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<h2>NAME</h2>
<em><b>r.watershed</b></em> - Watershed basin analysis program.
<h2>KEYWORDS</h2>
raster
<h2>SYNOPSIS</h2>
<b>r.watershed</b><br>
<b>r.watershed help</b><br>
<b>r.watershed</b> [-<b>m4</b>] <b>elevation</b>=<em>name</em> [<b>depression</b>=<em>string</em>] [<b>flow</b>=<em>string</em>] [<b>disturbed.land</b>=<em>string</em>] [<b>blocking</b>=<em>string</em>] [<b>threshold</b>=<em>integer</em>] [<b>max.slope.length</b>=<em>float</em>] [<b>accumulation</b>=<em>string</em>] [<b>drainage</b>=<em>string</em>] [<b>basin</b>=<em>string</em>] [<b>stream</b>=<em>string</em>] [<b>half.basin</b>=<em>string</em>] [<b>visual</b>=<em>string</em>] [<b>length.slope</b>=<em>string</em>] [<b>slope.steepness</b>=<em>string</em>] [--<b>overwrite</b>] [--<b>verbose</b>] [--<b>quiet</b>]
<h3>Flags:</h3>
<DL>
<DT><b>-m</b></DT>
<DD>Enable disk swap memory option: Operation is slow</DD>
<DT><b>-4</b></DT>
<DD>Allow only horizontal and vertical flow of water</DD>
<DT><b>--overwrite</b></DT>
<DD>Allow output files to overwrite existing files</DD>
<DT><b>--verbose</b></DT>
<DD>Verbose module output</DD>
<DT><b>--quiet</b></DT>
<DD>Quiet module output</DD>
</DL>
<h3>Parameters:</h3>
<DL>
<DT><b>elevation</b>=<em>name</em></DT>
<DD>Input map: elevation on which entire analysis is based</DD>
<DT><b>depression</b>=<em>string</em></DT>
<DD>Input map: locations of real depressions</DD>
<DT><b>flow</b>=<em>string</em></DT>
<DD>Input map: amount of overland flow per cell</DD>
<DT><b>disturbed.land</b>=<em>string</em></DT>
<DD>Input map or value: percent of disturbed land, for USLE</DD>
<DT><b>blocking</b>=<em>string</em></DT>
<DD>Input map: terrain blocking overland surface flow, for USLE</DD>
<DT><b>threshold</b>=<em>integer</em></DT>
<DD>Input value: minimum size of exterior watershed basin</DD>
<DT><b>max.slope.length</b>=<em>float</em></DT>
<DD>Input value: maximum length of surface flow, for USLE</DD>
<DT><b>accumulation</b>=<em>string</em></DT>
<DD>Output map: number of cells that drain through each cell</DD>
<DT><b>drainage</b>=<em>string</em></DT>
<DD>Output map: drainage direction</DD>
<DT><b>basin</b>=<em>string</em></DT>
<DD>Output map: unique label for each watershed basin</DD>
<DT><b>stream</b>=<em>string</em></DT>
<DD>Output map: stream segments</DD>
<DT><b>half.basin</b>=<em>string</em></DT>
<DD>Output map: each half-basin is given a unique value</DD>
<DT><b>visual</b>=<em>string</em></DT>
<DD>Output map: useful for visual display of results</DD>
<DT><b>length.slope</b>=<em>string</em></DT>
<DD>Output map: slope length and steepness (LS) factor for USLE</DD>
<DT><b>slope.steepness</b>=<em>string</em></DT>
<DD>Output map: slope steepness (S) factor for USLE</DD>
</DL>
<H2>DESCRIPTION</H2>
<EM>r.watershed</EM> generates a set of maps indicating:
1) the location of watershed basins, and
2) the LS and S factors of the Revised Universal Soil Loss Equation (RUSLE).
<P>
<!-- Interactive mode not activated in GRASS 6.
<EM>r.watershed</EM> can be run either interactively or non-interactively.
If the user types <tt>r.watershed</tt>
on the command line without program arguments, the program will prompt the user
with a verbose description of the input maps. The interactive version of
<EM>r.watershed</EM> can prepare inputs to lumped-parameter hydrologic models.
After a verbose interactive session, <EM>r.watershed</EM> will query the user
for a number of
map layers. Each map layer's values will be tabulated by watershed basin and sent
to an output file. This output file is organized to ease data entry into a
lumped-parameter hydrologic model program. The non-interactive version of
<EM>r.watershed</EM> cannot create this file.
<P>
The user can run the program non-interactively, by specifying input map names
on the command line. Parameter names may be specified by their
full names, or by any initial string that distinguish them from other parameter names.
In <EM>r.watershed</EM>'s case, the first two letters of each name sufficiently
distinguishes parameter names.
For example, the two expressions below are equivalent inputs to <EM>r.watershed</EM>:
<P>
<PRE>
el=elev.map th=100 st=stream.map ba=basin.map
elevation=elev.map threshold=100 stream=stream.map basin=basin.map
</PRE>
-->
<H2>OPTIONS</H2>
<dl>
<DT><EM>-m</EM>
<DD>Without this flag set, the entire analysis is run in memory
maintained by the operating system. This can be limiting, but is
relatively fast. Setting the flag causes the program to manage memory
on disk which allows larger maps to be processes but is considerably
slower.
<DT><EM>-4</EM>
<DD>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.
<DT><EM>elevation</EM>
<DD>Input map: Elevation on which entire analysis is based.
<DT><EM>depression</EM>
<DD>Input map: Map layer of actual depressions or sinkholes in the
landscape that are large enough to slow and store surface runoff from
a storm event. Any non-zero values indicate depressions.
<DT><EM>flow</EM>
<DD>Input map: 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. The algorithm is D8 flow accumulation.
<DT><EM>disturbed.land</EM>
<DD>Raster map input layer 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, <EM>r.watershed</EM>
assumes no disturbed land. This input is used for the RUSLE calculations.
<DT><EM>blocking</EM>
<DD>Input map: terrain that will block overland surface flow. Terrain
that will block overland surface flow and restart the slope length
for the RUSLE. Any non-zero values indicate blocking terrain.
<DT><EM>threshold</EM>
<DD>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 too read basin and half.basin results.
This parameter also controls the level of detail in the <em>stream</em>
segments map.
<DT><EM>max.slope.length</EM>
<DD>Input value indicating 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.
<DT><EM>accumulation</EM>
<DD>Output map: The absolute value of each cell in this output map layer is
the amount of overland flow that traverses the 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.
<DT><EM>drainage</EM>
<DD>Output map: drainage direction. Provides the "aspect" for each
cell. Multiplying positive values by 45 will give the direction in
degrees that the surface runoff will travel from that cell. The
value -1 indicates that the cell is a depression area (defined by
the depression input map). Other 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.
<DT><EM>basin</EM>
<DD>Output map: 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. 0 values
indicate that the cell is not part of a complete watershed basin
in the current geographic region.
<DT><EM>stream</EM>
<DD>Output map: stream segments. Values correspond to the watershed
basin values.
<DT><EM>half.basin</EM>
<DD>Output map: 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.
<DT><EM>visual</EM>
<DD>Output map: useful for visual display of results.
Surface runoff accumulation with the values
modified to provide for easy display. All negative accumulation values
are changed to zero. All positive values above the basin threshold
are given the value of the <em>threshold</em> parameter.
<DT><EM>length.slope</EM>
<DD>Output map: slope length and steepness (LS) factor. Contains the LS
factor for the Revised Universal Soil Loss Equation. Equations taken
from <EM>Revised Universal Soil Loss Equation for Western Rangelands</EM>
(Weltz et al. 1987).
Since the LS factor is a small number, it is multiplied by 100 for the
GRASS output map.
<DT><EM>slope.steepness</EM>
<DD>Output map: slope steepness (S) factor for RUSLE.
Contains the revised S factor for the Universal Soil
Loss Equation. Equations taken from article entitled
<EM>Revised Slope Steepness Factor for the Universal Soil
Loss Equation</EM> (McCool et al. 1987). Since the S factor
is a small number (usually less than one), it is multiplied
by 100 for the GRASS output map layer.
</dd>
</dl>
<H2>NOTES</H2>
<EM>r.watershed</EM> uses an algorithm designed to minimize the impact of
DEM data errors. This algorithm works slower than <EM>r.terraflow</EM> but
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
<EM>r.watershed</EM> against <EM>r.terraflow</EM> results in Panama.
<EM>r.terraflow</EM> was unable to replicate stream locations in the larger
valleys while <EM>r.watershed</EM> performed much better. Thus, if forest
canopy exists in valleys, SRTM, IFSAR, and similar data products will cause
major errors in <EM>r.terraflow</EM> stream output. Under similar conditions,
<EM>r.watershed</EM> will generate better <b>stream</b> and <b>half.basin</b>
results. If watershed divides contain flat to low slope, <EM>r.watershed</EM>
will generate better basin results than <EM>r.terraflow</EM>.
(<EM>r.terraflow</EM> 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, <EM>r.watershed</EM> will generate better basin
results than <EM>r.terraflow</EM>.
<P>
There are two versions of this program: <EM>ram</EM> and <EM>seg</EM>.
Which is version is run depends on whether the <EM>-m</EM> flag is set.
<BR>
The <EM>ram</EM> version uses virtual memory managed by the operating
system to store all the data structures and is faster than the <EM>seg</EM>
version;
<EM>seg</EM> uses the GRASS segmentation library which manages data in disk
files. Thus <EM>seg</EM> uses much less system memory (RAM) allowing other
processes to operate on the same CPU, even when the current geographic
region is huge.
<BR>
Due to memory requirements of both programs, it is quite easy to run out of
memory when working with huge map regions. If the <EM>ram</EM> 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 <EM>seg</EM> runs out of
memory, additional disk space needs to be freed up for the program to run.
<P>
Both versions use the A<sup>T</sup> least-cost search algorithm to determine
the flow of water over the landscape (see <a href="#seealso">SEE ALSO</a>
section).
The algorithm produces results similar to those obtained when running
<EM><A HREF="r.cost.html">r.cost</A></EM> and
<EM><A HREF="r.drain.html">r.drain</A></EM> on every cell on the map.
<P>
In many situations, the elevation data will be too finely detailed for
the amount of time or memory available. Running <EM>r.watershed</EM> may
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 <EM>g.region</EM>, and 2) Use the <EM>r.neighbors</EM> or
<EM>r.resamp.stats</EM> 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 <EM>r.watershed</EM> will be 90 meters: use the minimum
function for a 3 by 3 neighborhood. After changing to the resolution at
which <EM>r.watershed</EM> will be run, <EM>r.watershed</EM> should be run
using the values from the <EM>neighborhood</EM> output map layer that
represents the minimum elevation within the region of the coarser cell.
<P>
The minimum size of drainage basins, defined by the <em>threshold</em>
parameter, is only relevant for those watersheds with a single stream
having at least the <em>threshold</em> 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.
<P>
The <EM>r.watershed</EM> program does not require the user to have the
current geographic region filled with elevation values. Areas without
elevation data MUST be masked out, by creating a raster map (or raster
reclassification) named <tt>MASK</tt>. Areas
masked out will be treated as if they are off the edge of the region.
MASKs 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.
<P>
Zero data values will be treated as elevation data (not no_data).
<P>
To isolate an individual river network using the output of this module,
a number of approaches may be considered.
<OL>
<LI>Use a resample of the basins catchment raster map as a MASK.<BR>
The equivalent vector map method is similar using <em>v.select</em> or
<em>v.overlay</em>.
<LI>Use the <em>r.cost</em> module with a point in the river as a starting
point.
<LI>Use the <em>v.net.iso</em> module with a node in the river as a
starting point.
</OL>
<P>
To create <i>river mile</i> segmentation from a vectorized streams map,
try the <em>v.net.iso</em> or <em>v.lrs.segment</em> modules.
<H2>EXAMPLES</H2>
<i>These examples use the Spearfish sample dataset.</i>
<P>
Convert <em>r.watershed</em> streams map output to a vector layer.
<P>
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.
<div class="code"><pre>
r.watershed elev=elevation.dem stream=rwater.stream
r.to.vect -v in=rwater.stream out=rwater_stream
</pre></div>
<BR>
<P>
Set a nice color table for the accumulation map:
<div class="code"><pre>
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
</pre></div>
<BR>
<P>
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 <em>r.univar</em>)
as the cut-off value.
<div class="code"><pre>
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 MASK
# <i>Thinning is required before converting raster lines to vector</i>
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 feature=line
v.db.dropcol map=rwater_course column=label
</pre></div>
<!-- can't set line attribute to catchment it is in as v.what.rast and
v.distance only work for point features. Could create endpoint node
points map and upload to that ?? -->
<!-- Note value column containing accumulation cells in output vector
may not necessarily reference the downstream end of the line! drop it? -->
<BR>
<P>
Create watershed basins map and convert to a vector polygon map
<div class="code"><pre>
r.watershed elev=elevation.dem basin=rwater.basin thresh=15000
r.to.vect -s in=rwater.basin out=rwater_basins feature=area
v.db.dropcol map=rwater_basins column=label
v.db.renamecol map=rwater_basins column=value,catchment
</pre></div>
<BR>
<P>
Display output in a nice way
<div class="code"><pre>
r.shaded.relief map=elevation.dem
d.shadedmap rel=elevation.dem.shade drape=rwater.basin bright=40
d.vect rwater_course color=orange
</pre></div>
<BR>
<a name="references"></a>
<H2>REFERENCES</H2>
Ehlschlaeger, C. (1989). <i>Using the A<sup>T</sup> Search Algorithm
to Develop Hydrologic Models from Digital Elevation Data</i>,
<b>Proceedings of International Geographic Information Systems (IGIS)
Symposium '89</b>, pp 275-281 (Baltimore, MD, 18-19 March 1989).<BR>
URL: <a href="http://faculty.wiu.edu/CR-Ehlschlaeger2/older/IGIS/paper.html">
http://faculty.wiu.edu/CR-Ehlschlaeger2/older/IGIS/paper.html</a>
<P>
Kinner D., H. Mitasova, R. Harmon, L. Toma, R., Stallard. (2005).
<i>GIS-based Stream Network Analysis for The Chagres River Basin,
Republic of Panama</i>. <b>The Rio Chagres: A Multidisciplinary Profile of
a Tropical Watershed</b>, R. Harmon (Ed.), Springer/Kluwer, p.83-95.<BR>
URL: <a href="http://skagit.meas.ncsu.edu/%7Ehelena/measwork/panama/panama.html">
http://skagit.meas.ncsu.edu/~helena/measwork/panama/panama.html</a>
<P>
McCool et al. (1987). <i>Revised Slope Steepness Factor for the Universal
Soil Loss Equation</i>, <b>Transactions of the ASAE</b> Vol 30(5).
<P>
Weltz M. A., K. G. Renard, J. R. Simanton (1987). <i>Revised Universal Soil
Loss Equation for Western Rangelands</i>, <b>U.S.A./Mexico Symposium of
Strategies for Classification and Management of Native Vegetation for
Food Production In Arid Zones</b> (Tucson, AZ, 12-16 Oct. 1987).
<a name="seealso"></a>
<H2>SEE ALSO</H2>
<EM>
<a href="g.region.html">g.region</a>,
<a href="r.cost.html">r.cost</a>,
<a href="r.drain.html">r.drain</a>,
<a href="r.flow.html">r.flow</a>,
<!-- <a href="r.flowmd.html">r.flowmd</a>, -->
<a href="r.neighbors.html">r.neighbors</a>,
<a href="r.param.scale.html">r.param.scale</a>,
<a href="r.resamp.interp.html">r.resamp.interp</a>,
<a href="r.terraflow.html">r.terraflow</a>,
<a href="r.topidx.html">r.topidx</a>,
<a href="r.water.outlet.html">r.water.outlet</a>
</EM>
<H2>AUTHOR</H2>
Charles Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory
<p>
<i>Last changed: $Date: 2008-02-23 19:20:42 +0100 (Sat, 23 Feb 2008) $</i>
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