Delineate Subbasins Command

In GeoHECHMS, the Delineate Subbasins command delineates upstream drainage subbasins from user-selected locations, detailing the subbasin boundaries and upstream drainage tributaries that contribute runoff to the selected locations. If the selected locations are not on an existing stream, the locations are automatically “snapped” to the nearest stream, and then the corresponding subbasin is delineated.

Note that this command is similar to the Delineate Watershed command of the Watershed ribbon menu. These commands are different, however, in that the Delineate Subbasins command uses the user’s local elevation terrain surface while the Delineate Watershed command relies on the USGS WMS (Web Map Service) service and NED (National Elevation Data). To learn more about the Delineate Watershed command, refer to this article in our knowledge base.

To use the Delineate Subbasins command, follow the steps below:

From the Watershed ribbon menu, select the Delineate Subbasins command.
The Delineate Subbasins dialog box will be displayed.

The Delineate Subbasins command operates in three stages, as illustrated below. At each stage, the Delineate Subbasins dialog box provides the user with different options.
Delineate Subbasins command stages

The below sections explain the above three stages and describe how to interact with the Delineate Subbasins dialog box.

Clip and Smooth Surface

At this stage, the software performs clipping, thinning, and smoothing of the terrain surface. The following Delineate Subbasins dialog box will be displayed at this stage.
In the above dialog box, the user needs to specify the following:

Terrain Elevation Source

This section allows the user to select the elevation terrain surface. The Terrain surface dropdown combo box lists all the terrain surfaces that are already added to the project.

Terrain Surface Processing Limits

This section allows the user to define the area extent of the terrain surface that will be used to process terrain data.
The user can select from three available options, as described below:

Selecting the Elevation source extents option causes the software to process the entire terrain data.
Selecting the User-defined limits option allows the user to select a rectangular region of the terrain surface for processing terrain data.
To select a rectangular region, follow the steps below:
1. Click the [Pick] button.
2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select a rectangular region for processing the terrain data.
3. Select a rectangular region from the terrain surface in the Map View. The Delineate Subbasins dialog box will be redisplayed. The status of the User-defined limits read-only field will change to Defined.
4. The software will consider the selected rectangular region for processing terrain data.

Selecting the Assign clipping polygon option allows the user to select polygon shape regions from the terrain surface for processing terrain data.
To select polygon shape regions, follow the steps below:
1. Click the [Pick] button.
2. The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the clipping polygons.
3. Select the clipping polygons on the terrain surface from the Map View. Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
4. The Delineate Subbasins dialog box will be redisplayed. The status of the Assign clipping polygons read-only field will change to Selected.
5. The area underneath selected clipping polygons will be considered by the software for processing terrain data.

Terrain Surface Thinning Options

This section allows the user to define the cell size that will be used for thinning the terrain surface. The smaller the cell size value, the longer the processing time.
The user can select from three available options to define the cell size, as described below:

Selecting the Compute optimal cell size option causes the software to use the default cell size value computed by the software. By default, the software selects this option.
Selecting the Define cell size option allows the user to manually define the cell size value. The user can use the spin control button to define the cell size value.
Selecting the Use terrain surface cell size option causes the software to use the default cell size value of the terrain surface.

Once all the options in the Delineate Subbasins dialog box are configured, click the [Next] button. The processing of the Clip and Smooth Surface stage will begin. On completion, the control will automatically pass to the Stream Delineation stage.

Stream Delineation

At this stage, the software analyzes possible pits, filters, and extracts dissolved pits, as well as locates possible trenches, and performs stream network delineation. The following Delineate Subbasins dialog box will be displayed at this stage.
The software provides three options to choose from to perform stream delineation, as described below:

Automatic Stream Delineation

Selecting this option causes the software to automatically perform stream delineation using stream drop analysis.
The Bridge width threshold input field allows the user to define the threshold value for the width of geographic obstructions such as roadways, bridges, and culverts that are burned on the terrain surface. These obstructions can affect the surface flow and break stream networks. Using the threshold value, the software can detect these obstructions and maintain the correct stream network. The user can enter the threshold value manually in the Bridge width threshold input field or click the [Measure] button to measure the threshold value of geographic obstructions from the Map View.

Checking the Smooth stream trenches checkbox causes the software to level the trenches found on the terrain surface. By default, this check box is checked.

Manual Stream Delineation

Selecting this option allows the user to manually select or draw polylines as delineated streams.

The Manual Stream Delineation option is useful when the user wants the delineation process to be more precise. For example, the user can quickly “burn-in” streams along bridge or culvert connections where one is missing. The software will honor the manually drawn stream alignment and force the delineation to follow that culvert or bridge connection to show the actual water flow.
Example of using Manual Stream Delineation option

To assign already drawn polylines as delineated streams, follow the steps below:

Click the [Pick] button.
The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select the stream alignment polylines.
Select the polylines representing the stream network on the terrain surface in the Map View. The status bar will display the count of selected polylines.
Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
The Delineate Subbasins dialog box will be redisplayed. The stream count will appear in the Stream Polylines read-only field.

To manually draw polylines as delineated streams, follow the steps below:

Click the [Draw] button.
The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to draw stream alignment polylines.
Draw the polylines representing the stream network in the Map View. The status bar will display the count of drawn polylines.
Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
The Delineate Subbasins dialog box will be redisplayed. The stream count will appear in the Stream Polylines read-only field.

Checking the Smooth stream trenches checkbox causes the software to level the stream trenches on the terrain surface. By default, this checkbox is checked.

Do Not Apply Stream Delineation

Selecting this option allows the user to skip the Stream Delineation stage. Select this option when the relevant elevation terrain surface already has a stream network burned on it.

Once all the options in the Delineate Subbasins dialog box are configured, click the [Next] button. The Stream Delineation process will begin. On completion, the software will automatically proceed to the Watershed Delineation stage.

Watershed Delineation

This is the last stage of the delineation process. At this stage, the software performs multiple tasks such as forming stream network and drainage subbasins, smoothing subbasin boundaries, and computing subbasin hydrology. The following Delineate Subbasins dialog box will be displayed at this stage.
In the above dialog box, the user needs to specify the following:

Watershed Delineation

This section allows the user to define the extent of the drainage accumulation area. This is the area that must be amassed for runoff to be considered as streamflow. Smaller values result in minor and major streams; larger values result in only major streams. By default, the software uses a value of 100 acres. The user can enter a different value for drainage accumulation area in the Subbasin contributing area input field or click the […] button to interactively measure it from the Map View.

Manually Define Subbasin Outlets

This section allows the user to manually draw outlets or select the already drawn outlets from the Map View. Select the Manually Define Subbasin Outlets checkbox to enable this section.
Note that it is recommended to use the default settings. For example, if you specify the minimum subbasin area (Subbasin contributing area) to be too large and manually define the outlets, the smaller subbasins may not appear. The software discards the subbasins with smaller areas than the defined minimum subbasin area. As a result, rather than specifying large values for the minimum subbasin area, it is advisable to use the Merge adjacent subbasins option from the Computational Option section. If the subbasins still don’t show up, it is better to go with the default settings and let the software automatically find the placement of subbasins.

To draw the outlets on the Map View, follow the steps below:

Click the [Draw] button.
The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to place the outlet points.
Click on the Map View to place downstream outlets points. The status bar will display the count of placed outlet points.
Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
The Delineate Subbasins dialog box will be redisplayed. The count of drawn outlet points will appear in the Draw outlets read-only field.
The [Clear] button can be used to remove any previously defined outlets in order to redo the process.

To assign outlets from the Map View, follow the steps below:

Click the [Pick] button.
The Delineate Subbasins dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user to select outlets points from Map View.
Click on the outlets on Map View to select them (selected outlets will be highlighted). The count of selected outlets will be displayed on the status bar.
Once finished, press the [Enter] key or right-click and select Done from the displayed context menu.
The Delineate Subbasins dialog box will be redisplayed. The count of selected outlets will appear in the Assign Outlets read-only field.
The [Clear] button can be used to remove any previously defined outlets in order to redo the process.

The user can define a threshold value for maximum distance to traverse for snapping the outlet point to the nearest stream in the Snap to stream distance input field. Alternatively, the user can click the […] button to measure the threshold value from the Map View. By default, the software uses a value of 300 ft.

Computational Options

This section provides multiple options to affect the subbasin’s appearance and delineation process.
The following options are provided under the Computational Options section:

Checking the Results layer group checkbox allows the user to group the junctions, subbasin and stream network layers into one layer group in the Map Data Layers panel. If this checkbox is left unchecked, the software will put the junctions, subbasin, and stream network layers separately in the Map Data Layers panel. The user can define the names of these layers by clicking the corresponding pen icons next to the layer name.
Checking the Delete previously delineated subbasins checkbox causes the software to delete any previously delineated subbasins. By default, this checkbox is checked.
Checking the Compute basic hydrologic properties checkbox causes the software to compute the basic hydrologic properties of each delineated subbasin. By default, this checkbox is checked.
Checking the Compute detailed hydrologic properties checkbox causes the software to compute the detailed hydrologic properties of each delineated subbasin.
Checking the Smooth subbasin boundaries checkbox causes the software to smooth the boundaries of delineated subbasins. By default, this checkbox is checked.
Checking the Smooth stream alignments checkbox causes the software to smooth the stream alignment network. By default, this checkbox is checked.
Checking the Merge adjacent subbasins checkbox causes the software to merge smaller adjacent subbasins into a larger subbasin.
Checking the Fix junctions checkbox causes the software to accurately align the junction points towards the actual intersection point of the delineated subbasins. By default, this checkbox is checked.
The Subbasin Colorization section allows the user to define and edit the color scheme for delineated subbasins. The Scheme dropdown combo box allows the user to choose a subbasin colorization scheme from multiple color schemes listed under it.

Once all the options in the Delineate Subbasins dialog box are configured, click the [Next] button. The Watershed Delineation process will begin.

Note that the user can switch back and forth between completed stages and change their choices using the [Back] and [Next] buttons. To reset the entire delineation process, the user can click the [Reset All] button.

To learn more about the basic and detailed hydrologic properties computed during the Watershed Delineation stage, refer to this article in our knowledge base.

On completion of the Watershed Delineation stage, the delineated subbasins network will be displayed in the Map View.