What's the NRCS curve number method?

The NRCS / SCS runoff equation predicts direct runoff depth Q from total rainfall depth P, using a curve number CN that integrates land cover and soil hydrology. Standard for stormwater design across the US.

What's a typical curve number?

Open space, good cover: A=39, B=61, C=74, D=80. Residential 1/4 acre: A=61, B=75, C=83, D=87. Commercial: A=89, B=92, C=94, D=95. Forest: A=36, B=60, C=73, D=79. Source: TR-55 Table 2-2.

Should I use λ = 0.2 or 0.05 for initial abstraction?

Most US regulators still require λ = 0.2 (NRCS classic). λ = 0.05 (modern, NEH-630) is more accurate but accepts higher runoff at low rainfall and is less commonly accepted in regulatory submittals.

What is hydrologic soil group (HSG)?

NRCS classifies soils into four groups by infiltration capacity. Group A: high (sand, gravel). Group B: moderate (loam, silt loam). Group C: slow (sandy clay loam). Group D: very slow (clay). Use NRCS Web Soil Survey at websoilsurvey.nrcs.usda.gov for site-specific HSG.

How do I compute composite curve number for mixed land cover?

Area-weight: CN_composite = Σ(CN_i × A_i) / Σ(A_i). For example, 5 acres at CN=80 plus 5 acres at CN=70 = composite CN = 75. The CN method is non-linear, so always weight CN values directly — never weight runoff Q values.

What rainfall depth do I use?

Use the design storm rainfall for the project return period from NOAA Atlas 14 (https://hdsc.nws.noaa.gov/pfds/). Typical: 25-year, 24-hour for road crossings; 100-year, 24-hour for dam spillways. Apply the SCS Type II distribution for hydrograph routing.

What is antecedent runoff condition (ARC)?

NRCS defines three ARCs based on 5-day antecedent rainfall. ARC II = average condition (the default). ARC I = dry. ARC III = wet. Modern practice uses ARC II for all design unless specifically required otherwise. Older textbooks adjust CN by ARC; NRCS NEH-630 deprecates this.

When does the NRCS method NOT apply?

The CN method is empirical and was calibrated on small (<2,000 acre) agricultural watersheds in the eastern US. It under-predicts runoff for: very small storms (P < 0.5 in), urban watersheds with detention, snowmelt, frozen ground, and watersheds with significant baseflow.

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NRCS Curve Number Runoff

SCS / NRCS curve number method (TR-55) for computing runoff depth and volume from a design storm. The standard hydrologic-loss method for stormwater design across the US.

Units

Rainfall depth, Pin (24-hr design storm)

Curve number, CN— (40 to 98)

Initial abstraction ratio, λI_a/S

Drainage area, Aacres

Potential retention, S—in

Initial abstraction, I_a—in

Runoff depth, Q—in

Runoff volume—acre-ft

Runoff volume—ft³

Defaults: 3.5 in (typical 25-year, 24-hr in mid-Atlantic), CN = 75 (suburban residential, B soils), 10-acre site. SI mode converts inputs/outputs; the underlying empirical NRCS formula is solved in inches internally.

Potential maximum retention:

$$ S = \frac{1000}{CN} - 10 \quad (\text{inches}) $$

Initial abstraction:

$$ I_a = \lambda \, S, \qquad \lambda = 0.2 \text{ (classic) or } 0.05 \text{ (modern)} $$

Runoff depth (when P > I_a):

$$ Q = \frac{(P - I_a)^2}{P - I_a + S} $$

P total rainfall depth · S potential maximum retention after runoff begins · I_a initial abstraction (depression storage, interception, infiltration before runoff starts) · CN NRCS curve number · Q direct runoff depth.

Curve number table (TR-55 Table 2-2)

CN depends on land cover, hydrologic soil group (A/B/C/D), and antecedent runoff condition (ARC II is standard). Values shown are ARC II. For mixed-cover watersheds, area-weight: CN_w = Σ(CN_i × A_i) / Σ(A_i).

NRCS curve numbers for urban land uses (TR-55 Table 2-2a)
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 lots (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

NRCS curve numbers for agricultural and natural land uses (TR-55 Table 2-2c)
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
Row crops, contoured + terraced, good	62	71	78	81
Small grain, straight, poor	65	76	84	88
Small grain, contoured, good	61	73	81	84
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
Brush, good (> 75%)	30	48	65	73
Woods, fair cover	36	60	73	79
Woods, good cover	30	55	70	77
Farmsteads (buildings, lanes)	59	74	82	86

Hydrologic soil groups

NRCS hydrologic soil group classification
Group	Soil texture	Min. infiltration rate	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 loam, silty clay, clay, or shallow bedrock	< 0.05 in/hr	High

Site-specific HSG comes from NRCS Web Soil Survey at websoilsurvey.nrcs.usda.gov. Default to Group C or D when soils data is unavailable.

Worked examples

Example 1 — 10-acre subdivision, 25-yr 24-hr storm

Given: 10-ac residential ¼-ac development on HSG B soils (CN = 75), P₂₅ = 4.5 in (24-hr), λ = 0.2.

Find: Runoff depth Q and volume.

S = 1000/75 − 10 = 13.33 − 10 = 3.33 in

I_a = 0.2 · 3.33 = 0.667 in

P − I_a = 4.5 − 0.667 = 3.833 in > 0 ✓

Q = (3.833)² / (3.833 + 3.33) = 14.69 / 7.163 = 2.05 in

Volume = 2.05 in · 10 ac · (1 ft/12 in) = 1.71 ac-ft = 74,500 ft³

Q = 2.05 in · Volume = 1.71 ac-ft (74,500 ft³)

Example 2 — Mixed-cover watershed with composite CN

Given: 50-ac watershed = 20 ac residential ½-ac (CN=70, HSG B) + 15 ac woods good (CN=55) + 15 ac pasture fair (CN=69). P = 5.0 in.

Find: Composite CN and runoff depth Q.

CN_w = (20·70 + 15·55 + 15·69) / 50 = (1400 + 825 + 1035) / 50 = 3260/50 = 65.2

S = 1000/65.2 − 10 = 5.34 in; I_a = 1.07 in

Q = (5.0 − 1.07)² / (5.0 − 1.07 + 5.34) = 15.45 / 9.27 = 1.67 in

CN_w = 65 · Q = 1.67 in · Volume = 6.96 ac-ft

Initial abstraction: 0.2 vs 0.05

The classic NRCS method assumes I_a = 0.2 S — a coefficient set in the 1950s based on a small data set. More recent analysis (Hawkins et al. 2002, NRCS NEH-630 ch. 10, 2017) found I_a/S is closer to 0.05 on most watersheds.

Switching from λ = 0.2 to λ = 0.05 increases runoff depth at low rainfall events significantly. For a 1-inch storm on CN = 75, λ = 0.2 gives Q = 0.04 in; λ = 0.05 gives Q = 0.31 in (8× higher). The two methods converge at higher rainfall depths.

Most US regulatory agencies still require λ = 0.2 for stormwater design. NRCS-published documentation generally uses 0.05 for current research. Use 0.2 unless you're certain your reviewer accepts 0.05.

From runoff depth to peak flow

This calculator gives total runoff depth and volume. To convert to a peak flow rate, you need a hydrograph method:

TR-55 graphical peak discharge (Chapter 4): peak flow = q_u × A_m × Q × F_p, where q_u is unit peak from the chart based on Tc and Ia/P.
NRCS unit hydrograph (TR-20, HEC-HMS, HydroCAD): full hydrograph generation by routing rainfall through the dimensionless unit hydrograph.
SWMM (more flexible): subbasin-by-subbasin routing with non-linear reservoir or kinematic wave.

For a quick check on small watersheds, the Rational Method calculator gives a peak flow without the depth/volume — but it doesn't match what NRCS produces because the assumptions differ.

Reference: USDA NRCS (1986). Urban Hydrology for Small Watersheds (TR-55). Hawkins, R.H., et al. (2002). "Runoff Curve Number Method: Examination of the Initial Abstraction Ratio." EWRI World Water and Environmental Resources Congress 2002.