Cover description

Curve numbers for hydrologic soil group

Cover type & Hydrologic condition

% Impervious area

A

B

C

D

 Fully-developed urban areas 

 (vegetation established) Open space

 (lawns, 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, streets, roofs,

      etc. (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)

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

      ⅛ acre (500 m²) or less (townhouse)

65%

77

85

90

92

      ¼ acre (1000 m²)

48%

61

75

83

87

      ½ acre (2000 m²)

25%

54

70

80

85

      1 acre (4000 m²)

20%

51

68

79

84

      2 acres (8000 m²)

12%

46

65

77

82

 Agricultural lands:

      Meadow – continuous grass;

      protected from grazing & generally

      mowed for hay

30

58

71

78

      Woods – grass combination

      (orchard or tree farm)

            Poor

< 50% cover

57

73

82

86

            Fair

50% to 75% cover

43

65

76

82

            Good

> 75% cover

32

58

72

79

      Newly graded areas (pervious

      areas only, no vegetation)

77

86

91

94