Changing the Duration
Very often, it will be necessary to change the duration of the unit hydrograph.
If unit hydrographs are to be averaged, then they must be of the same duration.
Also, convolution of the unit hydrograph with a precipitation event requires that the duration of the unit hydrograph be equal to the time step of the incremental precipitation.
The most common method of altering the duration of a unit hydrograph is by the S-curve method.
The S-curve method involves continually lagging a unit hydrograph by its duration and adding the ordinates.
For the present example, the 6-hour unit hydrograph is continually lagged by 6 hours and the ordinates are added.