# WMS:OC Unit Hydro Unit Hydrograph Method

A lag time and S-graph are required to compute a unit hydrograph. Compute a unit hydrograph for a basin by first selecting the basin and then choosing the **Unit Hydrograph Method...** button from the *Edit Orange County Unit Hydrograph Parameters* dialog.

## Lag Time

Basin lag time (hrs) is usually based on the time of concentration (*T _{c}*) computed using the rational method analysis. Lag time is automatically calculated to be

*0.8*T*when entering a value for

_{c}*T*, but it is always possible to enter any value for lag time. Entering a

_{c}*T*value is not required because it is only used to compute lag time.

_{c}## S-graphs

There are four standard S-graphs that are used in Orange County: valley developed, valley undeveloped, foothill, and mountain. Turn on the check box on next to each S-graph that is to be used. A composite S-graph is computed using the decimal weights assigned to each of the standard S-graphs.

The **Plot Unit Hydrograph** button computes the unit hydrograph using the lag time and composite S-graph and displays the results in both a graphical and tabular format.

Clicking on **OK** will write the unit hydrograph data to the UI card for analysis using HEC-1.

## Related Topics

WMS – Watershed Modeling System | ||
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Modules: | Terrain Data • Drainage • Map • Hydrologic Modeling • River • GIS • 2D Grid • 2D Scatter | |

Models: | CE-QUAL-W2 • GSSHA • HEC-1 • HEC-HMS • HEC-RAS • HSPF • MODRAT • NSS • OC Hydrograph • OC Rational • Rational • River Tools • Storm Drain • SMPDBK • SWMM • TR-20 • TR-55 | |

Toolbars: | Modules • Macros • Units • Digitize • Static Tools • Dynamic Tools • Drawing • Get Data Tools | |

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