928. S-curve, or S-curve hydrograph : A graph showing the summation of the ordinates of a series of unit hydrographs spaced at unit-rainfall duration intervals. It represents the hydrograph of unit rate of rainfall excess continued indefinitely. If the ordinates of the S-curve are expressed as a percentage of the total unit hydrograph volume and the abscissa as unit of time or as percentage of lag time, the S-curve is known as a ‘summation graph’.
