What curve number should I use for a residential area?

It depends on the lot size and the soil group. Quarter-acre lots (about 1 000 m², 38 % impervious) are CN 61, 75, 83 and 87 for soil groups A, B, C and D. Commercial and business districts (85 % impervious) are 89 to 95. The tabulated urban values assume the impervious area is directly connected to the drainage system and the pervious part is open space in good condition.

What is the curve number for grassland, pasture and woods?

Pasture or range in good condition (more than 75 % ground cover, lightly grazed) is 39, 61, 74 and 80 for groups A to D. Woods in good condition are 32, 58, 72 and 79. Lawns and parks with 50–75 % grass cover are 49, 69, 79 and 84.

How do I find the hydrologic soil group?

In the United States, the NRCS Web Soil Survey gives the hydrologic soil group for each mapped soil. Elsewhere, assign the group from the soil texture and depth or from measured infiltration: sands and loamy sands are usually A, silt loams and loams B, sandy clay loams C, and clays or shallow soils over rock or a high water table D. National soil maps in many countries carry a hydrological grouping that can be mapped to A–D.

What is antecedent runoff condition (AMC/ARC)?

The tabulated curve numbers are for average conditions (ARC II). For a catchment that is dry before the storm (ARC I) or already wet (ARC III), Chow et al. (1988) give CN(I) = 4.2 CN / (10 − 0.058 CN) and CN(III) = 23 CN / (10 + 0.13 CN). For CN 75 these give 56 and 87. NRCS now treats the average value as the design case and the others as bounds for sensitivity checks.

How do I calculate a composite curve number?

Weight each sub-area's curve number by its area. Area weighting works well when the curve numbers are close together; where they differ widely (a large paved area within open ground), compute the runoff from each part separately and add the volumes, because runoff is not linear in CN. For urban areas whose impervious fraction differs from the table, TR-55 Figures 2-3 and 2-4 adjust the composite CN for connected and unconnected impervious area.

SCS Curve Number Table: Runoff CN by Land Use and Soil Group

The runoff equation

The NRCS method computes the depth of direct runoff Q from the storm rainfall P and the potential maximum retention S, which the curve number sets:

S = \frac{25 400}{C N} - 254 (mm) I_{a} = 0.2 S Q = \frac{( P - I _{a} ) ^{2}}{P - I _{a} + S} for P > I_{a}

In US customary units S = 1000/CN − 10 in inches. No runoff occurs until the rainfall exceeds the initial abstraction I_a, which covers interception, depression storage and infiltration before runoff begins. The ratio I_a/S = 0.2 is the one the tables were developed with; Hawkins et al. (2002) found 0.05 fits observed data better, but the published curve numbers then need converting, so use 0.2 unless your manual says otherwise.

For example, 100 mm of rainfall on a catchment with CN 75: S = 25 400/75 − 254 = 84.7 mm, I_a = 16.9 mm, and Q = (100 − 16.9)² / (100 − 16.9 + 84.7) = 41.1 mm of direct runoff, 41 % of the rainfall.

Hydrologic soil groups

Every soil is placed in one of four groups by its minimum infiltration rate when thoroughly wet (TR-55, Appendix A):

Group	Typical soils	Minimum infiltration rate	Runoff potential
A	Sand, loamy sand, sandy loam	> 7.6 mm/h (0.30 in/h)	Low
B	Silt loam, loam	3.8 – 7.6 mm/h (0.15 – 0.30 in/h)	Moderately low
C	Sandy clay loam	1.3 – 3.8 mm/h (0.05 – 0.15 in/h)	Moderately high
D	Clay loam, silty clay loam, sandy clay, silty clay, clay; shallow soils; high water table	< 1.3 mm/h (0.05 in/h)	High

NEH Part 630 Chapter 7 (2009) now assigns the group from the saturated hydraulic conductivity and the depth to an impermeable layer or water table, and adds dual groups (A/D, B/D, C/D) for soils that are D only while undrained. Where the drainage is in place, use the first letter.

Tables

Curve numbers for average antecedent runoff condition (ARC II), with I_a = 0.2 S. Urban rows assume the impervious area is directly connected to the drainage system; residential percentages are the average impervious fraction for that lot size. Use them in the TR-55 calculator, the unit hydrograph generator, or the NRCS lag method of the time of concentration calculator.

Jump to: Urban — Open Space · Urban — Impervious · Urban Districts · Residential Districts · Developing Urban · Cultivated — Fallow · Cultivated — Row Crops · Cultivated — Small Grain · Cultivated — Legumes/Meadow · Other Agricultural

Urban — Open Space

Cover description	Hydrologic condition	A	B	C	D
Lawns, parks, golf courses, cemeteries	Poor (grass < 50%)	68	79	86	89
Lawns, parks, golf courses, cemeteries	Fair (grass 50–75%)	49	69	79	84
Lawns, parks, golf courses, cemeteries	Good (grass > 75%)	39	61	74	80

Urban — Impervious

Cover description	Hydrologic condition	A	B	C	D
Paved parking lots, roofs, driveways (excl. right-of-way)	–	98	98	98	98
Streets & roads — paved; curbs & storm sewers	–	98	98	98	98
Streets & roads — paved; open ditches	–	83	89	92	93
Streets & roads — gravel	–	76	85	89	91
Streets & roads — dirt	–	72	82	87	89

Urban Districts

Cover description	Hydrologic condition	A	B	C	D
Commercial & business (85% impervious)	–	89	92	94	95
Industrial (72% impervious)	–	81	88	91	93

Residential Districts

Cover description	Hydrologic condition	A	B	C	D
1/8 acre or less (65% impervious)	–	77	85	90	92
1/4 acre (38% impervious)	–	61	75	83	87
1/3 acre (30% impervious)	–	57	72	81	86
1/2 acre (25% impervious)	–	54	70	80	85
1 acre (20% impervious)	–	51	68	79	84
2 acres (12% impervious)	–	46	65	77	82

Developing Urban

Cover description	Hydrologic condition	A	B	C	D
Newly graded area (no permanent vegetation)	–	77	86	91	94

Cultivated — Fallow

Cover description	Hydrologic condition	A	B	C	D
Bare soil	–	77	86	91	94
Crop residue cover (CR)	Poor	76	85	90	93
Crop residue cover (CR)	Good	74	83	88	90

Cultivated — Row Crops

Cover description	Hydrologic condition	A	B	C	D
Straight row (SR)	Poor	72	81	88	91
Straight row (SR)	Good	67	78	85	89
Contoured (C)	Poor	70	79	84	88
Contoured (C)	Good	65	75	82	86
Contoured & terraced (C&T)	Poor	66	74	80	82
Contoured & terraced (C&T)	Good	62	71	78	81

Cultivated — Small Grain

Cover description	Hydrologic condition	A	B	C	D
Straight row (SR)	Poor	65	76	84	88
Straight row (SR)	Good	63	75	83	87
Contoured (C)	Poor	63	74	82	85
Contoured (C)	Good	61	73	81	84
Contoured & terraced (C&T)	Poor	61	72	79	82
Contoured & terraced (C&T)	Good	59	70	78	81

Cultivated — Legumes/Meadow

Cover description	Hydrologic condition	A	B	C	D
Straight row (SR)	Poor	66	77	85	89
Straight row (SR)	Good	58	72	81	85
Contoured (C)	Poor	64	75	83	85
Contoured (C)	Good	55	69	78	83
Contoured & terraced (C&T)	Poor	63	73	80	83
Contoured & terraced (C&T)	Good	51	67	76	80

Other Agricultural

Cover description	Hydrologic condition	A	B	C	D
Pasture, grassland, or range — continuous forage	Poor (<50% cover, heavy grazed)	68	79	86	89
Pasture, grassland, or range — continuous forage	Fair (50–75% cover)	49	69	79	84
Pasture, grassland, or range — continuous forage	Good (>75% cover, light grazed)	39	61	74	80
Meadow — continuous grass, no grazing	–	30	58	71	78
Brush — brush-weed-grass mixture	Poor (<50% cover)	48	67	77	83
Brush — brush-weed-grass mixture	Fair (50–75% cover)	35	56	70	77
Brush — brush-weed-grass mixture	Good (>75% cover)	30	48	65	73
Woods-grass combination (orchard / tree farm)	Poor	57	73	82	86
Woods-grass combination (orchard / tree farm)	Fair	43	65	76	82
Woods-grass combination (orchard / tree farm)	Good	32	58	72	79
Woods	Poor (heavy grazing / burning)	45	66	77	83
Woods	Fair (some grazing, no burning)	36	60	73	79
Woods	Good (protected from grazing)	30	55	70	77
Farmsteads — buildings, lanes, driveways	–	59	74	82	86

Hydrologic condition

For cultivated and grazed land, the condition describes how well the cover protects the soil and encourages infiltration: the density of the vegetation or residue, the amount of grazing and, for crops, the rotation. Good means the cover favours infiltration and the runoff potential is lower; poor means sparse or heavily grazed cover. For woods, the condition reflects grazing and burning, which remove the litter layer that holds water.

Outside the United States

The curve numbers were derived from small US catchments, but the method is used worldwide because the inputs are a land cover and a soil class. The land covers transfer reasonably well; the soil group is the harder part, since few national soil maps use the NRCS classification directly. Where a national adaptation exists (South Africa's SCS-SA method, for one, adjusts the curve numbers and the antecedent moisture for local conditions), use it in preference to the US tables.

SCS curve number table