You can directly associate a user-defined unit hydrograph to a subarea (catchment) for runoff calculations. In dynamic flow routing, the hydraulics of the system are modeled over time.

The coefficients for the generic unit hydrographs are usually developed based on a linear correlation analysis between precipitation data and flow measurement. Usually flow monitoring data are only available at a handful of locations in a collection system. Therefore, when using generic unit hydrograph, you only need to load a model at points corresponding to flow monitoring locations. This lumping of wet weather loads will model flows downstream of the monitoring point accurately but will underestimate flow immediately upstream of the monitoring location.

In this method, the rainfall is converted into catchment outflow in discrete time intervals, called convolution time steps. Usually, it is best to set the convolution time step to the same value as the increment in the unit hydrograph that you enter. Smaller intervals won't produce greater accuracy while larger intervals will lose resolution.

For more information on unit hydrographs, see "Generic Unit Hydrographs"-438 and "Soil Conservation Service (SCS)"-440.

To add a unit hydrograph to a catchment:

1. Click a catchment in your model to display the Property Editor, or right-click a catchment and select Properties from the shortcut menu.
2. In the Runoff section of the Property Editor, select Unit Hydrograph as the Runoff Method.
3. Select Generic Unit Hydrograph in the Unit Hydrograph Method drop-down.
4. Click the Ellipses (...) button next to the Runoff Hydrograph field. The Unit Hydrograph Data dialog box appears.
5. Click the New button to add a row to the Time vs. Flow table.
6. Enter Time vs. Flow data into the table. Press Enter after typing a value to add a new row to the table (or click the New button to add a new row).
7. Click the Graph button to view a plot of the Time vs. Flow data.
8. Click OK to close the dialog box and add the hydrograph to the Property Editor for the catchment.

Unit Hydrograph Data Dialog Box

This dialog box allows you to define Time vs. Flow unit hydrographs. The dialog box contains the time-vs.-flow table and the following buttons:

Setting	Description
New	Creates a new row in the hydrograph table.
Delete	Deletes the currently highlighted row from the hydrograph table.
Report	Opens a print preview window containing a report that details the input data for this dialog box.
Graph	Opens a graph window plotting the unit hydrograph defined by the points in the table.

The table contains the following columns: