The Generate Streams menu option is used to map vector streams for a project. Vector streams are an intersecting network of polyline segments with Horton ordering as described in the algorithm section. Vector streams can be generated using three main stream initiation methods to develop a set of potential channel heads which are then used for mapping flow paths and intersections to develop a vector stream network. These methods are:
•Stream Area Threshold (number of pixels or area). The SAT value refers to the number of pixel's flow paths that must accumulate in a single pixel before it will be considered a potential channel head.
•Use an existing vector layer containing points. Each point in this layer will be considered a potential channel head.
•Use an existing grid layer (decimal or integer) and consider all grid cell with values exceeding a threshold value as a potential channel head.
The forms for each of these methods are shown below.
The 'Create point layer based on existing watercourses' button on the 'Import Points' tab will analyse the existing streams layer and try to identify the upstream extremities of the unconnected stream segments to use as stream initiation points. This method is useful if you wish to generate a Vector Stream network that is very similar to an observed (mapped) stream network.
All methods of stream initiation allow the Vector Stream network to be further constrained by restricting the mapping of vector streams to previously delineated subcatchment areas by selecting the 'Restrict Streams to Catchments' check box. The form also prompts the user for an optional Minimum Source Channel Length (MSCL) parameters with which to generate the vector stream network. The MSCL parameter (if used – see checkbox) defines how long a stream segment must be to be considered valid. Thus if many potential channel heads exist in an area only the highest elevation pixel will be an actual channel head. No other channel heads will be accepted unless their source channels are greater than the MSCL before intersecting the vector stream network.
The effect of using the MSCL parameter can be seen below. The left figure does not use the MSCL parameter whereas the right figure has used a MSCL of 100 metres. Both vector stream networks were generated with the same SAT value. It can be seen that using the MSCL parameter can reduce the duplication of streams from nearby channel heads. It will also effect the calculated bifurcation of the stream network.
The stream network includes derivation of Horton orders for each segment of the stream network. The network can be coloured based on the derived Horton orders in the View Attributes form by selecting Properties from the Vector Layers panel when Synthetic Stream Network is selected.
Vector stream networks are also useful for analysis of subcatchment drainage properties as well as many of CatchmentSIM's charts and calculated subcatchment parameters.
