Geoscience Reference
In-Depth Information
Table B6.3.1
Curve Numbers (after USDA-SCS, 1985): soils of Group A have high
infiltration rates even if thoroughly wetted; group B soils have moderate infiltration
rates and are moderately well drained; group C soils have low infiltration rates with
an impeding layer for downward drainage; group D soils have low infiltration rates
when wetted, including swelling clays and soils with a permanently high water table
Land Use
Hydrologic Condition
Hydrologic
Soil Group
ABCD
Fallow
77 86 91 94
Row Crops: Contoured
Poor
70 79 84 88
Good
65 75 82 86
Small grain: Contoured
Poor
63 74 82 85
Good
61 73 81 84
Rotation Meadow: Contoured
Poor
64 75 83 85
Good
55 69 78 83
Pasture or Range
Poor
68 79 86 89
Fair
49 69 79 84
Good
39 61 74 80
Meadow
Good
30 58 71 78
Woods
Good
25 55 70 77
Farmsteads
-
59 74 82 86
Roads: Dirt
-
72 82 87 89
Roads: Surfaced
-
74 84 90 92
B6.3.3 Curve Numbers, Antecedent Moisture Condition, and Contributing Areas
The curve number approach, in its simplest form, takes no explicit account of the effect of the
antecedent moisture condition of the catchment in calculating the storm runoff. Yet it is known
that there can be a highly nonlinear dependence of runoff on antecedent state. The SCS-CN
model allows for this by suggesting that the curve number be modified according to whether
the catchment is wet or dry (Table B6.3.2), noting that the curve number is inversely related to
Table B6.3.2
Adjusting curve numbers for an-
tecedent moisture condition (after USDA-SCS, 1985):
Condition 2 represents the normal curve number con-
dition prior to the annual maximum flood; Condition
1 represents dry conditions; Condition 3 represents
soil that is nearly saturated
Curve Number
Condition 1
Condition 3
(Condition 2)
(Dry)
(Wet)
100
100
100
90
78
96
80
63
91
70
51
85
60
40
78
50
31
70
40
22
60
30
15
50
