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.

Definition

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:

where

is runoff ([L]; in)
is rainfall ([L]; in)
is the potential maximum soil moisture retention after runoff begins ([L]; in)
is the initial abstraction ([L]; in), or the amount of water before runoff, such as infiltration, or rainfall interception by vegetation; historically, it has generally been assumed that , although more recent research has found that may be a more appropriate relationship in urbanized watersheds where the CN is updated to reflect developed conditions.[2]

The runoff curve number, , is then related

has a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower the curve number, the more permeable the soil is. As can be seen in the curve number equation, runoff cannot begin until the initial abstraction has been met. It is important to note that the curve number methodology is an event-based calculation, and should not be used for a single annual rainfall value, as this will incorrectly miss the effects of antecedent moisture and the necessity of an initial abstraction threshold.

Selection

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]

  • HSG Group A (low runoff potential): Soils with high infiltration rates even when thoroughly wetted. These consist chiefly of deep, well-drained sands and gravels. These soils have a high rate of water transmission (final infiltration rate greater than 0.30 in (7.6 mm) per hour).
  • HSG Group B: Soils with moderate infiltration rates when thoroughly wetted. These consist chiefly of soils that are moderately deep to deep, moderately well drained to well drained with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (final infiltration rate of 0.15–0.30 in (3.8–7.6 mm) per hour).
  • HSG Group C: Soils with slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine textures. These soils have a slow rate of water transmission (final infiltration rate 0.05–0.15 in (1.3–3.8 mm) per hour).
  • HSG Group D (high runoff potential): Soils with very slow infiltration rates when thoroughly wetted. These consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious materials. These soils have a very slow rate of water transmission (final infiltration rate less than 0.05 in (1.3 mm) per hour).

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.

Values

Fully developed urban areas (vegetation established)
Cover description Curve numbers for hydrologic soil group
ABCD
Open space (lawns, parks, golf courses, cemeteries, etc.) Poor condition (grass cover <50%) 68798689
Fair condition (grass cover 50 to 75%) 49697984
Good condition (grass cover >75%) 39617480
Impervious areas Paved parking lots, roofs, driveways, etc. (excluding right of way) 98989898
Streets and roads Paved; curbs and storm sewers (excluding right-of-way) 98989898
Paved; open ditches (including right-of-way) 83899293
Gravel (including right of way) 76858991
Dirt (including right-of-way) 72828789
Western desert urban areas Natural desert landscaping (pervious area only) 63778588
Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders) 96969696
Urban districts Commercial and business (85% imp.) 89929495
Industrial (72% imp.) 81889193
Residential districts by average lot size 18 acre or less (town houses) (65% imp.) 77859092
14 acre (38% imp.) 61758387
13 acre (30% imp.) 57728186
12 acre (25% imp.) 54708085
1 acre (20% imp.) 51687984
2 acres (12% imp.) 46657782
Developing urban areas
Cover description Curve numbers for hydrologic soil group
ABCD
Newly graded areas (pervious areas only, no vegetation) 77869194
Cultivated agricultural lands
Cover description Curve numbers for hydrologic soil group
Cover type Treatment[A] Hydrologic condition ABCD
Fallow Bare soil 77869194
Crop residue cover (CR) Poor 76859093
Good 74838890
Row crops Straight row (SR) Poor 72818891
Good 67788589
SR + CRPoor 71808790
Good 64758285
Contoured (C)Poor 70798488
Good 65758286
C + CRPoor 69788387
Good 64748185
Contoured & terraced (C&T)Poor 66748082
Good 62717881
C&T + R Poor 65737981
Good 61707780
Small grain SR Poor 65768488
Good 63758387
SR + CR Poor 64758386
Good 60728084
C Poor 63748285
Good 61738184
C + CR Poor 62738184
Good 60728083
C&T Poor 61727982
Good 59707881
C&T + R Poor 60717881
Good 58697780
Close-seeded or broadcast legumes or rotation meadow SR Poor 66778589
Good 58728185
C Poor 64758385
Good 55697883
C&T Poor 63738083
Good 51677680
A Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
Other agricultural lands
Cover description Curve numbers for hydrologic soil group
Cover type Hydrologic conditionABCD
Pasture, grassland, or range—continuous forage for grazing.A Poor 68798689
Fair 49697984
Good 39617480
Meadow—continuous grass, protected from grazing and generally mowed for hay. 30587178
Brush—brush-weed-grass mixture with brush the major element.B Poor 48677783
Fair 35567077
Good 30C486573
Woods—grass combination (orchard or tree farm).D Poor 57738286
Fair 43657682
Good 32587279
Woods.E Poor 45667783
Fair 36607379
Good 30557077
Farmsteads—buildings, lanes, driveways, and surrounding lots. 59748286
A 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.
B Poor: <50% ground cover; Fair: 50-75% ground cover; Good: >75% ground cover.
C Actual curve number is less than 30; use CN = 30 for runoff computation.
D 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.
E 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.
Arid and semiarid rangelands
Cover description Curve numbers for hydrologic soil group
Cover type Hydrologic conditionA ABBCD
Herbaceuous—mixture of grass, weeds, and low-growing brush, with brush the minor element Poor 808793
Fair 718189
Good 627485
Oak-aspen—mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush Poor 667479
Fair 485763
Good 304148
Pinyon-juniper—pinyon, juniper, or both; grass understory Poor 758589
Fair 587380
Good 416171
Sagebrush with grass understory Poor 678085
Fair 516370
Good 354755
Desert shrub—major plants include saltbush, geasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus. Poor 63778588
Fair 55728186
Good 49687984
A Poor: <30% ground cover (litter, grass, and brush overstory); Fair: 30 to 70% ground cover; Good: >70% ground cover.
B Curve numbers for group A have been developed only for desert shrub.

Adjustments


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 , or average soil moisture. The other moisture conditions are dry, AMC I or , and moist, AMC III or . The curve number can be adjusted by factors to , where factors are less than 1 (reduce and potential runoff), while factor are greater than 1 (increase and potential runoff). The AMC factors can be looked up in the reference table below. Find the CN value for AMC II and multiply it by the adjustment factor based on the actual AMC to determine the adjusted curve number.

Adjustments to select curve number for soil moisture conditions.[3]
Curve Number (AMC II) Factors to Convert Curve Number for AMC II to AMC I or III
AMC I (dry) AMC III (wet)
100.402.22
200.451.85
300.501.67
400.551.50
500.621.40
600.671.30
700.731.21
800.791.14
900.871.07
1001.001.00

Initial abstraction ratio adjustment

The relationship was derived from the study of many small, experimental watersheds . Since the history and documentation of this relationship are relatively obscure, more recent analysis used model fitting methods to determine the ratio of to with hundreds of rainfall-runoff data from numerous U.S. watersheds. In the model fitting done by Hawkins et al. (2002)[2] found that the ratio of to varies from storm to storm and watershed to watershed and that the assumption of is usually high. More than 90 percent of ratios were less than 0.2. Based on this study, use of ratios of 0.05 rather than the commonly used value of 0.20 would seem more appropriate. Thus, the CN runoff equation becomes:

In this equation, note that the values of are not the same as the one used in estimating direct runoff with an ratio of 0.20, because 5 percent of the storage is assumed to be the initial abstraction, not 20 percent. The relationship between and was obtained from model fitting results, giving the relationship:

The user, then, must do the following to use the adjusted 0.05 initial abstraction ratio:

  1. Use the traditional tables of curve numbers to select the value appropriate for your watershed.
  2. Calculate using the traditional equation:
  3. Convert this S value to using the relationship above.
  4. Calculate the runoff depth using the CN runoff equation above (with 0.05 substituted for the initial abstraction ratio).

See also

References

  1. 1 2 3 United States Department of Agriculture (1986). Urban hydrology for small watersheds (PDF). Technical Release 55 (TR-55) (Second ed.). Natural Resources Conservation Service, Conservation Engineering Division.
  2. 1 2 Hawkins, R.H.; Jiang, R.; Woodward, D.E.; Hjelmfelt, A.T.; Van Mullem, J.A. (2002). "Runoff Curve Number Method: Examination of the Initial Abstraction Ratio". Proceedings of the Second Federal Interagency Hydrologic Modeling Conference, Las Vegas, Nevada. 42 (3): 629–643. doi:10.1111/j.1752-1688.2006.tb04481.x. S2CID 130013737.
  3. Ward, Andy D.; Trimble, Stanley W. (2004). Environmental Hydrology. Boca Raton, Florida 33431: CRC Press LLC. ISBN 9781566706162.{{cite book}}: CS1 maint: location (link)
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