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.


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