The four pages in this section are reproduced from the SCS (now NRCS) report Urban hydrology for small watersheds. This report is commonly known as TR-55. The tables provide estimates of the curve number (CN) as a function of hydrologic soil group (HSG), cover type, treatment, hydrologic condition, antecedent runoff condition (ARC), and impervious area in the catchment.
TR-55 provides the following guidance for use of these tables:

  • Soils are classified into four HSG's (A, B, C, and D) according to their minimum infiltration rate, which is obtained for bare soil after prolonged wetting. Appendix A \[of TR-55\] defines the four groups and provides a list of most of the soils in the United States and their group classification. The soils in the area of interest may be identified from a soil survey report, which can be obtained from local SCS offices or soil and water conservation district offices.

  • There are a number of methods for determining cover type. The most common are field reconnaissance, aerial photographs, and land use maps.
  • Treatment is a cover type modifier (used only in Table 2-2b) to describe the management of cultivated agricultural lands. It includes mechanical practices, such as contouring and terracing, and management practices, such as crop rotations and reduced or no tillage.
  • Hydrologic condition indicates the effects of cover type and treatment on infiltration and runoff and is generally estimated from density of plant and residue cover on sample areas. Good hydrologic condition indicates that the soil usually has a low runoff potential for that specific hydrologic soil group, cover type and treatment. Some factors to consider in estimating the effect of cover on infiltration and runoff are: (a) canopy or density of lawns, crops, or other vegetative areas; (b) amount of year-round cover; (c) amount of grass or close-seeded legumes in rotations; (d) percent of residue cover; and (e) degree of surface roughness.
  • The index of runoff potential before a storm event is the antecedent runoff condition (ARC). The CN for the average ARC at a site is the median value as taken from sample rainfall and runoff data. The curve numbers in table 2-2 are for the average ARC, which is used primarily for design applications.
  • The percentage of impervious area and the means of conveying runoff from impervious areas to the drainage systems should be considered in computing CN for urban areas. An impervious area is considered connected if runoff from it flows directly into the drainage systems. It is also considered connected if runoff from it occurs as shallow concentrated shallow flow that runs over a pervious area and then into a drainage system. Runoff from unconnected impervious areas is spread over a pervious area as sheet flow.

SCS TR-55 Table 2-2a – Runoff curve numbers for urban areas1

Cover description

Curve numbers for hydrologic soil group

Cover type and hydrologic condition

Average percent impervious area2

A

B

C

D

Fully developed urban areas






Open space (lawns, parks, golf courses, cemeteries, etc.)3:






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 areas only)4 . .


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

89

92

94

95

Industrial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

72

81

88

91

93

Residential districts by average lot size






1/8 acre or less (town houses) . . . . . . . . . . . . . . . . . .

65

77

85

90

92

1/4 acre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

38

61

75

83

87

1/3 acre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

57

72

81

86

1/2 acre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

54

70

80

85

1 acre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20

51

68

79

84

2 acre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12

46

65

77

82

Developing urban areas






Newly graded areas (pervious areas only,
no vegetation)5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .


77

86

91

94

Idle lands (CN's are determined using cover types
similar to those in table 2-2c






1 Average runoff condition, and Ia = 0.2S.
2 The average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CN's for other combinations of conditions may be computed using figure 2-3 or 2-4.
3 CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type.
4 Composite CN's for natural desert landscaping should be computed using figures 2-3 or 2-4 based on the impervious area percentage (CN = 98) and the pervious area CN. The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition.
5 Composite CN's to use for the design of temporary measures during grading and construction should be computed using figure 2-3 or 2-4, based on the degree of development (imperviousness area percentage) and the CN's for the newly graded pervious areas.

SCS TR-55 Table 2-2b – Runoff curve numbers for cultivated agricultural lands1

Cover description

Curve numbers for hydrologic soil group

Cover type

Treatment2

Hydrologic condition3

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 + CR

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 + CR

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

1 Average runoff condition, and Ia = 0.2S.
2 Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
3 Hydrologic condition is based on combination of factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or close-seeded legumes in rotations, (d) percent of residue cover on the land surface (good  20%), and (e) degree of surface roughness.
Good: Factors impair infiltration and tend to increase runoff.
Poor: Factors encourage average and better than average infiltration and tend to decrease runoff.

SCS TR-55 Table 2-2c – Runoff curve numbers for other agricultural lands1

Cover description



Curve numbers for hydrologic soil group








Cover type and hydrologic condition

Hydrologic condition


A


B


C


D

Pasture, grassland, or range – continuous

Poor


68

79


86


89



forage for graving.2

Fair


49

69


79


84




Good


39

61


74


80














Meadow – continuous grass, protected from


30

58


71


78



grazing and generally mowed for hay.






















Brush – brush-weed mixture with brush

Poor


48

67


77


83



the major element.3

Fair


35

56


70


77




Good


304

48


65


73














Woods – grass combination (orchard

Poor


57

73


82


86



or tree farm).5

Fair


43

65


76


82




Good


32

58


72


79














Woods.6

Poor


45

66


77


83




Fair


36

60


73


79




Good


304

55


70


77














Farmsteads – buildings, lanes, driveways,


59

74


82


86



and surrounding lots.











1 Average runoff condition, and Ia = 0.2S.
2 Poor: <50% ground cover or heavily grazed with no mulch.
Fair: 50 to 75% ground cover and not heavily grazed.
Good: >75% ground cover and lightly or only occasionally grazed.
3 Poor: <50% ground cover.
Fair: 50 to 75% ground cover.
Good: >75% ground cover.
4 Actual curve number is less than 30; use CN=30 for runoff computations.
5 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.
6 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.

SCS TR-55 Table 2-2d – Runoff curve numbers for arid and semiarid rangelands1

Cover description

Curve numbers for hydrologic soil group

Cover type

Hydrologic condition2

A3

B

C

D

Herbaceous – 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 saltbrush,

greasewood, creosotebush, blackbrush, bursage,

palo verde, mesquite, and cactus.

Poor

63

77

85

88

Fair

55

72

81

86

Good

49

68

79

84

1 Average runoff condition, and Ia = 0.2S.
2 Poor: <30% ground cover (litter, grass, and brush overstory).
Fair: 30 to 70% ground cover.
Good: >70% ground cover.
3 Curve numbers for group A have been developed only for desert shrub.

APPENDIX B