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The minimum sub-catchment area defines the drainage area at which headwater catchments can be formed (also known as drainage threshold or stream threshold). As a result, any link in the generated network will have a total upstream catchment area of at least this amount. Individual sub-catchment sizes will vary and some will be significantly smaller. Notably, where two connected nodes are very close together, the link between them will be short and the corresponding catchment can be very small.
DEMs that have been derived only from contour maps or from other remote sensing technologies such as light detection and ranging (LIDAR) or shuttle terrain radar mapping (STRM) are usually not hydrologically correct. Such DEMs need to be adjusted to represent the flow of streams in the right direction. This is because they create a sense that there are spurious pits in the landscape when actually, none are present. |
The Draw Network method (shown in Figure 2) allows you to define the networks manually, by loading a sub-catchment map (a raster with cell values set according to sub-catchment number) and then connecting sub-catchments by clicking and dragging using the mouse. This specifies how those sub-catchments are linked together by the node-link. You should use this method when you have previously defined the boundaries of your sub-catchments, either by using Source or an external program.
You can create a draft of your sub-catchment boundaries using the DEM based network generation method, export those boundaries from Source, edit them in an external GIS program (for example combining or splitting sub-catchments), and then import your edited sub-catchment boundaries back into Source to create a revised sub-catchment node-link network using the Draw Network method. |
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| Note: Any spatial layers displayed in the Geographic Editor must have the same projection as the sub-catchment map or DEM that was used to create the node-link network. |
DEM-based Network Generation
To use the DEM-based Network Generation method (shown in Figure 1):
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Figure 3. Recording Manager, View Catchment Area
Draw Network Method
To use the Draw Network method (Figure 3):
- Choose Draw Network from the Available Methods drop-down menu;
- Click Load Sub-catchment Map and open the desired sub-catchment raster file;
- Define the the direction of flow between the catchments by using one of two methods:
- Manual: Click and drag on the map to create a stream network by specifying the direction of flow between sub-catchments. Ensure a connection is also made from the lower-most sub-catchment to a point outside the sub-catchment.This is the catchment outlet, highlighted with a circle in Figure 4.
- Automatic: Click Add Links from Shp File (shapefile) to load a shapefile of links that are then mapped to the sub-catchment map automatically. Each link must be defined in the shapefile as a polyline. The link (polyline) may have internal vertices, but these will usually be ignored. By default, the Geographic Wizard allows one link per catchment only, and will generate an error if the selected shapefile does not satisfy this rule.
- Checking Allow Multiple Links per Catchment allows the user to load a shapefile with more than one link per catchment. Bifurcations are not permitted.
- Checking Use Internal Vertices will create separate links between internal vertices of a polyline (usually, links are only created between the polyline end points, and internal vertices are ignored).
- Click Next once the network includes every sub-catchment and a corresponding outlet. You can rename the sub-catchments in the table under the Sub-catchment button.
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Figure 3. Geographic Wizard, Network definition by drawing
The other options in the list (Pan, Zoom, Copy graph and Properties) have the same function as that described for using the DEM based network generation method.
Additional buttons under Load Sub-catchment Map Reference Points assist you in adding further detail to the network map. Load Gauge Map and Load Background Maps allow you to upload a shape file of gauges or an image like a stream flow map to assist in drawing the node-link network. Add Links from Shp file allows you to load a shape file of links that is then mapped to the sub-catchment map automatically.
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| Note: If all the sub-catchments have not been connected, they will be shaded in blue. All sub-catchments must be connected by links before you can click Next (Figure 4). |
Figure 4. Geographic Wizard, Incomplete connection of sub-catchments
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Regardless of which method you use to define the network (DEM based network generation or Draw Network), the subcatchments you create are automatically numbered with a prefix of "SC #"; and the numbers have leading zeros based on the total number of subcatchments. That is, SC #1, SC #2 … SC# 9 is used for a model with 1 to 9 subcatchments; SC #01, SC #02 … SC #99 is used for a model with 10 to 99 subcatchments; SC #001, SC #002 … SC #999 is used for a model with 100 to 999 subcatchments. |
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