: How do I compute a composite curve number?

Area-weight the CN values directly: CN_composite = Σ(CN_i × A_i) / Σ(A_i). Always weight CN, never weight the runoff depth Q, because the runoff equation is non-linear.

Q: How do I adjust CN for wet or dry antecedent conditions?

Tabulated CN values are ARC II (average). For dry ARC I: CN_I = 4.2·CN_II / (10 − 0.058·CN_II). For wet ARC III: CN_III = 23·CN_II / (10 + 0.13·CN_II). Modern NRCS practice (NEH-630) uses ARC II for all design and deprecates ARC adjustment.

Question 1

What is a curve number (CN)?

Accepted Answer

The NRCS curve number is a dimensionless index from 30 to 98 that integrates land cover and hydrologic soil group into a single runoff parameter. Higher CN means more runoff. It sets the potential maximum retention S = 1000/CN − 10 (inches) used in the NRCS runoff equation Q = (P − Ia)² / (P − Ia + S).

Question 2

What is a typical curve number?

Accepted Answer

For hydrologic soil group B: open space good cover = 61, residential 1/4-acre = 75, commercial = 92, woods good cover = 55, pasture good = 61, row crops straight good = 78. Paved surfaces = 98 for all soil groups. Source: TR-55 Table 2-2.

Question 3

What are the four hydrologic soil groups?

Accepted Answer

Group A: sand/loamy sand, infiltration > 0.30 in/hr, low runoff. Group B: silt loam/loam, 0.15–0.30 in/hr. Group C: sandy clay loam, 0.05–0.15 in/hr. Group D: clay/shallow bedrock, < 0.05 in/hr, high runoff. Look up site-specific HSG on NRCS Web Soil Survey.

Question 4

How do I compute a composite curve number?

Accepted Answer

Area-weight the CN values directly: CN_composite = Σ(CN_i × A_i) / Σ(A_i). Always weight CN, never weight the runoff depth Q, because the runoff equation is non-linear.

Question 5

How do I adjust CN for wet or dry antecedent conditions?

Accepted Answer

Tabulated CN values are ARC II (average). For dry ARC I: CN_I = 4.2·CN_II / (10 − 0.058·CN_II). For wet ARC III: CN_III = 23·CN_II / (10 + 0.13·CN_II). Modern NRCS practice (NEH-630) uses ARC II for all design and deprecates ARC adjustment.

Land use	% Imp.	A	B	C	D
Open space, poor cover (< 50% grass)	—	68	79	86	89
Open space, fair cover (50–75% grass)	—	49	69	79	84
Open space, good cover (> 75% grass)	—	39	61	74	80
Paved parking, roofs, driveways	100	98	98	98	98
Paved streets, curb & gutter	100	98	98	98	98
Paved streets, open ditches	—	83	89	92	93
Gravel roads	—	76	85	89	91
Dirt roads	—	72	82	87	89
Commercial / business	85	89	92	94	95
Industrial	72	81	88	91	93
Residential, 1/8-ac (townhouse)	65	77	85	90	92
Residential, 1/4-ac lots	38	61	75	83	87
Residential, 1/3-ac lots	30	57	72	81	86
Residential, 1/2-ac lots	25	54	70	80	85
Residential, 1-ac lots	20	51	68	79	84
Residential, 2-ac lots	12	46	65	77	82

Land use / treatment	A	B	C	D
Fallow, bare soil	77	86	91	94
Row crops, straight, poor	72	81	88	91
Row crops, straight, good	67	78	85	89
Row crops, contoured, good	64	75	82	85
Small grain, straight, poor	65	76	84	88
Pasture, poor (< 50% cover)	68	79	86	89
Pasture, fair (50–75% cover)	49	69	79	84
Pasture, good (> 75% cover)	39	61	74	80
Meadow, continuous grass	30	58	71	78
Brush, fair	35	56	70	77
Woods, fair cover	36	60	73	79
Woods, good cover	30	55	70	77
Farmsteads (buildings, lanes)	59	74	82	86

Group	Soil texture	Min. infiltration	Runoff potential
A	Sand, loamy sand, sandy loam	> 0.30 in/hr	Low
B	Silt loam, loam	0.15–0.30 in/hr	Moderate
C	Sandy clay loam	0.05–0.15 in/hr	Moderately high
D	Clay, silty clay, shallow bedrock	< 0.05 in/hr	High

NRCS Curve Number (CN) — TR-55 Reference

Urban / Developed Land Uses (TR-55 Table 2-2a)

Agricultural & Natural Land Uses (TR-55 Table 2-2c)

Hydrologic Soil Groups (HSG)

Composite (Weighted) CN for a Mixed Watershed

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