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       r.watershed  - Watershed basin analysis program.


       raster, hydrology


       r.watershed help
       r.watershed       [-f4ma]       elevation=name        [depression=name]        [flow=name]
       []      [blocking=name]      [accumulation=name]      [drainage=name]
       [basin=name]    [stream=name]    [half.basin=name]    [visual=name]    [length.slope=name]
       [slope.steepness=name]            [threshold=integer]             [max.slope.length=float]
       [convergence=integer]   [memory=integer]   [--overwrite]  [--verbose]  [--quiet]

           Enable MFD flow (default is SFD (D8))
           SFD: single flow direction, MFD: multiple flow direction

           Allow only horizontal and vertical flow of water

           Enable disk swap memory option: Operation is slow
           Only  needed  if  memory  requirements  exceed  available  RAM;  see  manual on how to
           calculate memory requirements

           Use positive flow accumulation even for likely underestimates
           See manual for a detailed description of flow accumulation output

           Allow output files to overwrite existing files

           Verbose module output

           Quiet module output

           Input map: elevation on which entire analysis is based

           Input map: locations of real depressions

           Input map: amount of overland flow per cell
           Input map or value: percent of disturbed land, for USLE

           Input map: terrain blocking overland surface flow, for USLE

           Output map: number of cells that drain through each cell

           Output map: drainage direction

           Output map: unique label for each watershed basin

           Output map: stream segments

           Output map: each half-basin is given a unique value

           Output map: useful for visual display of results

           Output map: slope length and steepness (LS) factor for USLE

           Output map: slope steepness (S) factor for USLE

           Input value: minimum size of exterior watershed basin

           Input value: maximum length of surface flow, for USLE

           Convergence factor for MFD (1-10)
           1 = most diverging flow, 10 = most converging flow. Recommended: 5
           Default: 5

           Maximum memory to be used with -m flag (in MB)
           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 this flag set, the entire analysis is  run  in  memory  maintained  by  the
              operating system.  This can be limiting, but is very fast.  Setting the flag causes
              the program to manage memory on disk which allows larger maps to be  processed  but
              is considerably slower.

              Use  multiple flow direction (MFD) instead of single flow direction (SFD, D8).  SFD
              is enabled by default.

              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.

              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.

              Maximum amount of memory in MB to be used with -m set. More memory  speeds  up  the

              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.

              Input  map:  Elevation  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.

              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.  All cells that
              are not NULL and not zero indicate depressions. Water will flow into but not out of

              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.
              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, r.watershed assumes  no  disturbed  land.   This
              input is used for the RUSLE calculations.

              Input  map: terrain that will block overland surface flow.  Terrain that will block
              overland surface flow and restart the slope length for the RUSLE.  All  cells  that
              are not NULL and not zero indicate blocking terrain.

              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.

              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.

              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.

              Output  map:  drainage direction.  Provides the "aspect" for each cell measured CCW
              from East.  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.

              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.

              Output map: stream segments.  Values correspond to the watershed basin values.  Can
              be vectorized after thinning (r.thin) with

              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.

              DEPRECATED  A  colortable  is  generated by default for the accumulation output for
              easy visual inspection.

              Output map: 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), it is multiplied by 100.

              Output  map:  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), it is multiplied by 100.


   AT least-cost search algorithm
       r.watershed uses an AT 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

   Multiple flow direction (MFD)
       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 AT least-cost path is always  included.  As  a  result,
       depressions  and  obstacles  are  traversed  with  a gracefull 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.

   In-memory mode and disk swap mode
       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.

   Large regions with many 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.

   Basin threshold
       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.

   MASK and no data
       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).

   Further processing of output layers
       To isolate an individual river network using the  output  of  this  module,  a  number  of
       approaches may be considered.

       1      Use a resample of the basins catchment raster map as a MASK.
              The equivalent vector map method is similar using or v.overlay.

       2      Use the r.cost module with a point in the river as a starting point.

       3      Use the 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

       To create river mile segmentation from a vectorized streams  map,  try  the  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.


       These examples use the Spearfish sample dataset.

       Convert r.watershed streams map output to a vector layer.

       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 -v flag
       preserves the catchment ID as the vector category number.
         r.watershed elev=elevation.dem -v out=rwater_stream

       Set a different color table for the accumulation map:
         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

       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 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 in=rwater.course.Thin out=rwater_course feature=line
         v.db.dropcol map=rwater_course column=label

       Create watershed basins map and convert to a vector polygon map
         r.watershed elev=elevation.dem basin=rwater.basin thresh=15000 -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

       Display output in a nice way
         r.shaded.relief map=elevation.dem
         d.shadedmap rel=elevation.dem.shade drape=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).

       Holmgren, P. (1994). Multiple flow direction algorithms for runoff modelling in grid based
       elevation models: An empirical evaluation.  Hydrological Processes Vol 8(4), p.327-334.
       DOI: 10.1002/hyp.3360080405

       Kinner D., H. Mitasova, R. Harmon,  L.  Toma,  R.,  Stallard.  (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,

       McCool et al. (1987). Revised Slope Steepness Factor for the Universal Soil Loss Equation,
       Transactions of the ASAE Vol 30(5).

       Weltz M. A., K. G. Renard, J. R. Simanton (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.


         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


       Original version:  Charles  Ehlschlaeger,  U.S.  Army  Construction  Engineering  Research
       Faster sorting algorithm and MFD support: Markus Metz

       Last changed: $Date: 2010-09-16 00:25:59 -0700 (Thu, 16 Sep 2010) $

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