Problem

Assume the watershed in Example has gone through extensive commercial and industrial growt...

Assume the watershed in Example has gone through extensive commercial and industrial growth on the wooded area. Now 50% of the formerly wooded areas have become urbanized, so of that portion, 40% is commercial and business and 60% is fair condition lawn space. Assume the soil is 50% group B and 50% group C for all areas. Using Figure, determine the runoff volume for a rainfall of 6 in.

EXAMPLE

SCS CURVE-NUMBER METHOD

A watershed is 40% wooded (good condition) and 60% residential (1/4-ac lots). The watershed has 50% soil group B and 50% soil group C. Determine the runoff volume if the rainfall is 7 in. Assume antecedent moisture condition number II (Table).

Land Use	Soil Group	Fraction of Area	CN
Wooded	B	0.4(0.5) = 0.2	55
	C	0.4(0.5) = 0.2	70
Residential	B	0.6(0.5) = 0.3	75
	C	0.6(0.5) = 0.3	83

The weighted CN is

CN = 0.2(55) + 0.2(70) + 0.3(75) + 0.3(83) or

CN = 11 + 14 + 22.5 + 24.9 = 72.4

or, using CN = 72, runoff volume is 3.9 in for the given rainfall (Figure).

Figure

Graphical solution of rainfall-runoff equation.

Table

Runoff Curve Numbers for Selected Agricultural, Suburban, and Urban Land Use (Antecedent Moisture Condition II; Ia = 0.2S)

Land Use Description		Hydrologic Soil Group
		A	B	C	D
Cultivated land1
Without conservation treatment		72	81	88	91
With conservation treatment		62	71	78	81
Pasture or range land
Poor condition		68	79	86	89
Good condition		39	61	74	80
Meadow
Good condition		30	58	71	78
Wood or forest land
Thin stand, poor cover, no mulch		45	66	77	83
Good cover2		25	55	70	77
Open spaces, lawns, parks, golf courses, cemeteries, etc.
Good condition: grass cover on 75% or more of the area		39	61	74	80
Fair condition: grass cover on 50%-75% of the area		49	69	79	84
Commercial and business areas (85% impervious)		89	92	94	95
Industrial districts (72% impervious)		81	88	91	93
Residential3
Average lot size	Average % impervious4
1/8 ac or less	65	77	85	90	92
1/4 ac	38	61	75	83	87
1/3 ac	30	57	72	81	86
1/2 ac	25	54	70	80	85
1 ac	20	51	68	79	84
Paved parking lots, roofs, driveways, etc.5		2 8	98	98	98
Streets and roads
Paved with curbs and storm sewers5		2 8	98	98	98
Gravel		76	85	89	91
Dirt		72	82	87	89

1For a more detailed description of agricultural land use curve numbers, refer to National Engineering Handbook, Section 4, “Hydrology,” Chapter 9, Aug. 1972.

2Good cover is protected from grazing, and litter and brush cover soil.

3Curve numbers are computed assuming that the runoff from the house and driveway is directed toward the street with a minimum of roof water directed to lawns where additional infiltration could occur.

4The remaining pervious areas (lawn) are considered to be in good pasture condition for these curve numbers.

5In some warmer climates of the country, a curve number of 95 may be used.

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