|
Interpreting L-THIA's Results: SCS Curve Number Method
The SCS curve
number method is a simple, widely used and efficient
method for determining the approximate amount
of runoff from a rainfall even in a particular
area. Although the method is designed for a
single storm event, it can be scaled to find
average annual runoff values. The data requirements
for this method are very low, rainfall amount
and curve number. The curve number is based
on the area's hydrologic soil group, land use, treatment and hydrologic condition.
The
general equation for the SCS curve number method
is as follows:
The
initial equation (1) is based on trends observed
in data from collected sites, therefore it is
an empirical equation instead of a physically
based equation. After further empirical evaluation
of the trends in the data base, the initial abstractions,
Ia, could be defined as a percentage of S (2).
With this assumption, the equation (3) could be
written in a more simplified form with only 3
variables. The parameter CN is a transformation
of S, and it is used to make interpolating, averaging,
and weighting operations more linear (4).
With
the following chart, the amount of runoff can
be found if the rainfall amount (in inches) and
curve number is known.
There
are two advantages of using L-THIA over a manual
method. One, the availability of the data; L-THIA
provides the rainfall data for any area in the
United States. Two, L-THIA completes this calculation
for every rainfall event for thirty years and
then reports the average annual runoff value.
|
Land
Use Description on Input Screen
|
Description
and Curve Numbers from TR-55
|
|
Cover
Description
|
Curve
Number for Hydrologic Soil Group |
|
Cover
Type and Hydrologic Condition
|
%
Impervious Areas
|
A
|
B
|
C
|
D
|
| Agricultural |
Row
Crops - Straight Rows + Crop Residue Cover-
Good Condition(1) |
|
64
|
75
|
82
|
85
|
| Commercial |
Urban
Districts: Commercial and Business |
85
|
89
|
92
|
94
|
95
|
| Forest |
Woods(2)
- Good Condition |
|
30
|
55
|
70
|
77
|
| Grass/Pasture |
Pasture,
Grassland, or Range(3)
- Good Condition |
|
39
|
1
|
74
|
80
|
| High
Density Residential |
Residential
districts by average lot size: 1/8 acre or
less |
65
|
77
|
85
|
90
|
92
|
| Industrial |
Urban
district: Industrial |
72
|
81
|
88
|
91
|
93
|
| Low
Density Residential |
Residential
districts by average lot size: 1/2 acre lot |
25
|
54
|
70
|
80
|
85
|
| Open
Spaces |
Open
Space (lawns, parks, golf courses, cemeteries,
etc.)(4)
Fair Condition (grass cover 50% to 70%) |
|
49
|
69
|
79
|
84
|
| Parking
and Paved Spaces |
Impervious
areas: Paved parking lots, roofs, driveways,
etc. (excluding right-of-way) |
100
|
98
|
98
|
98
|
98
|
| Residential
1/8 acre |
Residential
districts by average lot size: 1/8 acre or
less |
65
|
77
|
85
|
90
|
92
|
| Residential
1/4 acre |
Residential
districts by average lot size: 1/4 acre |
38
|
61
|
75
|
83
|
87
|
| Residential
1/3 acre |
Residential
districts by average lot size: 1/3 acre |
30
|
57
|
72
|
81
|
86
|
| Residential
1/2 acre |
Residential
districts by average lot size: 1/2 acre |
25
|
54
|
70
|
80
|
85
|
| Residential
1 acre |
Residential
districts by average lot size: 1 acre |
20
|
51
|
68
|
79
|
84
|
| Residential
2 acres |
Residential
districts by average lot size: 2 acre |
12
|
46
|
65
|
77
|
82
|
| Water/
Wetlands |
|
0
|
0
|
0
|
0
|
0
|
Color Key
|
Basic
Input Value
|
Detailed
Input Value
|
Basic
and Detailed Input Type Value
|
|
