NAME
r.watershed - Calculates hydrological parameters and RUSLE factors.
KEYWORDS
raster,
hydrology,
watershed,
accumulation,
drainage,
stream network,
stream power index,
topographic index
SYNOPSIS
r.watershed
r.watershed --help
r.watershed [-s4mab] elevation=name [depression=name] [flow=name] [disturbed_land=name] [blocking=name] [retention=name] [threshold=integer] [max_slope_length=float] [accumulation=name] [tci=name] [spi=name] [drainage=name] [basin=name] [stream=name] [half_basin=name] [length_slope=name] [slope_steepness=name] [convergence=integer] [memory=memory in MB] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags:
- -s
- SFD (D8) flow (default is MFD)
- SFD: single flow direction, MFD: multiple flow direction
- -4
- Allow only horizontal and vertical flow of water
- -m
- Enable disk swap memory option: Operation is slow
- Only needed if memory requirements exceed available RAM; see manual on how to calculate memory requirements
- -a
- Use positive flow accumulation even for likely underestimates
- See manual for a detailed description of flow accumulation output
- -b
- Beautify flat areas
- Flow direction in flat areas is modified to look prettier
- --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
- depression=name
- Name of input depressions raster map
- All non-NULL and non-zero cells are considered as real depressions
- flow=name
- Name of input raster representing amount of overland flow per cell
- disturbed_land=name
- Name of input raster map percent of disturbed land
- For USLE
- blocking=name
- Name of input raster map blocking overland surface flow
- For USLE. All non-NULL and non-zero cells are considered as blocking terrain.
- retention=name
- Name of input raster map with percentages for flow accumulation.
- threshold=integer
- Minimum size of exterior watershed basin
- max_slope_length=float
- Maximum length of surface flow in map units
- For USLE
- accumulation=name
- Name for output accumulation raster map
- Number of cells that drain through each cell
- tci=name
- Name for output topographic index ln(a / tan(b)) map
- spi=name
- Name for output stream power index a * tan(b)
- Name for output raster map
- drainage=name
- Name for output drainage direction raster map
- Directions numbered from 1 to 8
- basin=name
- Name for output basins raster map
- stream=name
- Name for output stream segments raster map
- half_basin=name
- Name for output half basins raster map
- Each half-basin is given a unique value
- length_slope=name
- Name for output slope length raster map
- Slope length and steepness (LS) factor for USLE
- slope_steepness=name
- Name for output slope steepness raster map
- Slope steepness (S) factor for USLE
- convergence=integer
- Convergence factor for MFD (1-10)
- 1 = most diverging flow, 10 = most converging flow. Recommended: 5
- Default: 5
- memory=memory in MB
- Maximum memory to be used (in MB)
- Cache size for raster rows
- Default: 300
r.watershed generates a set of maps indicating: 1) flow
accumulation, drainage direction, the location of streams and
watershed basins, and 2) the LS and S factors of the Revised Universal
Soil Loss Equation (RUSLE).
Without flag
-m set, the entire analysis is run in memory
maintained by the operating system. This can be limiting, but is very
fast. Setting this flag causes the program to manage memory on disk
which allows very large maps to be processed but is slower.
Flag -s force the module to use single flow direction (SFD, D8)
instead of multiple flow direction (MFD). MFD is enabled by default.
By -4 flag the user allow only horizontal and vertical flow of
water. Stream and slope lengths are approximately the same as outputs
from default surface flow (allows horizontal, vertical, and diagonal
flow of water). This flag will also make the drainage basins look
more homogeneous.
When -a flag is specified the module will use positive flow
accumulation even for likely underestimates. When this flag is not
set, cells with a flow accumulation value that is likely to be an
underestimate are converted to the negative. See below for a detailed
description of flow accumulation output.
Option convergence specifies convergence factor for MFD. Lower
values result in higher divergence, flow is more widely
distributed. Higher values result in higher convergence, flow is less
widely distributed, becoming more similar to SFD.
Option elevation specifies the elevation data on which entire
analysis is based. NULL (nodata) cells 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. The elevation map need not be sink-filled because
the module uses a least-cost algorithm.
Option depression specifies the optional map of actual
depressions or sinkholes in the landscape that are large
enough to slow and store surface runoff from a storm event. All cells
that are not NULL and not zero indicate depressions. Water will flow
into but not out of depressions.
Raster flow map specifies amount of overland flow per cell.
This map indicates the amount of overland flow units that each cell
will contribute to the watershed basin model. Overland flow units
represent the amount of overland flow each cell contributes to surface
flow. If omitted, a value of one (1) is assumed.
Raster retention map specifies correction factors per cell for
flow distribution. This map indicates the percentage of overland flow
units leaving each cell. Values should be between zero and 100. If
omitted, a value of 100 is assumed.
Input Raster map or value containing the percent of disturbed land
(i.e., croplands, and construction sites) where the raster or input
value of 17 equals 17%. If no map or value is
given, r.watershed assumes no disturbed land. This input is
used for the RUSLE calculations.
Option blocking specifies terrain that will block overland
surface flow. Blocking cells and streams stop the slope length for the
RUSLE. All cells that are not NULL and not zero indicate blocking
terrain.
Option threshold specifies the minimum size of an exterior
watershed basin in cells, if no flow map is input, or overland flow
units when a flow map is given. Warning: low threshold values will
dramactically increase run time and generate difficult to read basin
and half_basin results. This parameter also controls the level of
detail in the stream segments map.
Value given by max_slope_length option indicates the maximum
length of overland surface flow in meters. If overland flow travels
greater than the maximum length, the program assumes the maximum
length (it assumes that landscape characteristics not discernible in
the digital elevation model exist that maximize the slope length).
This input is used for the RUSLE calculations and is a sensitive
parameter.
Output accumulation map contains the absolute value of the
amount of overland flow that traverses each cell. This value will be
the number of upland cells plus one if no overland flow map is given.
If the overland flow map is given, the value will be in overland flow
units. Negative numbers indicate that those cells possibly have surface
runoff from outside of the current geographic region. Thus, any cells
with negative values cannot have their surface runoff and sedimentation
yields calculated accurately.
Output tci raster map contains topographic index TCI,
computed as
ln(α / tan(β)) where α is the cumulative
upslope area draining through a point per unit contour length and
tan(β) is the local slope angle. The TCI reflects the
tendency of water to accumulate at any point in the catchment and the
tendency for gravitational forces to move that water downslope (Quinn
et al. 1991). This value will be negative if α /
tan(β) < 1.
Output spi raster map contains stream power index SPI,
computed as
α * tan(β) where α is the cumulative
upslope area draining through a point per unit contour length and
tan(β) is the local slope angle. The SPI reflects the
power of water flow at any point in the catchment and the
tendency for gravitational forces to move that water downslope (Moore
et al. 1991). This value will be negative if α < 0,
i.e. for cells with possible surface runoff from outside of the current
geographic region.
Output drainage raster map contains drainage direction.
Provides the "aspect" for each cell measured CCW from East.
Figure: Drainage is 8 directions numbered counter-clockwise
starting from 1 in north-east direction
(source)
Multiplying positive values by 45 will give the direction in degrees
that the surface runoff will travel from that cell. The value 0
(zero) indicates that the cell is a depression area (defined by the
depression input map). Negative values indicate that surface runoff
is leaving the boundaries of the current geographic region. The
absolute value of these negative cells indicates the direction of
flow. For MFD, drainage indicates the direction of maximum flow.
The output basin map contains unique label for each watershed
basin. Each basin will be given a unique positive even integer. Areas
along edges may not be large enough to create an exterior watershed
basin. NULL values indicate that the cell is not part of a complete
watershed basin in the current geographic region.
The output stream contains stream segments. Values correspond
to the watershed basin values. Can be vectorized after thinning
(r.thin) with
r.to.vect.
The output half_basin raster map stores each half-basin is
given a unique value. Watershed basins are divided into left and right
sides. The right-hand side cell of the watershed basin (looking
upstream) are given even values corresponding to the values in basin.
The left-hand side cells of the watershed basin are given odd values
which are one less than the value of the watershed basin.
The output length_slope raster map stores slope length and
steepness (LS) factor for the Revised Universal Soil Loss Equation
(RUSLE). Equations taken from Revised Universal Soil Loss
Equation for Western Rangelands (Weltz et al. 1987). Since the LS
factor is a small number (usually less than one), the GRASS output map
is of type DCELL.
The output slope_steepness raster map stores slope steepness
(S) factor for the Universal Soil Loss Equation (RUSLE). Equations
taken from article entitled
Revised Slope Steepness Factor for the Universal Soil
Loss Equation (McCool et al. 1987). Since the S factor is a small
number (usually less than one), the GRASS output map is of type DCELL.
r.watershed uses an A
T least-cost search algorithm
(see REFERENCES section) designed to minimize the impact of DEM data
errors. Compared
to
r.terraflow, this algorithm
provides more accurate results in areas of low slope as well as DEMs
constructed with techniques that mistake canopy tops as the ground
elevation. Kinner et al. (2005), for example, used SRTM and IFSAR DEMs
to compare
r.watershed
against
r.terraflow results in
Panama.
r.terraflow was unable
to replicate stream locations in the larger valleys
while
r.watershed performed much better. Thus, if forest
canopy exists in valleys, SRTM, IFSAR, and similar data products will
cause major errors in
r.terraflow stream output. Under
similar conditions,
r.watershed will generate
better
stream and
half_basin results. If watershed
divides contain flat to low slope,
r.watershed will generate
better basin results
than
r.terraflow. (
r.terraflow
uses the same type of algorithm as ESRI's ArcGIS watershed software
which fails under these conditions.) Also, if watershed divides
contain forest canopy mixed with uncanopied areas using SRTM, IFSAR,
and similar data products,
r.watershed will generate better
basin results
than
r.terraflow. The
algorithm produces results similar to those obtained when running
r.cost and
r.drain on every cell on the raster map.
r.watershed offers two methods to calculate surface flow:
single flow direction (SFD, D8) and multiple flow direction
(MFD). With MFD, water flow is distributed to all neighbouring cells
with lower elevation, using slope towards neighbouring cells as a
weighing factor for proportional distribution. The A
T
least-cost path is always included. As a result, depressions and
obstacles are traversed with a graceful flow convergence before the
overflow. The convergence factor causes flow accumulation to converge
more strongly with higher values. The supported range is 1 to 10,
recommended is a convergence factor of 5 (Holmgren, 1994). If many
small sliver basins are created with MFD, setting the convergence
factor to a higher value can reduce the amount of small sliver basins.
There are two versions of this program:
ram and
seg.
ram is used by default,
seg can be used by setting
the
-m flag.
The ram version requires a maximum of 31 MB of RAM for 1
million cells. Together with the amount of system memory (RAM)
available, this value can be used to estimate whether the current
region can be processed with the ram version.
The ram version uses virtual memory managed by the operating
system to store all the data structures and is faster than
the seg version; seg uses the GRASS segmentation
library which manages data in disk files. seg uses only as
much system memory (RAM) as specified with the memory option,
allowing other processes to operate on the same system, even when the
current geographic region is huge.
Due to memory requirements of both programs, it is quite easy to run
out of memory when working with huge map regions. If the ram
version runs out of memory and the resolution size of the current
geographic region cannot be increased, either more memory needs to be
added to the computer, or the swap space size needs to be
increased. If seg runs out of memory, additional disk space
needs to be freed up for the program to run.
The r.terraflow module was
specifically designed with huge regions in mind and may be useful here
as an alternative, although disk space requirements
of r.terraflow are several times higher than of seg.
The upper limit of the
ram version is 2 billion
(2
31 - 1) cells, whereas the upper limit for
the
seg version is 9 billion-billion (2
63 - 1 =
9.223372e+18)
cells.
In some situations, the region size (number of cells) may
be too large for the amount of time or memory
available. Running
r.watershed may then require use of a
coarser resolution. To make the results more closely resemble the
finer terrain data, create a map layer containing the lowest elevation
values at the coarser resolution. This is done by: 1) Setting the
current geographic region equal to the elevation map layer
with
g.region, and 2) Use
the
r.neighbors or
r.resamp.stats command to
find the lowest value for an area equal in size to the desired
resolution. For example, if the resolution of the elevation data is 30
meters and the resolution of the geographic region
for
r.watershed will be 90 meters: use the minimum function
for a 3 by 3 neighborhood. After changing to the resolution at
which
r.watershed will be run,
r.watershed should be
run using the values from the
neighborhood output map layer
that represents the minimum elevation within the region of the coarser
cell.
The minimum size of drainage basins, defined by the
threshold
parameter, is only relevant for those watersheds with a single stream
having at least the
threshold of cells flowing into it.
(These watersheds are called exterior basins.) Interior drainage
basins contain stream segments below multiple tributaries. Interior
drainage basins can be of any size because the length of an interior
stream segment is determined by the distance between the tributaries
flowing into it.
The
r.watershed program does not require the user to have the
current geographic region filled with elevation values. Areas without
elevation data (masked or NULL cells) are ignored. It is NOT necessary
to create a raster map (or raster reclassification)
named
MASK for NULL cells. Areas without elevation data will
be treated as if they are off the edge of the region. Such areas will
reduce the memory necessary to run the program. Masking out
unimportant areas can significantly reduce processing time if the
watersheds of interest occupy a small percentage of the overall area.
Gaps (NULL cells) in the elevation map that are located within the
area of interest will heavily influence the analysis: water will flow
into but not out of these gaps. These gaps must be filled beforehand,
e.g. with r.fillnulls.
Zero (0) and negative values will be treated as elevation data (not
no_data).
Problem areas, i.e. those parts of a basin with a likely underestimate
of flow accumulation, can be easily identified with e.g.
r.mapcalc "problems = if(flow_acc < 0, basin, null())"
If the region of interest contains such problem areas, and this is not
desired, the computational region must be expanded until the catchment
area for the region of interest is completely included.
To isolate an individual river network using the output of this
module, a number of approaches may be considered.
- Use a resample of the basins catchment raster map as a MASK.
The equivalent vector map method is similar
using v.select or
v.overlay.
- Use the r.cost module with a
point in the river as a starting point.
- Use the v.net.iso module
with a node in the river as a starting point.
All individual river networks in the stream segments output can be
identified through their ultimate outlet points. These points are all
cells in the stream segments output with negative drainage direction.
These points can be used as start points
for
r.water.outlet or
v.net.iso.
To create river mile segmentation from a vectorized streams
map, try
the v.net.iso
or v.lrs.segment
modules.
The stream segments output can be easily vectorized after thinning
with
r.thin. Each stream segment in the
vector map will have the value of the associated basin. To isolate
subbasins and streams for a larger basin, a MASK for the larger basin
can be created with
r.water.outlet. The stream
segments output serves as a guide where to place the outlet point used
as input to r.water.outlet.
The basin threshold must have been sufficiently small to isolate a
stream network and subbasins within the larger basin.
Given that the drainage is 8 directions numbered
counter-clockwise starting from 1 in north-east direction,
multiplying the output
by 45 (by 45. to get a double precision floating point raster
map in r.mapcalc) gives
the directions in degrees. For most applications, zeros
which indicate depressions specified by depression
and negative values which indicate runoff leaving the region
should be replaced by NULL (null() in
r.mapcalc).
The following command performs these replacements:
r.mapcalc "drainage_degrees = if(drainage > 0, 45. * drainage, null())"
Alternatively, the user can use the
-a flag or later the
abs() function in
r.mapcalc if the runoff is leaving
the region.
These examples use the Spearfish sample dataset.
If you want a detailed stream network, set the threshold option small
to create lots of catchment basins, as only one stream is presented
per catchment. The
r.to.vect -v flag preserves the catchment
ID as the vector category number.
r.watershed elev=elevation.dem stream=rwater.stream
r.to.vect -v in=rwater.stream out=rwater_stream
Set a different color table for the accumulation map:
MAP=rwater.accum
r.watershed elev=elevation.dem accum=$MAP
eval `r.univar -g "$MAP"`
stddev_x_2=`echo $stddev | awk '{print $1 * 2}'`
stddev_div_2=`echo $stddev | awk '{print $1 / 2}'`
r.colors $MAP col=rules << EOF
0% red
-$stddev_x_2 red
-$stddev yellow
-$stddev_div_2 cyan
-$mean_of_abs blue
0 white
$mean_of_abs blue
$stddev_div_2 cyan
$stddev yellow
$stddev_x_2 red
100% red
EOF
Create a more detailed stream map using the accumulation map and
convert it to a vector output map. The accumulation cut-off, and
therefore fractal dimension, is arbitrary; in this example we use the
map's mean number of upstream catchment cells (calculated in the above
example by r.univar) as the
cut-off value. This only works with SFD, not with MFD.
r.watershed elev=elevation.dem accum=rwater.accum
r.mapcalc 'MASK = if(!isnull(elevation.dem))'
r.mapcalc "rwater.course = \
if( abs(rwater.accum) > $mean_of_abs, \
abs(rwater.accum), \
null() )"
r.colors -g rwater.course col=bcyr
g.remove -f type=raster name=MASK
# Thinning is required before converting raster lines to vector
r.thin in=rwater.course out=rwater.course.Thin
r.colors -gn rwater.course.Thin color=grey
r.to.vect in=rwater.course.Thin out=rwater_course type=line
v.db.dropcolumn map=rwater_course column=label
r.watershed elev=elevation.dem basin=rwater.basin thresh=15000
r.to.vect -s in=rwater.basin out=rwater_basins type=area
v.db.dropcolumn map=rwater_basins column=label
v.db.renamecolumn map=rwater_basins column=value,catchment
Display output in a nice way
r.relief map=elevation.dem
d.shade shade=elevation.dem.shade color=rwater.basin bright=40
d.vect rwater_course color=orange
- 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://chuck.ehlschlaeger.info/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), 327-334.
DOI: 10.1002/hyp.3360080405
- Kinner D., Mitasova H., Harmon R., Toma L., Stallard R. (2005).
GIS-based Stream Network Analysis for The Chagres River Basin,
Republic of Panama. The Rio Chagres: A Multidisciplinary Profile of
a Tropical Watershed, R. Harmon (Ed.), Springer/Kluwer, p.83-95.
URL:
http://fatra.cnr.ncsu.edu/~hmitaso/measwork/panama/panama.html
- McCool et al. (1987). Revised Slope Steepness Factor for the Universal
Soil Loss Equation, Transactions of the ASAE Vol 30(5).
- Metz M., Mitasova H., Harmon R. (2011). Efficient extraction of
drainage networks from massive, radar-based elevation models with least
cost path search, Hydrol. Earth Syst. Sci. Vol 15, 667-678.
DOI: 10.5194/hess-15-667-2011
-
Moore I.D., Grayson R.B., Ladson A.R. (1991). Digital terrain
modelling: a review of hydrogical, geomorphological, and biological
applications, Hydrological Processes, Vol 5(1), 3-30
DOI: 10.1002/hyp.3360050103
- Quinn P., K. Beven K., Chevallier P., Planchon O. (1991). The
prediction of hillslope flow paths for distributed hydrological modelling
using Digital Elevation Models, Hydrological Processes Vol 5(1),
p.59-79.
DOI: 10.1002/hyp.3360050106
- Weltz M. A., Renard K.G., Simanton J. R. (1987). Revised Universal Soil
Loss Equation for Western Rangelands, U.S.A./Mexico Symposium of
Strategies for Classification and Management of Native Vegetation for
Food Production In Arid Zones (Tucson, AZ, 12-16 Oct. 1987).
g.region,
r.cost,
r.drain,
r.fillnulls,
r.flow,
r.mask,
r.neighbors,
r.param.scale,
r.resamp.interp,
r.terraflow,
r.topidx,
r.water.outlet,
r.stream.extract
Original version:
Charles Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory
Faster sorting algorithm and MFD support:
Markus Metz <markus.metz.giswork at gmail.com>
Retention for flow distribution by Andreas Gericke (IGB Berlin)
SOURCE CODE
Available at:
r.watershed source code
(history)
Latest change: Wednesday Nov 27 22:53:26 2024 in commit: 3b5486c0463a7103ab2109e28bf860fe34539868
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© 2003-2024
GRASS Development Team,
GRASS GIS 8.5.0dev Reference Manual