Putnam (1972) developed a lag equation for watersheds around the Wichita, Kansas area as follows:
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.
Related Topics:
Computing Lag Time or Time of Concentration from Basin Data