# WMS:SCS Lag Time Equation

Perhaps the most commonly used equation for lag time is the SCS equation (1972) as shown. This equation may be used when computing the unit hydrograph using Snyder's method and any of the preceding equations for lag time may also be used when computing the unit hydrograph using the SCS method. Also remember that the SCS used the relationship relating lag time and time of concentration (Overview of Basin Data Equation) to compute lag time (and the other way around) from any of the time of concentration equations which follow.

$T_{LAG} = L^{0.8} \frac {(S + 1)^{0.7}}{1900 \sqrt {Y}}$

where:

TLAG = lag time in hours.
L = hydraulic length of watershed in feet.
Y = watershed slope in percent.
S = maximum retention in the watershed in inches as defined by:
$S = \frac {1000}{CN} -10$
CN = SCS curve number for the watershed as defined by the loss method.