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       r.flow  - Constructs flowlines.
       Computes  flowlines,  flowpath  lengths,  and flowaccumulation (contributing areas) from a
       elevation raster map.


       raster, hydrology


       r.flow --help
       r.flow   [-u3m]   elevation=name     [aspect=name]      [barrier=name]      [skip=integer]
       [bound=integer]       [flowline=name]       [flowlength=name]      [flowaccumulation=name]
       [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

           Compute upslope flowlines instead of default downhill flowlines

           3D lengths instead of 2D

           Use less memory, at a performance penalty

           Allow output files to overwrite existing files

           Print usage summary

           Verbose module output

           Quiet module output

           Force launching GUI dialog

       elevation=name [required]
           Name of input elevation raster map

           Name of input aspect raster map

           Name of input barrier raster map

           Number of cells between flowlines

           Maximum number of segments per flowline

           Name for output flow line vector map

           Name for output flow path length raster map

           Name for output flow accumulation raster map


       r.flow generates flowlines using a  combined  raster-vector  approach  (see  Mitasova  and
       Hofierka  1993  and  Mitasova  et al. 1995) from an input elevation raster map (integer or
       floating point), and optionally an input aspect raster map and/or an input barrier  raster

       There  are  three  possible  output  raster  maps which can be produced in any combination
       simultaneously: a vector map flowline of flowlines, a raster map  flowlength  of  flowpath
       lengths,  and a raster map flowaccumulation of flowline densities (which are equal upslope
       contributed areas per unit width, when multiplied by resolution).


       Aspect used for input must follow the same rules as aspect computed in other modules  (see or r.slope.aspect).

       Output  flowline  is  generated  downhill.  The  line  segments  of  flowline vectors have
       endpoints on edges of a grid formed by drawing imaginary lines through the centers of  the
       cells  in  the  elevation map. Flowlines are generated from each cell downhill by default;
       they can be generated uphill using the flag -u. A flowline stops if its next segment would
       reverse  the direction of flow (from up to down or vice-versa), cross a barrier, or arrive
       at a cell with undefined elevation or aspect. Another option, skip,  indicates  that  only
       the  flowlines from every val-th cell are to be included in flowline.  The default skip is
       max(1, <rows in elevation>/50, <cols in elevation>/50).  A high  skip  usually  speeds  up
       processing time and often improves the readability of a visualization of flowline.

       Flowpath length output is given in a raster map flowlength. The value in each grid cell is
       the sum of the planar lengths of all segments of the flowline generated from that cell. If
       the  flag  -3  is given, elevation is taken into account in calculating the length of each

       Flowline density downhill or uphill output is given in a raster map flowaccumulation.  The
       value in each grid cell is the number of flowlines which pass through that grid cell, that
       means the number of flowlines from the entire map which have segment endpoints within that
       cell.  With the -m flag less memory is used as aspect at each cell is computed on the fly.
       This option incurs a severe performance penalty. If this flag is given, the  aspect  input
       map  (if  any)  will be ignored.  The barrier parameter is a raster map name with non-zero
       values representing barriers as input.

       For best results, use input elevation maps with high precision units  (e.g.,  centimeters)
       so  that flowlines do not terminate prematurely in flat areas.  To prevent the creation of
       tiny flowline segments with imperceivable changes in elevation, an  endpoint  which  would
       land  very  close  to  the  center of a grid cell is quantized to the exact center of that
       cell. The maximum distance between the intercepts along each axis  of  a  single  diagonal
       segment and another segment of 1/2 degree different aspect is taken to be "very close" for
       that axis. Note that this distance (the so-called "quantization error") is about  1-2%  of
       the resolution on maps with square cells.

       The  values  in  length  maps computed using the -u flag represent the distances from each
       cell to an upland flat or singular point. Such  distances  are  useful  in  water  erosion
       modeling  for  computation of the LS factor in the standard form of USLE. Uphill flowlines
       merge on ridge lines; by redirecting the order of the flowline points in the output vector
       map,  dispersed waterflow can be simulated. The density map can be used for the extraction
       of ridge lines.

       Computing the flowlines downhill simulates the actual flow (also  known  as  the  raindrop
       method).  These  flowlines  tend to merge in valleys; they can be used for localization of
       areas with waterflow accumulation and  for  the  extraction  of  channels.  The  downslope
       flowline  density  multiplied  by  the  resolution  can be used as an approximation of the
       upslope contributing area per unit contour width. This area  is  a  measure  of  potential
       water  flux  for  the  steady  state  conditions  and can be used in the modeling of water
       erosion for the computation of the unit stream power based LS factor or sediment transport

       r.flow  has  been  designed  for  modeling  erosion  on  hillslopes  and has rather strict
       conditions for ending flowlines. It is therefore not very suitable for the  extraction  of
       stream  networks  or  delineation of watersheds unless a DEM without pits or flat areas is
       available (r.fill.dir can be used to fill pits).

       To label the vector flowlines automatically, the user can use v.category (add categories).

   Algorithm background
       r.flow uses an original vector-grid algorithm which uses an infinite number of  directions
       between 0.0000... and 360.0000...  and traces the flow as a line (vector) in the direction
       of gradient (rather than from cell to cell  in  one  of  the  8  directions  =  D-infinity
       algorithm).  They are traced in any direction using aspect (so there is no limitation to 8
       directions here). The D8 algorithm produces zig-zag lines. The value in the outlet is very
       similar  for  r.flow algorithm (because it is essentially the watershed area), however the
       spatial distribution of flow, especially on hillslopes is quite different. It is  still  a
       1D  flow  routing so the dispersal flow is not accurately described, but still better than

       r.flow uses a single flow algorithm, i.e.  all  flow  is  transported  to  a  single  cell

       Elevation raster map resolution differs from current region resolution
       The resolutions of all input raster maps and the current region must match (see g.region).
       Resolution too unbalanced
       The difference in length between the two axes of a grid cell is so great that quantization
       error is larger than one of the dimensions. Resample the map and try again.


       In this example a flow line vector  map,  a  flow  path  length  raster  map  and  a  flow
       accumulation  raster  map are computed from an elevation raster map (North Carolina sample
       g.region raster=elevation -p
       r.flow elevation=elevation skip=3 flowline=flowline flowlength=flowlength \

       Figure: Flow lines with underlying elevation map; flow lines  with  underlying  flow  path
       lengths (in map units: meters); flow accumulation map (zoomed view)


           ·   Mitasova,  H.,  L. Mitas, 1993, Interpolation by regularized spline with tension :
               I. Theory and implementation. Mathematical Geology 25, p. 641-655.  (online)

           ·   Mitasova and Hofierka 1993 : Interpolation by Regularized Spline with Tension: II.
               Application  to  Terrain  Modeling  and  Surface  Geometry Analysis.  Mathematical
               Geology 25(6), 657-669 (online).

           ·   Mitasova, H., Mitas, L., Brown, W.M., Gerdes, D.P.,  Kosinovsky,  I.,  Baker,  T.,
               1995:  Modeling  spatially  and  temporally distributed phenomena: New methods and
               tools for GRASS GIS. International Journal  of  Geographical  Information  Systems
               9(4), 433-446.

           ·   Mitasova,  H.,  J.  Hofierka,  M. Zlocha, L.R. Iverson, 1996, Modeling topographic
               potential for erosion and deposition  using  GIS.  Int.  Journal  of  Geographical
               Information  Science, 10(5), 629-641. (reply to a comment to this paper appears in
               1997 in Int. Journal of Geographical Information Science, Vol. 11, No. 6)

           ·   Mitasova, H.(1993): Surfaces and  modeling.  Grassclippings  (winter  and  spring)


        r.basins.fill, r.drain, r.fill.dir, r.water.outlet, r.watershed, v.category,


       Original  version  of  program:  Maros  Zlocha and Jaroslav Hofierka, Comenius University,
       Bratislava, Slovakia

       The current version of the program (adapted for GRASS 5.0): Joshua Caplan,  Mark  Ruesink,
       Helena  Mitasova,  University  of Illinois at Urbana-Champaign with support from USA CERL.
       GMSL/University of Illinois at Urbana-Champaign

       Last changed: $Date: 2018-10-18 21:05:15 +0200 (Thu, 18 Oct 2018) $


       Available at: r.flow source code (history)

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