Environmental Engineering Reference
In-Depth Information
Table 6-1 ( Continued )
Curve Numbers for
Hydrologic Soil Group
Hydrologic
Cover Type
Condition
A
B
C
D
Runoff Curve Numbers for Other Agricultural Lands c
Pasture, grassland, or range—continuous
forage for grazing d
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 e
Poor
48
67
77
83
Fair
35
56
70
77
30 f
Good
48
65
73
Woods-grass combination (orchard or tree
farm) g
Poor
57
73
82
86
Fair
43
65
76
82
Good
32
58
72
79
Woods h
Poor
45
66
77
83
Fair
36
60
73
79
30 f
Good
55
70
77
Farmsteads—buildings, lanes, driveways,
and surrounding lots
59
74
82
86
Source: [2, Table 2-2].
a Average runoff condition, and I a = 0 . 2 S .
b CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combi-
nations of open space cover type.
c Average runoff condition, and I a = 0 . 2 S .
d 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.
e Poor: < 50% ground cover.
Fair: 50 to 75% ground cover.
Good: > 75% ground cover.
f Actual curve number is less than 30; use CN = 30 for runoff computations.
g CNs shown were computed for areas with 50% woods and 50% grass (pasture) cover. Other
combinations of conditions may be computed from the CNs for woods and pasture.
h 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.
acceptable if the curve numbers are similar, if the difference in curve num-
bers is more than 5, the use of a weighted curve number reduces significantly
the estimated amount of runoff from the watershed. This is especially problem-
atic with pervious-impervious combinations: “combination of impervious areas
with pervious areas can imply a significant initial loss that may not take place.”
Therefore, the runoff from different subareas should be calculated separately and
then combined or weighted appropriately. At a minimum, runoff volume from
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