Unit Hydrograph

Most hydrologic models use the unit hydrograph as the basis of converting a computed effective rainfall depth into a runoff hydrograph.  A unit hydrograph is defined by the runoff associated with a unit (1 inch in English units or 1 cm in Metric units) effective precipitation depth over the entire watershed. 

The unit hydrograph is also associated with a time duration, such as 30 minutes, 90 minutes, etc. meaning that the unit flowrates would be different for different durations of the 1 inch (cm) of effective precipitation.

The unit hydrograph is also affected by the different properties of the watershed such as shape, slope, and land cover.

A unit hydrograph for a basin is derived by measuring the rainfall hyetograph and the runoff at the outlet.  The runoff hydrograph is then normalized by the area of the watershed and rainfall depth, with the duration corresponding to the rainfall event.  The unit hydrograph can be used to estimate the runoff for any event size.  For example an effective precipitation of .5 inches over the duration of the unit hydrograph would result in a total runoff hydrograph where each ordinate is exactly half of the unit hydrograph ordinate.  Likewise a depth of 2 inches over the same duration would result in a hydrograph that is double the unit hydrograph (move your cursor over the different storm depths to see how the hydrographs are changed).