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Draw and Assign Subbasins Command

In hydrology, a subbasin is used to represent the physical watershed. Subbasins define drainage area polygons that produce runoff to the other elements in the model. In GeoHECHMS, subbasins can be defined by either drawing or assigning polygons on the Map View using the following commands:

  • Draw Subbasins
  • AssignSubbasins

When creating a subbasin, the software performs the following tasks:

  • It creates the subbasin polygon on the HEC-HMS model-specific base layer contained within the Map Data Layers panel.
  • It assigns and computes the necessary attribute information of the created subbasin polygon.

Drawing Subbasins

The Draw Subbasins command allows the user to interactively draw polygons on the Map View as subbasins.

Follow the steps below to use the Draw Subbasins command:

  1. From the Input ribbon menu, select the Drainage Subbasins dropdown menu and then choose the Draw Subbasins command.
    Draw Subbasins ribbon menu command
  2. The Draw Subbasins dialog box will be displayed.
    Draw Subbasins dialog box

Note that pressing the [Ctrl+B] keys will run the Draw Subbasins command directly on the Map View.

The following sections describe how to use the Draw Subbasins command and interact with the above dialog box.

Drawing Subbasin Polygon

The Draw Subbasin Polygons section is used to draw the subbasins on the Map View using polygons. To draw a subbasin polygon, follow the steps below:

  1. Click on the [Draw] button and the dialog box will temporarily disappear. Use the Draw curvilinear polygon checkbox option to draw the polygon using curvilinear segments.
    [Draw] button
  2. The status bar (shown under the Map View) will prompt you to draw a subbasin on the Map View. Draw the subbasin polygon on the Map View. While drawing elements, the user can use the [Ctrl] key to switch between the curvilinear and the linear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.
  3. To finish digitizing the subbasin polygon, either press the [Enter] key or right-click and select Done from the displayed context menu. The user can press the [Esc] key to abort the current draw command.
  4. After completing the subbasin digitization, the user is immediately returned to the Draw Subbasins dialog box, and the Subbasin polygons read-only field will be changed from Not Drawn to Drawn.
    Subbasins polygon read-only field

Subbasin Specifications

This section is used to specify each drawn subbasin by providing each with an ID. The user can assign these IDs either manually or automatically by using some predefined formats.

Follow the steps below to assign subbasin IDs to the subbasins:

  1. If a subbasin has been drawn while the Subbasin ID option is enabled, the user can manually enter the subbasin ID in the corresponding field as shown below.
    Subbasin ID radio button option
  2. Alternately, the user can enable the Auto-name subbasin ID radio button option in order to automatically name every newly drawn subbasin as per the user’s predefined naming formats as shown below.
    Auto-name subbasin ID radio button optionThe different subbasin naming formats present in the Auto-name subbasin ID option are as follows:

    • Subbasin ID prefix: This option allows a prefix to be added to the front of the subbasin ID.
    • Subbasin ID digits: This option permits specification of a set number of digits to use for the subbasin ID. For example, using 3 digits causes the subbasin ID to be of the format 001, 002, etc.
    • Next available subbasin ID: This entry defines the next element ID number to be used.
    • Subbasin ID increment: This entry defines the increment to use when numbering elements. The default value is 1.
    • Subbasin ID suffix: This option allows a suffix to be added to the end of the subbasin ID.
    • Subbasin ID preview: This entry provides a preview of the subbasin naming specifications defined above.
  3. After providing the Subbasin ID, press the [Enter] key or click on the [Apply] button.
    [Apply] buttonNote that if the auto-name option is enabled and the user returns to the dialog box, the [Apply] button is disabled since the just drawn subbasins have already been named and created.
  4. The drawn subbasin will be displayed on the Map View.
    Map View subbasins

Assigning Subbasins

The Assign Subbasins command allows the user to manually associate previously drawn polylines or polygons as subbasins.

Follow the steps below to use the Assign Subbasins command:

  1. From the Input ribbon menu, select the Drainage Subbasins dropdown menu and then choose the Assign Subbasins command.
    Assign Subbasins ribbon menu command
  2. The Assign Subbasins dialog box will be displayed.
    Assign Subbasins dialog box

The following sections describe how to use the Assign Subbasins command and interact with the above dialog box.

Selecting Subbasin Polygons/Polylines

The Select Subbasin Polygons/Polylines section is used to select the previously drawn polylines/polygons from the Map View to assign them as subbasins.

To assign a subbasin polygon, follow the steps below:

  1. Click the [Pick] button, and the dialog box will temporarily disappear.
    [Pick] button
  2. The status bar will prompt you to select the polygon/polyline on the Map View. Select the subbasin polygon/polyline on the Map View.
  3. After selecting the polygon/polyline, the user is immediately returned to the Assign Subbasins dialog box. The Subbasin polygons/polylines read-only field will be changed from Not Selected to Selected.

Subbasins Specifications

This section is used to specify an ID for each selected subbasin polygon. The user can assign these IDs either manually or automatically by using some predefined formats.

Note that this section is similar to the Subbasin Specifications of the Draw Subbasins dialog box. Hence, see the second subsection under Drawing Subbasins to learn more about it.

About the Author Chris Maeder

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