Provided by: grass-doc_7.8.2-1build3_all 
NAME
r.stream.extract - Performs stream network extraction.
KEYWORDS
raster, hydrology, stream network
SYNOPSIS
r.stream.extract
r.stream.extract --help
r.stream.extract elevation=name [accumulation=name] [depression=name] threshold=float [d8cut=float]
[mexp=float] [stream_length=integer] [memory=integer] [stream_raster=name] [stream_vector=name]
[direction=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags:
--overwrite
Allow output files to overwrite existing files
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
elevation=name [required]
Name of input elevation raster map
accumulation=name
Name of input accumulation raster map
Stream extraction will use provided accumulation instead of calculating it anew
depression=name
Name of input raster map with real depressions
Streams will not be routed out of real depressions
threshold=float [required]
Minimum flow accumulation for streams
Must be > 0
d8cut=float
Use SFD above this threshold
If accumulation is larger than d8cut, SFD is used instead of MFD. Applies only if no accumulation map
is given.
mexp=float
Montgomery exponent for slope, disabled with 0
Montgomery: accumulation is multiplied with pow(slope,mexp) and then compared with threshold
Default: 0
stream_length=integer
Delete stream segments shorter than stream_length cells
Applies only to first-order stream segments (springs/stream heads)
Default: 0
memory=integer
Maximum memory to be used (in MB)
Cache size for raster rows
Default: 300
stream_raster=name
Name for output raster map with unique stream ids
stream_vector=name
Name for output vector map with unique stream ids
direction=name
Name for output raster map with flow direction
DESCRIPTION
r.stream.extract extracts streams in both raster and vector format from a required input elevation map
and optional input accumulation map.
NOTES
NULL (nodata) cells in the input elevation map are ignored, zero and negative values are valid elevation
data. Gaps in the elevation map that are located within the area of interest must be filled beforehand,
e.g. with r.fillnulls, to avoid distortions.
All non-NULL and non-zero cells of depression map will be regarded as real depressions. Streams will not
be routed out of depressions. If an area is marked as depression but the elevation model has no
depression at this location, streams will not stop there. If a flow accumulation map and a map with real
depressions are provided, the flow accumulation map must match the depression map such that flow is not
distributed out of the indicated depressions. It is recommended to use internally computed flow
accumulation if a depression map is provided.
Option threshold defines the minimum (optionally modified) flow accumulation value that will initiate a
new stream. If Montgomery’s method for channel initiation is used, the cell value of the accumulation
input map is multiplied by (tan(local slope))mexp and then compared to threshold. If mexp is given than
the method of Montgomery and Foufoula-Georgiou (1993) to initiate a stream with this value. The cell
value of the accumulation input map is multiplied by (tan(local slope))mexp and then compared to
threshold. If threshold is reached or exceeded, a new stream is initiated. The default value 0 disables
Montgomery. Montgomery and Foufoula-Georgiou (1993) generally recommend to use 2.0 as exponent. mexp
values closer to 0 will produce streams more similar to streams extracted with Montgomery disabled.
Larger mexp values decrease the number of streams in flat areas and increase the number of streams in
steep areas. If weight is given, the weight is applied first.
Option d8cut defines minimum amount of overland flow (accumulation) when SFD (D8) will be used instead of
MFD (FD8) to calculate flow accumulation. Only applies if no accumulation map is provided. Setting to 0
disables MFD completely.
Option stream_length defines minimum stream length in number of cells for first-order (head/spring)
stream segments. All first-order stream segments shorter than stream_length will be deleted.
Output direction raster map contains flow direction for all non-NULL cells in input elevation. Flow
direction is of D8 type with a range of 1 to 8. Multiplying values with 45 gives degrees CCW from East.
Flow direction was adjusted during thinning, taking shortcuts and skipping cells that were eliminated by
the thinning procedure.
Stream extraction
If no accumulation input map is provided, flow accumulation is determined with a hydrological analysis
similar to r.watershed. The algorithm is MFD (FD8) after Holmgren 1994, as for r.watershed. The threshold
option determines the number of streams and detail of stream networks. Whenever flow accumulation
reaches threshold, a new stream is started and traced downstream to its outlet point. As for r.watershed,
flow accumulation is calculated as the number of cells draining through a cell.
If accumulation is given than the accumulation values of the provided accumulation map are used and not
calculated from the input elevation map. In this case the elevation map must be exactly the same map used
to calculate accumulation. If accumulation was calculated with r.terraflow, the filled elevation output
of r.terraflow must be used. Further on, the current region should be aligned to the accumulation map.
Flow direction is first calculated from elevation and then adjusted to accumulation. It is not necessary
to provide accumulation as the number of cells, it can also be the optionally adjusted or weighed total
contributing area in square meters or any other unit. When an original flow accumulation map is adjusted
or weighed, the adjustment or weighing should not convert valid accumulation values to NULL (nodata)
values.
In case of getting the error message ERROR: Accumulation raster map is NULL but elevation map is not NULL
the computational region must be carefully adjusted to exclude NULL pixels in the accumulation raster map
prior to stream extraction.
Weighed flow accumulation
Flow accumulation can be calculated first, e.g. with r.watershed, and then modified before using it as
input for r.stream.extract. In its general form, a weighed accumulation map is generated by first
creating a weighing map and then multiplying the accumulation map with the weighing map using r.mapcalc.
It is highly recommended to evaluate the weighed flow accumulation map first, before using it as input
for r.stream.extract.
This allows e.g. to decrease the number of streams in dry areas and increase the number of streams in wet
areas by setting weight to smaller than 1 in dry areas and larger than 1 in wet areas.
Another possibility is to restrict channel initiation to valleys determined from terrain morphology.
Valleys can be determined with r.param.scale method=crosc (cross-sectional or tangential curvature).
Curvature values < 0 indicate concave features, i.e. valleys. The size of the processing window
determines whether narrow or broad valleys will be identified (See example below).
Defining a region of interest
The stream extraction procedure can be restricted to a certain region of interest, e.g. a subbasin, by
setting the computational region with g.region and/or creating a MASK. Such region of interest should be
a complete catchment area, complete in the sense that the complete area upstream of an outlet point is
included and buffered with at least one cell.
Stream output
The output raster and vector contains stream segments with unique IDs. Note that these IDs are different
from the IDs assigned by r.watershed. The vector output also contains points at the location of the start
of a stream segment, at confluences and at stream network outlet locations.
Output stream_raster raster map stores extracted streams. Cell values encode a unique ID for each stream
segment.
Output stream_vector vector map stores extracted stream segments and points. Points are written at the
start location of each stream segment and at the outlet of a stream network. In layer 1, categories are
unique IDs, identical to the cell value of the raster output. The attribute table for layer 1 holds
information about the type of stream segment: start segment, or intermediate segment with tributaries,
and about the stream network this stream or node belongs to. Columns are cat int,stream_type
varchar(),type_code int,network int. The network attribute is the network ID of the stream/node. The
encoding for type_code is 0 = start, 1 = intermediate. In layer 2, categories are identical to type_code
in layer 1 with additional category 2 = outlet for outlet points. Points with category 1 = intermediate
in layer 2 are at the location of confluences.
EXAMPLE
This example is based on the elevation map "elev_ned_30m" in the North Carolina sample dataset and uses
valleys determined with r.param.scale to weigh an accumulation map produced with r.watershed.
# set region
g.region -p raster=elev_ned_30m@PERMANENT
# calculate flow accumulation
r.watershed ele=elev_ned_30m@PERMANENT acc=elev_ned_30m.acc
# curvature to get narrow valleys
r.param.scale input=elev_ned_30m@PERMANENT output=tangential_curv_5 size=5 method=crosc
# curvature to get a bit broader valleys
r.param.scale input=elev_ned_30m@PERMANENT output=tangential_curv_7 size=7 method=crosc
# curvature to get broad valleys
r.param.scale input=elev_ned_30m@PERMANENT output=tangential_curv_11 size=11 method=crosc
# create weight map
r.mapcalc "weight = if(tangential_curv_5 < 0, -100 * tangential_curv_5, \
if(tangential_curv_7 < 0, -100 * tangential_curv_7, \
if(tangential_curv_11 < 0, -100 * tangential_curv_11, 0.000001)))"
# weigh accumulation map
r.mapcalc expr="elev_ned_30m.acc.weighed = elev_ned_30m.acc * weight"
# copy color table from original accumulation map
r.colors map=elev_ned_30m.acc.weighed raster=elev_ned_30m.acc
Weight map (spatial subset with lake in the southern half)
Original flow accumulation map (spatial subset with lake in the southern half)
Weighed flow accumulation map (spatial subset with lake in the southern half)
Display both the original and the weighed accumulation map. Compare them and proceed if the weighed
accumulation map makes sense.
# extract streams using the original accumulation map
r.stream.extract elevation=elev_ned_30m@PERMANENT \
accumulation=elev_ned_30m.acc \
threshold=1000 \
stream_rast=elev_ned_30m.streams.noweight
# extract streams from weighed map
# note that the weighed map is a bit smaller than the original map
r.stream.extract elevation=elev_ned_30m@PERMANENT \
accumulation=elev_ned_30m.acc.weighed \
threshold=1000 \
stream_rast=elev_ned_30m.streams
Now display both stream maps and decide which one is more realistic.
Extracted streams from original flow accumulation map
Extracted streams from weighed flow accumulation map
REFERENCES
• Ehlschlaeger, C. (1989). Using the AT Search Algorithm to Develop Hydrologic Models from Digital
Elevation Data, Proceedings of International Geographic Information Systems (IGIS) Symposium ’89,
pp 275-281 (Baltimore, MD, 18-19 March 1989). URL:
http://faculty.wiu.edu/CR-Ehlschlaeger2/older/IGIS/paper.html
• Holmgren, P. (1994). Multiple flow direction algorithms for runoff modelling in grid based
elevation models: An empirical evaluation. Hydrological Processes Vol 8(4), pp 327-334. DOI:
10.1002/hyp.3360080405
• Montgomery, D.R., Foufoula-Georgiou, E. (1993). Channel network source representation using
digital elevation models. Water Resources Research Vol 29(12), pp 3925-3934.
SEE ALSO
r.mapcalc, r.param.scale, r.stream.channel (Addon), r.stream.distance (Addon), r.stream.order (Addon),
r.stream.segment (Addon), r.stream.slope (Addon), r.stream.snap (Addon), r.stream.stats (Addon),
r.terraflow, r.thin, r.to.vect, r.watershed
See also r.streams.* modules wiki page.
AUTHOR
Markus Metz
SOURCE CODE
Available at: r.stream.extract source code (history)
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© 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual