Stream Reach And Watershed

Title  Stream Reach And Watershed

Summary

This tool produces a vector network and OGR file from the stream raster grid. The flow direction grid is used to connect flow paths along the stream raster. The Strahler order of each stream segment is computed. The subwatershed draining to each stream segment (reach) is also delineated and labeled with the value identifier that corresponds to the WSNO (watershed number) attribute in the Stream Reach OGR file.

This tool orders the stream network according to the Strahler ordering system. Streams that don't have any other streams draining in to them are order 1. When two stream reaches of different order join the order of the downstream reach is the order of the highest incoming reach. When two reaches of equal order join the downstream reach order is increased by 1. When more than two reaches join the downstream reach order is calculated as the maximum of the highest incoming reach order or the second highest incoming reach order + 1. This generalizes the common definition to cases where more than two reaches join at a point. The network topological connectivity is stored in the Stream Network Tree file, and coordinates and attributes from each grid cell along the network are stored in the Network Coordinates file.

The stream raster grid is used as the source for the stream network, and the flow direction grid is used to trace connections within the stream network. Elevations and contributing area are used to determine the elevation and contributing area attributes in the network coordinate file. Points in the outlets file are used to logically split stream reaches to facilitate representing watersheds upstream and downstream of monitoring points. The program uses the attribute field "id" in the outlets file as identifiers in the Network Tree file. This tool then translates the text file vector network representation in the Network Tree and Coordinates files into a polyline OGR file. Further attributes are also evaluated. The program has an option to delineate a single watershed by representing the entire area draining to the Stream Network as a single value in the output watershed grid.


Usage

Command Prompt Syntax:

mpiexec -n <number of processes> StreamNet -fel <felfile> -p <pfile> -ad8 <ad8file> -src <srcfile> -ord <ordfile> -tree <treefile> -coord<coordfile> -net <netfile> [ -netlyr netlayername] [ -o <outletfile>] [ -lyrname <layer name>] [ -lyrno <layer number>] -w <wfile> [ -sw]

pfile: Input D8 flow directions grid 'p'

srcfile: Input stream raster grid 'src'

ordfile: Output grid of channel network Strahler order 'ord'

ad8file: Input grid with D8 contributing area 'ad8'

felfile: Input pit filled elevation data grid 'fel'

treefile: Output textfile with list of links in channel network tree

coordfile: Output textfile with list of coordinates in channel network tree

outletfile: input outlets file (OGR readable dataset).This must contain a field named "id" that is

used to identify the links upstream of outlet points

layer name: OGR layer name if outlets are not the first layer in outletfile (optional)

layer number: OGR layer number if outlets are not the first layer in outletfile (optional)

Layer name and layer number should not both be specified.

wfile: Output grid of watershed identifiers 'w'

netfile: Output file (OGR writeable data set) of resultant channel network.

netlayername: layer name of netfile (Optional)


Syntax

StreamReachAndWatershed (Input_Pit_Filled_Elevation_Grid, Input_D8_Flow_Direction_Grid, Input_D8_Drainage_Area, Input_Stream_Raster_Grid, {Input_Outlets}, Delineate_Single_Watershed, Input_Number_of_Processes, Output_Stream_Order_Grid, Output_Network_Connectivity_Tree, Output_Network_Coordinates, Output_Stream_Reach_file, Output_Watershed_Grid)

Parameter Explanation Data Type
Input_Pit_Filled_Elevation_Grid Dialog Reference

This input is a grid of elevation values. This is usually the output of the "Pit Remove" tool, in which case it is elevations with pits removed. Pits are low elevation areas in digital elevation models (DEMs) that are completely surrounded by higher terrain. They are generally taken to be artifacts that interfere with the processing of flow across DEMs, so are removed by raising their elevation to the point where they just drain. This step is not essential if you have reason to believe that the pits in your DEM are real. Elevation values are used in this function to compute the slope of stream segments. Not removing pits can result in negative (i.e. uphill) stream segment slopes.

 Raster Layer
Input_D8_Flow_Direction_Grid Dialog Reference

This input is a grid of flow directions that are encoded using the D8 method where all flow from a cells goes to a single neighboring cell in the direction of steepest descent.

 Raster Layer
Input_D8_Drainage_Area Dialog Reference

A grid giving the contributing area value in terms of the number of grid cells (or the summation of weights) for each cell taken as its own contribution plus the contribution from upslope neighbors that drain in to it using the D8 algorithm. This is usually the output of the "D8 Contributing Area" tool and is used to determine the contributing area attribute in the Network Coordinate file.

 Raster Layer
Input_Stream_Raster_Grid Dialog Reference

An indicator grid indicating streams, by using a grid cell value of 1 on streams and 0 off streams. Several of the "Stream Network Analysis" tools produce this type of grid. The Stream Raster Grid is used as the source for the stream network.

 Raster Layer
Input_Outlets (Optional) Dialog Reference

A point feature defining points of interest. If this input used, the tool will only deliiniate the stream network upstream of these outlets. Additionally, points in the Outlets feature are used to logically split stream reaches to facilitate representing watersheds upstream and downstream of monitoring points. This tool REQUIRES THAT THERE BE an integer attribute field "id" in the Outlets feature layer, because the "id" values are used as identifiers in the Network Tree file.

 Feature Layer
Delineate_Single_Watershed Dialog Reference

This option causes the tool to delineate a single watershed by representing the entire area draining to the Stream Network as a single value in the output watershed grid. Otherwise a seperate watershed is delineated for each stream reach.

 Boolean
Input_Number_of_Processes Dialog Reference

This input indicates the number of stripes that the domain will be divided into and the number of MPI parallel processes that will be spawned to evaluate each piece of the domain.

 Long
Output_Stream_Order_Grid Dialog Reference

The Stream Order Grid has cells values of streams ordered according to the Strahler order system. The Strahler ordering system defines order 1 streams as stream reaches that don't have any other reaches draining in to them. When two stream reaches of different order join the order of the downstream reach is the order of the highest incoming reach. When two reaches of equal order join the downstream reach order is increased by 1. When more than two reaches join the downstream reach order is calculated as the maximum of the highest incoming reach order or the second highest incoming reach order + 1. This generalizes the common definition to cases where more than two reaches join at a point. This generalizes the common definition to cases where more than two flow paths join at a point.

 Raster Dataset
Output_Network_Connectivity_Tree Dialog Reference

This output is a text file that details the network topological connectivity is stored in the Stream Network Tree file. Columns are as follows:

  • Link Number (Arbitrary - will vary depending on number of processes used)

  • Start Point Number in Network coordinates (*coord.dat) file (Indexed from 0)

  • End Point Number in Network coordinates (*coord.dat) file (Indexed from 0)

  • Next (Downstream) Link Number. Points to Link Number. -1 indicates no links downstream, i.e. a terminal link

  • First Previous (Upstream) Link Number. Points to Link Number. -1 indicates no upstream links.

  • Second Previous (Upstream) Link Numbers. Points to Link Number. -1 indicates no upstream links. Where only one previous link is -1, it indicates an internal monitoring point where the reach is logically split, but the network does not bifurcate.

  • Strahler Order of Link

  • Monitoring point identifier at downstream end of link. -1 indicates downstream end is not a monitoring point.

  • Network magnitude of the link, calculated as the number of upstream sources (following Shreve).

File
Output_Network_Coordinates Dialog Reference

This output is a text file that contains the coordinates and attributes of points along the stream network. Columns are as follows:

  • X coordinate

  • Y Coordinate

  • Distance along channels to the downstream end of a terminal link

  • Elevation

  • Contributing area

File
Output_Stream_Reach_file Dialog Reference

This output is a polyline OGR file giving the links in a stream network. The columns in the attribute table are:

LINKNO- Link Number. A unique number associated with each link (segment of channel between junctions). This is arbitrary and will vary depending on number of processes used.

DSLINKNO - Link Number of the downstream link. -1 indicates that this does not exist.

USLINKNO1 - Link Number of first upstream link. (-1 indicates no link upstream, i.e. for a source link)

USLINKNO2 - Link Number of second upstream link. (-1 indicates no second link upstream, i.e. for a source link or an internal monitoring point where the reach is logically split but the network does not bifurcate.)

DSNODEID - Node identifier for node at downstream end of stream reach. This identifier corresponds to the "id" attribute from the Outlets shapefile used to designate nodes.

strmOrder - Strahler Stream Order

Length - Length of the link. The units are the horizontal map units of the underlying DEM grid.

Magnitude - Shreve Magnitude of the link. This is the total number of sources upstream

DSContArea - Drainage area at the downstream end of the link. Generally this is one grid cell upstream of the downstream end because the drainage area at the downstream end grid cell includes the area of the stream being joined.

strmDrop - Drop in elevation from the start to the end of the link

Slope - Average slope of the link (computed as drop/length)

StraightL - Straight line distance from the start to the end of the link

USContArea - Drainage area at the upstream end of the link

WSNO - Watershed number. Cross reference to the *w.shp and *w grid files giving the identification number of the watershed draining directly to the link.

DOUTEND - Distance to the eventual outlet (i.e. the most downstream point in the stream network) from the downstream end of the link.

DOUTSTART - Distance to the eventual outlet from the upstream end of the link

DOUTMID - Distance to the eventual outlet from the midpoint of the link

 File
Output_Watershed_Grid Dialog Reference

This output grid identified each reach watershed with a unique ID number, or in the case where the delineate single watershed option was checked, the entire area draining to the stream network is identified with a single ID.

 Raster Dataset

Code Samples

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