LID DESIGN CALCULATIONS AND METHODOLOGY - Low Impact Development and Sustainable Stormwater Management - page 113

Initial abstraction (I a ) includes all losses before the start of surface runoff: depres-
sion storage, interception, evaporation, and infiltration. I a can be highly variable,
but the SCS has found that it can be approximated empirically by

I a = 0 . 2 S

Therefore, the runoff equation becomes (when P> 0 . 2 S )

− 0 . 2 S) 2
P − 0 . 8 S

(P

Q =

Finally, S is a function of the watershed soil and cover conditions as represented
by the runoff curve number (CN):

1000
CN − 10

S =

Therefore, runoff can be calculated using only the curve number and rain-
fall. Curve numbers are determined by land cover type, hydrologic condition,
antecedent runoff condition (ARC; sometimes referred to as antecedent moisture
condition), and hydrologic soil group. Curve numbers for various land covers
based on an average ARC for annual floods and I a = 0 . 2 S can be found in Urban
Hydrology for Small Watersheds [2], and various other references. Table 6-1
includes some of the more commonly used curve numbers from Urban Hydrology
for Small Watersheds .
Often, a single, area-weighted curve number is used to represent a watershed
consisting of subareas with different curve numbers. Although this approach is

Table 6-1 Commonly Used Curve Numbers

Curve Numbers for
Hydrologic Soil Group

Cover Type and Hydrologic Condition A B C D

Runoff Curve Numbers for Urban Areas a
Open spaces (parks, golf courses, cemeteries, etc.) b
Poor condition (grass cover < 50%) 68 79 86 89

Fair condition (grass cover 50% to 75%) 49 69 79 84

Good condition (grass cover > 75%) 39 61 74 80

Impervious areas
Paved parking lots, roofs, driveways, etc.
(excluding right-of-way) Streets and roads

98 98 98 98

paved; curbs and storm sewers (excluding
right-of-way)

98 98 98 98

Paved, open ditches (including right-of-way) 83 89 92 93

Gravel (including right-of-way) 76 85 89 91

( continued )