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Unit Hydrograph Transformer

The standard SCS dimensionless unit hydrograph (DUH) can be transformed into a discharge versus time hydrograph (UH) for any watershed, given drainage area, time of concentration, and peak rate factor.  The duration associated with the UH is computed by the formula provided in the NRCS document, NEH 630 Hydrology Chapter 16, equation 16A-13.  According to the document, the duration should be near 13 to 20 percent of the time of concentration.

The spreadsheet downloadable here also creates an S-graph by lagging the UH several times by successive durations.  The S-graph enables runoff modeling with any duration of excess rainfall. (For more info, see any standard hydrology textbook.)

Finally, the spreadsheet computes the S-graph in a form which can be copied and pasted into the hydrology model HEC-HMS to enable it to compute runoff transformation using the SCS method and any peak rate factor.

The following Microsoft Excel spreadsheet was developed by the NRCS Water Quality and Quantity Technology Development Team, has been tested, and correctly produces the unit hydrograph transformation. The spreadsheet employs macros which are safe in this version at the time downloaded. The user is cautioned that proper use of the spreadsheet may require training in hydrology.



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