In hydrology, a statistically derived hydrograph for a storm of specified duration, graphically representing the percent of total direct runoff passing a point on a stream, as a function of time. This is usually presented as a histogram or table of percent runoff within each of successive short time intervals. In principle it is the same as the unit hydrograph; both are used as tools in river forecasting and for other purposes such as the comparison of runoff characteristics of different drainage areas.