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SCS Unit Hydrograph

SCS Unit Hydrograph

SCS Unit Hydrograph

I have seen several reports that refer to a calculation method called the SCS Unit Hydrograph.  Is this the same as the Tabular method, or the graphical method as described in the TR-55 manual, both, or something completely different?


RE: SCS Unit Hydrograph

I generally associate the unit hydrograph with the TR-55 method of computing flow rates.  The typical unit hydrograph is adjusted for the particular conditions (RCN Tc Area etc).

RE: SCS Unit Hydrograph

The SCS (now NRCS) Unit Hydrograph is typically used with the SCS runoff procedure, as embodied in TR-20 and other programs.  The UH is used to predict the runoff from a single burst of rainfall.  Complete runoff hydrographs are produced by combining a large number of UH's for an entire rainfall distribution.  (Its actually a bit more complicated, but this is the general outline.) For further info see http://www.hydrocad.net/tr-20.htm

The TR-55 Tabular Method is derived from TR-20, and is designed to approximate the runoff hydrographs that would be obtained with TR-20 itself.  TR-55 does't use the UH directly, since it is already embodied in the TR-20 results that it employs.   For details see http://www.hydrocad.net/tr-55.htm

A number of hydrology applications, such as HydroCAD, employ the actual SCS runoff procedure, making it easy to use a different UH if required for special applications.  For details see www.hydrocad.net

RE: SCS Unit Hydrograph

National Engineering Handbook Section 4-Hydrology, USDA Soil Conservation Service, 1972

This covers the engineering and science behind the unit hydrograph.  Type in a search engine and you can download a copy.  It is sometime listed as NEH 4

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