WMS:Customized Lag Time Equation

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

 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.


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