WMS:Putnam Lag Time Equation

Putnam (1972) developed a lag equation for watersheds around the Wichita, Kansas area as follows:

$T_{LAG} = 0.49 \sqrt { \frac {L}{ \sqrt {S}}} I_a^{-0.57}$

where:

TLAG = lag time in hours.
L = maximum flow length in miles.
S = weighted slope along the maximum flow path in ft/mile.
Ia = Impervious cover as a fraction.

This equation was used for watersheds ranging in size from 0.3 to 150 sq. miles, impervious covers less than 0.3 and a ratio of between 1.0 and 9.0.