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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. |
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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 #". The subcatchment numbers have leading zeros added 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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