Unit Hydrograph

The direct runoff hydrograph resulting from a unit depth of excess rainfall produced by a storm of uniform intensity and specified duration. A hydrograph with a volume of 1 inch of runoff resulting from a storm of a specified duration and areal distribution. Hydrographs from other storms of the same duration and distribution are assumed to have the same time base but with ordinates of flow in proportion to the runoff volumes.

The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period