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:

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.

 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.