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There are at least two versions of the old SCS lag equation. Table 1.1 in McCuen gives:
If we note that tc = tL/0.6 and that there are 60 minutes in an hour, and we replace HL with L and Y with S (slope, now expressed as a number rather than as a percentage), for the time of concentration, tc, in minutes, we get:
This second one is the form shown as Equation 3.49 on page 158 of McCuen. Table 9.18 in the ASCE Hydrology Handbook, 2nd Edition, gives the above equation in a slightly different form. Time of concentration, Tc, in minutes is given as:
The note in Table 9.18 for this entry states that the "equation [was] developed by SCS from agricultural watershed data. It has been adapted to small urban basins under 2000 acres. Found generally good where area is completely paved. For mixed areas, it tends to overestimate."
American Society of Civil Engineers (ASCE), Hydrology Handbook, 2nd Ed. ASCE, New York, NY, 1996.
McCuen, R.H., Hydrologic Analysis and Design, 3rd Ed. Pearson Prentice Hall, Upper Saddle River, NJ, 2005.