SCS Curve Number Method

The SCS curve number method is a simiple, widely used and efficient method for determining the approxient 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 stat requirments 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 2 former being of greatest importance.

The general equation for the SCS curve number method is as follows:

Curve Number Equations (3 KB)

The initial equation (1) is based on trends observed in data from collected sites, therefore it is an emperical equation instead of a physically based equation. After further empirical evaulation 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 availablity of the data. L-THIA provides the rainfall data for any area in the United States. Two, L-THIA completes this caluculation for every rainfall event for thirty years and then reports the average annual runoff value.

 


Curve Number Chart (12 KB)

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 - Staight Rows + Crop Residue Cover- Good Condition (1)
 
64
75
82
85
Commercial Urban Districts: Commerical 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
61
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, drivesways, 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

Notes

(1) Hydraulic condition is based on combination 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, (d) percent of residue on the land surface (good>=20%), and (e) degree of surface roughness.

(2) Good: Woods are protected form grazing, and litter and brush adequately cover the soil.

(3) Good: >75% ground cover and lightly or only occasionally grazed.

(4) CN's shown sre equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type.