The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess.[1] The curve number method was developed by the USDA Natural Resources Conservation Service, which was formerly called the Soil Conservation Service or SCS — the number is still popularly known as a "SCS runoff curve number" in the literature. The runoff curve number was developed from an empirical analysis of runoff from small catchments and hillslope plots monitored by the USDA. It is widely used and is an efficient method for determining the approximate amount of direct runoff from a rainfall event in a particular area.
The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition. References, such as from USDA[1] indicate the runoff curve numbers for characteristic land cover descriptions and a hydrologic soil group.
The runoff equation is:
Q=\begin{cases} 0&forP\leqIa\\
| |||||||
{P-Ia |
+S}&forP>Ia\end{cases}
where
Q
P
S
Ia
Ia=0.2S
Ia=0.05S
The runoff curve number,
CN
S=
1000 | |
CN |
-10
CN
The NRCS curve number is related to soil type, soil infiltration capability, land use, and the depth of the seasonal high water table. To account for different soils' ability to infiltrate, NRCS has divided soils into four hydrologic soil groups (HSGs). They are defined as follows.[1]
Selection of a hydrologic soil group should be done based on measured infiltration rates, soil survey (such as the NRCS Web Soil Survey), or judgement from a qualified soil science or geotechnical professional. The table below presents curve numbers for antecedent soil moisture condition II (average moisture condition). To alter the curve number based on moisture condition or other parameters, see Adjustments.
Cover description | Curve numbers for hydrologic soil group | ||||
---|---|---|---|---|---|
A | B | C | D | ||
Open space (lawns, parks, golf courses, cemeteries, etc.) | Poor condition (grass cover <50%) | 68 | 79 | 86 | 89 |
Fair condition (grass cover 50 to 75%) | 49 | 69 | 79 | 84 | |
Good condition (grass cover >75%) | 39 | 61 | 74 | 80 | |
Impervious areas | Paved parking lots, roofs, driveways, etc. (excluding right of way) | 98 | 98 | 98 | 98 |
Streets and roads | Paved; curbs and storm sewers (excluding right-of-way) | 98 | 98 | 98 | 98 |
Paved; open ditches (including right-of-way) | 83 | 89 | 92 | 93 | |
Gravel (including right of way) | 76 | 85 | 89 | 91 | |
Dirt (including right-of-way) | 72 | 82 | 87 | 89 | |
Western desert urban areas | Natural desert landscaping (pervious area only) | 63 | 77 | 85 | 88 |
Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders) | 96 | 96 | 96 | 96 | |
Urban districts | Commercial and business (85% imp.) | 89 | 92 | 94 | 95 |
Industrial (72% imp.) | 81 | 88 | 91 | 93 | |
Residential districts by average lot size | acre or less (town houses) (65% imp.) | 77 | 85 | 90 | 92 |
acre (38% imp.) | 61 | 75 | 83 | 87 | |
acre (30% imp.) | 57 | 72 | 81 | 86 | |
acre (25% imp.) | 54 | 70 | 80 | 85 | |
1 acre (20% imp.) | 51 | 68 | 79 | 84 | |
2 acres (12% imp.) | 46 | 65 | 77 | 82 |
Cover description | Curve numbers for hydrologic soil group | |||||
---|---|---|---|---|---|---|
Cover type | Treatment | Hydrologic condition | A | B | C | D |
Fallow | Bare soil | — | 77 | 86 | 91 | 94 |
Crop residue cover (CR) | Poor | 76 | 85 | 90 | 93 | |
Good | 74 | 83 | 88 | 90 | ||
Row crops | Straight row (SR) | Poor | 72 | 81 | 88 | 91 |
Good | 67 | 78 | 85 | 89 | ||
SR + CR | Poor | 71 | 80 | 87 | 90 | |
Good | 64 | 75 | 82 | 85 | ||
Contoured (C) | Poor | 70 | 79 | 84 | 88 | |
Good | 65 | 75 | 82 | 86 | ||
C + CR | Poor | 69 | 78 | 83 | 87 | |
Good | 64 | 74 | 81 | 85 | ||
Contoured & terraced (C&T) | Poor | 66 | 74 | 80 | 82 | |
Good | 62 | 71 | 78 | 81 | ||
C&T + R | Poor | 65 | 73 | 79 | 81 | |
Good | 61 | 70 | 77 | 80 | ||
Small grain | SR | Poor | 65 | 76 | 84 | 88 |
Good | 63 | 75 | 83 | 87 | ||
SR + CR | Poor | 64 | 75 | 83 | 86 | |
Good | 60 | 72 | 80 | 84 | ||
C | Poor | 63 | 74 | 82 | 85 | |
Good | 61 | 73 | 81 | 84 | ||
C + CR | Poor | 62 | 73 | 81 | 84 | |
Good | 60 | 72 | 80 | 83 | ||
C&T | Poor | 61 | 72 | 79 | 82 | |
Good | 59 | 70 | 78 | 81 | ||
C&T + R | Poor | 60 | 71 | 78 | 81 | |
Good | 58 | 69 | 77 | 80 | ||
Close-seeded or broadcast legumes or rotation meadow | SR | Poor | 66 | 77 | 85 | 89 |
Good | 58 | 72 | 81 | 85 | ||
C | Poor | 64 | 75 | 83 | 85 | |
Good | 55 | 69 | 78 | 83 | ||
C&T | Poor | 63 | 73 | 80 | 83 | |
Good | 51 | 67 | 76 | 80 | ||
Crop residue cover applies only if residue is on at least 5% of the surface throughout the year. |
Cover description | Curve numbers for hydrologic soil group | |||||
---|---|---|---|---|---|---|
Cover type | Hydrologic condition | A | B | C | D | |
Pasture, grassland, or range—continuous forage for grazing. | Poor | 68 | 79 | 86 | 89 | |
Fair | 49 | 69 | 79 | 84 | ||
Good | 39 | 61 | 74 | 80 | ||
Meadow—continuous grass, protected from grazing and generally mowed for hay. | — | 30 | 58 | 71 | 78 | |
Brush—brush-weed-grass mixture with brush the major element. | Poor | 48 | 67 | 77 | 83 | |
Fair | 35 | 56 | 70 | 77 | ||
Good | 30 | 48 | 65 | 73 | ||
Woods—grass combination (orchard or tree farm). | Poor | 57 | 73 | 82 | 86 | |
Fair | 43 | 65 | 76 | 82 | ||
Good | 32 | 58 | 72 | 79 | ||
Woods. | Poor | 45 | 66 | 77 | 83 | |
Fair | 36 | 60 | 73 | 79 | ||
Good | 30 | 55 | 70 | 77 | ||
Farmsteads—buildings, lanes, driveways, and surrounding lots. | — | 59 | 74 | 82 | 86 |
Poor: <50% ground cover or heavily grazed with no mulch; Fair: 50-75% ground cover and not heavily grazed; Good: >75% ground cover and light or only occasionally grazed. | |
Poor: <50% ground cover; Fair: 50-75% ground cover; Good: >75% ground cover. | |
Actual curve number is less than 30; use CN = 30 for runoff computation. | |
CN's shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other combinations of conditions may be computed from the CN's for woods and pasture. | |
Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning; Fair: Woods are grazed but not burned, and some forest litter covers the soil; Good: Woods are protected from grazing, and litter and brush adequately cover the soil. |
Cover description | Curve numbers for hydrologic soil group | ||||
---|---|---|---|---|---|
Cover type | Hydrologic condition | A | B | C | D |
Herbaceuous—mixture of grass, weeds, and low-growing brush, with brush the minor element | Poor | — | 80 | 87 | 93 |
Fair | — | 71 | 81 | 89 | |
Good | — | 62 | 74 | 85 | |
Oak-aspen—mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush | Poor | — | 66 | 74 | 79 |
Fair | — | 48 | 57 | 63 | |
Good | — | 30 | 41 | 48 | |
Pinyon-juniper—pinyon, juniper, or both; grass understory | Poor | — | 75 | 85 | 89 |
Fair | — | 58 | 73 | 80 | |
Good | — | 41 | 61 | 71 | |
Sagebrush with grass understory | Poor | — | 67 | 80 | 85 |
Fair | — | 51 | 63 | 70 | |
Good | — | 35 | 47 | 55 | |
Desert shrub—major plants include saltbush, geasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus. | Poor | 63 | 77 | 85 | 88 |
Fair | 55 | 72 | 81 | 86 | |
Good | 49 | 68 | 79 | 84 | |
Poor: <30% ground cover (litter, grass, and brush overstory); Fair: 30 to 70% ground cover; Good: >70% ground cover. | |
Curve numbers for group A have been developed only for desert shrub. |
Runoff is affected by the soil moisture before a precipitation event, the antecedent moisture condition (AMC). A curve number, as calculated above, may also be termed AMC II or
CNII
CNI
CNIII
CNII
CNI
CN
CNIII
CN
Curve Number (AMC II) | Factors to Convert Curve Number for AMC II to AMC I or III | |
---|---|---|
AMC I (dry) | AMC III (wet) | |
10 | 0.40 | 2.22 |
20 | 0.45 | 1.85 |
30 | 0.50 | 1.67 |
40 | 0.55 | 1.50 |
50 | 0.62 | 1.40 |
60 | 0.67 | 1.30 |
70 | 0.73 | 1.21 |
80 | 0.79 | 1.14 |
90 | 0.87 | 1.07 |
100 | 1.00 | 1.00 |
The relationship
Ia=0.2S
Ia
S
Ia
S
Ia/S=0.20
Ia/S
Ia/S
Q=\begin{cases} 0&forP\leq0.05S\\
(P-0.05S0.05)2 | |
P+0.95S0.05 |
&forP>0.05S\end{cases}
In this equation, note that the values of
S0.05
Ia/S
S0.05
S0.20
S0.05=1.33{S0.20
The user, then, must do the following to use the adjusted 0.05 initial abstraction ratio:
S0.20
S=
1000 | |
CN |
-10
S0.05