C. E. Gregory[[27]] states that I, in the expression Q = AIR is a function of the time of concentration or the duration of the storm. If t represents the time of concentration and T represents the duration of the storm, then when T is less than t
I = 0.175t⅓,
but when T is greater than t,
I = 0.175
t(T4
3 − (T − t)4
3).
Gregory condenses Kuichling’s rules with regard to the per cent run-off, as follows:
1. The per cent of rainfall discharged from any given drainage area is nearly constant for heavy rains lasting equal periods of time.
2. This per cent varies directly with the area of impervious surface.
3. This per cent increases rapidly and directly or uniformly with the duration of the maximum intensity of the rainfall until a period is reached which is equal to the time required for the concentration of the drainage waters from the entire area at the point of observation, but if the rainfall continues at the same intensity for a longer period this per cent will continue to increase at a much smaller rate.
4. This per cent becomes larger when a moderate rain has immediately preceded a heavy shower on a partially permeable territory.
Gregory’s formulas have not been generally accepted and are not widely used in practice. Marston stated:[[28]]