River tracing and catchment delineation

If the checkbox, ‘Use DEM for river tracing and catchment delineation’ is selected, the DEM can be used to delineate rivers and catchments based on the elevations in the DEM. Delineation is carried out using the ‘Add trace’ and ‘Add delineate’ tools in ‘Branch’ and ‘Catchment’ ribbons. More details about these tools are available in Section 6 (River Network) and Section 7 (Catch­ments). Before these tools can be used, however, the DEM must be pro­cessed.

Processing elevation data

The DEM is preprocessed to create a raster file with flow direction information that is used to delineate river branches and catchments.

Note: It is recommended that the size of the DEM should not exceed 25 mil­lion pixels. A warning message will be shown if the size of the DEM is above this limit.

The size of the DEM can be reduced by changing the spatial extent or by res­ampling.

Spatial extent

The spatial extent of the DEM can be limited to the defined working area (see below), thereby reducing the number of pixels.

Resampling

Resampling requires specification of a resampling factor. If a resampling fac­tor of 2 is used, then the minimum elevation of a 2x2 set of pixels is assigned to one new pixel with the same area as the 2x2 set. A resampling factor of 3 assigns the minimum value of a 3x3 pixel set to one new pixel with the same area as the 3x3 set.

Preprocess digital elevation data

In order to use the river tracing and catchment delineation on the map, the digital elevation data must be preprocessed by clicking the button ‘Preproc­ess digital elevation data’. After changing the spatial extent or resampling the elevation data, the DEM must be preprocessed again.

The processed data is saved in a ppdd-file. If no location is specified, then the ppdd file is saved by default in the same location as the model setup.

Many DEMs contain inaccuracies because of their resolution or artifacts resulting from the method of creation. When delineating catchments and riv­ers from the DEM, you may find some unexpected results. This can happen in flat areas where DEMs may not have the vertical resolution to correctly delineate the rivers and catchments.

Note: In many digital elevation models, all pixels in lakes have undefined (or blank) pixel values.

If the option ‘Assume internal undefined areas as local depressions’ is set, all internal undefined areas are handled as a local depression. When a traced branch is added, an artificial channel will be traced through the depression along the shortest path between the upstream and downstream ends. For example, if a lake is represented in a DEM as an undefined area, then an arti­ficial flow path will be created through the lake along the shortest path to the outlet.

If undefined areas are not set as local depressions, a traced branch will stop upon reaching an undefined area.