Transformation

Knowing the volume is an important part of hydrology as it helps in designing storage facilities like detention basins.  However, just as important as the volume is the timing of the runoff, including the peak flow, time to peak, and duration.  The transformation of the effective precipitation into a time variation of runoff is described by a hydrograph.

If the effective precipitation is 1 inch, then the resulting hydrograph is referred to as a unit hydrograph.