# WMS:Customized Lag Time Equation

Almost all of the lag time equations are of the form:

${\displaystyle T_{LAG}=C_{t}\ast \left({\frac {(L\ast L_{ca})}{\sqrt {S}}}\right)^{m}}$

where:

TLAG = watershed lag time in hours.
Ct = coefficient accounting for differences in watershed slope and storage.
L = the maximum flow length of the watershed along the main channel from the point of reference to the upstream boundary of the watershed, in miles.
Lca = the distance along the main channel from the point of reference to a point opposite the centroid, in miles.
S = slope of the maximum flow distance path in ft/mile.
m = lag exponent

Therefore, if the equation the state, county, etc. uses to compute lag time is not available, it can often be set up using this equation by entering a custom coefficient, Ct, and exponent, m.