# 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 (Tc)
computed using the rational method analysis. Lag time is automatically
calculated to be 0.8*Tc when you enter a value for Tc, but you can always
enter any value for lag time. Entering a Tc value is not required because
it is only used to compute lag time.

## S-graphs

There are four standard S-graphs that are used in Orange County: valley
developed, valley undeveloped, foothill, and mountain. You must toggle
the check box on next to each S-graph that you would like to use. 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:**

Derived Unit Hydrograph

Tc from Rational Method