1r.flow(1)                     Grass User's Manual                    r.flow(1)
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NAME

6       r.flow  - Constructs flowlines.
7       Computes  flowlines, flowpath lengths, and flowaccumulation (contribut‐
8       ing areas) from a elevation raster map.
9

KEYWORDS

11       raster, hydrology
12

SYNOPSIS

14       r.flow
15       r.flow --help
16       r.flow   [-u3m]    elevation=name     [aspect=name]      [barrier=name]
17       [skip=integer]    [bound=integer]   [flowline=name]   [flowlength=name]
18       [flowaccumulation=name]      [--overwrite]     [--help]     [--verbose]
19       [--quiet]  [--ui]
20
21   Flags:
22       -u
23           Compute upslope flowlines instead of default downhill flowlines
24
25       -3
26           3D lengths instead of 2D
27
28       -m
29           Use less memory, at a performance penalty
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31       --overwrite
32           Allow output files to overwrite existing files
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34       --help
35           Print usage summary
36
37       --verbose
38           Verbose module output
39
40       --quiet
41           Quiet module output
42
43       --ui
44           Force launching GUI dialog
45
46   Parameters:
47       elevation=name [required]
48           Name of input elevation raster map
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50       aspect=name
51           Name of input aspect raster map
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53       barrier=name
54           Name of input barrier raster map
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56       skip=integer
57           Number of cells between flowlines
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59       bound=integer
60           Maximum number of segments per flowline
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62       flowline=name
63           Name for output flow line vector map
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65       flowlength=name
66           Name for output flow path length raster map
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68       flowaccumulation=name
69           Name for output flow accumulation raster map
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DESCRIPTION

72       r.flow generates flowlines using a combined raster-vector approach (see
73       Mitasova and Hofierka 1993 and Mitasova et al. 1995) from an input ele‐
74       vation  raster map (integer or floating point), and optionally an input
75       aspect raster map and/or an input barrier raster map.
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77       There are three possible output raster maps which can  be  produced  in
78       any  combination  simultaneously: a vector map flowline of flowlines, a
79       raster map flowlength of flowpath lengths, and a raster map flowaccumu‐
80       lation of flowline densities (which are equal upslope contributed areas
81       per unit width, when multiplied by resolution).
82

NOTES

84       Aspect used for input must follow the same rules as aspect computed  in
85       other modules (see v.surf.rst or r.slope.aspect).
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87       Output  flowline  is  generated downhill. The line segments of flowline
88       vectors have endpoints on edges of a grid formed by  drawing  imaginary
89       lines  through the centers of the cells in the elevation map. Flowlines
90       are generated from each cell downhill by default; they can be generated
91       uphill  using  the  flag -u. A flowline stops if its next segment would
92       reverse the direction of flow (from up to down or vice-versa), cross  a
93       barrier,  or  arrive  at  a  cell  with  undefined elevation or aspect.
94       Another option, skip, indicates that  only  the  flowlines  from  every
95       val-th cell are to be included in flowline.  The default skip is max(1,
96       <rows in elevation>/50, <cols in elevation>/50).  A high  skip  usually
97       speeds up processing time and often improves the readability of a visu‐
98       alization of flowline.
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100       Flowpath length output is given in a raster map flowlength.  The  value
101       in  each  grid cell is the sum of the planar lengths of all segments of
102       the flowline generated from that cell. If the flag -3 is given,  eleva‐
103       tion is taken into account in calculating the length of each segment.
104
105       Flowline  density  downhill  or  uphill output is given in a raster map
106       flowaccumulation. The value in each grid cell is the  number  of  flow‐
107       lines which pass through that grid cell, that means the number of flow‐
108       lines from the entire map which  have  segment  endpoints  within  that
109       cell.   With  the -m flag less memory is used as aspect at each cell is
110       computed on the fly. This option incurs a severe  performance  penalty.
111       If  this  flag is given, the aspect input map (if any) will be ignored.
112       The barrier parameter is a raster map name with non-zero values  repre‐
113       senting barriers as input.
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115       For  best  results,  use input elevation maps with high precision units
116       (e.g., centimeters) so that flowlines do not terminate  prematurely  in
117       flat  areas.   To  prevent  the creation of tiny flowline segments with
118       imperceivable changes in elevation, an endpoint which would  land  very
119       close  to the center of a grid cell is quantized to the exact center of
120       that cell. The maximum distance between the intercepts along each  axis
121       of  a single diagonal segment and another segment of 1/2 degree differ‐
122       ent aspect is taken to be "very close" for that axis.  Note  that  this
123       distance (the so-called "quantization error") is about 1-2% of the res‐
124       olution on maps with square cells.
125
126       The values in length maps computed using the -u flag represent the dis‐
127       tances  from  each  cell to an upland flat or singular point. Such dis‐
128       tances are useful in water erosion modeling for computation of  the  LS
129       factor  in  the  standard form of USLE. Uphill flowlines merge on ridge
130       lines; by redirecting the order of the flowline points  in  the  output
131       vector  map,  dispersed waterflow can be simulated. The density map can
132       be used for the extraction of ridge lines.
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134       Computing the flowlines downhill simulates the actual flow (also  known
135       as the raindrop method). These flowlines tend to merge in valleys; they
136       can be used for localization of areas with waterflow  accumulation  and
137       for  the  extraction of channels. The downslope flowline density multi‐
138       plied by the resolution can be used as an approximation of the  upslope
139       contributing  area  per  unit  contour width. This area is a measure of
140       potential water flux for the steady state conditions and can be used in
141       the  modeling  of  water erosion for the computation of the unit stream
142       power based LS factor or sediment transport capacity.
143
144       r.flow has been designed for modeling erosion  on  hillslopes  and  has
145       rather strict conditions for ending flowlines. It is therefore not very
146       suitable for the extraction of stream networks or delineation of water‐
147       sheds  unless a DEM without pits or flat areas is available (r.fill.dir
148       can be used to fill pits).
149
150       To label the vector flowlines automatically, the user can  use  v.cate‐
151       gory (add categories).
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153   Algorithm background
154       r.flow  uses  an  original vector-grid algorithm which uses an infinite
155       number of directions between 0.0000... and 360.0000...  and traces  the
156       flow  as a line (vector) in the direction of gradient (rather than from
157       cell to cell in one of the 8 directions = D-infinity  algorithm).  They
158       are  traced in any direction using aspect (so there is no limitation to
159       8 directions here). The D8 algorithm produces zig-zag lines. The  value
160       in  the  outlet  is  very  similar  for r.flow algorithm (because it is
161       essentially the watershed area), however the  spatial  distribution  of
162       flow,  especially  on  hillslopes  is quite different. It is still a 1D
163       flow routing so the dispersal flow is  not  accurately  described,  but
164       still better than D8.
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166       r.flow  uses a single flow algorithm, i.e. all flow is transported to a
167       single cell downslope.
168
169   Diagnostics
170       Elevation raster map resolution differs from current region resolution
171       The resolutions of all input raster maps and the  current  region  must
172       match (see g.region).
173       Resolution too unbalanced
174       The  difference  in  length  between  the two axes of a grid cell is so
175       great that quantization error is larger than  one  of  the  dimensions.
176       Resample the map and try again.
177

EXAMPLE

179       In  this  example a flow line vector map, a flow path length raster map
180       and a flow accumulation raster  map  are  computed  from  an  elevation
181       raster map (North Carolina sample dataset):
182       g.region raster=elevation -p
183       r.flow elevation=elevation skip=3 flowline=flowline flowlength=flowlength \
184              flowaccumulation=flowaccumulation
185
186       Figure:  Flow  lines  with  underlying  elevation  map; flow lines with
187       underlying flow path lengths (in map units: meters); flow  accumulation
188       map (zoomed view)
189

REFERENCES

191           ·   Mitasova,  H.,  L.  Mitas,  1993,  Interpolation by regularized
192               spline with tension : I. Theory and implementation.  Mathemati‐
193               cal Geology 25, p. 641-655.  (online)
194
195           ·   Mitasova  and  Hofierka  1993  :  Interpolation  by Regularized
196               Spline with Tension: II. Application to  Terrain  Modeling  and
197               Surface Geometry Analysis.  Mathematical Geology 25(6), 657-669
198               (online).
199
200           ·   Mitasova, H., Mitas, L., Brown, W.M., Gerdes, D.P., Kosinovsky,
201               I., Baker, T., 1995: Modeling spatially and temporally distrib‐
202               uted phenomena: New methods and tools for GRASS  GIS.  Interna‐
203               tional   Journal  of  Geographical  Information  Systems  9(4),
204               433-446.
205
206           ·   Mitasova, H., J. Hofierka, M. Zlocha, L.R. Iverson, 1996,  Mod‐
207               eling  topographic  potential  for erosion and deposition using
208               GIS. Int. Journal of Geographical Information  Science,  10(5),
209               629-641.  (reply  to a comment to this paper appears in 1997 in
210               Int. Journal of Geographical Information Science, Vol. 11,  No.
211               6)
212
213           ·   Mitasova, H.(1993): Surfaces and modeling. Grassclippings (win‐
214               ter and spring) p.18-19.
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SEE ALSO

217         r.basins.fill,  r.drain,  r.fill.dir,  r.water.outlet,   r.watershed,
218       v.category, v.to.rast
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AUTHORS

221       Original  version of program: Maros Zlocha and Jaroslav Hofierka, Come‐
222       nius University, Bratislava, Slovakia
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224       The current version of the program  (adapted  for  GRASS  5.0):  Joshua
225       Caplan,  Mark  Ruesink,  Helena  Mitasova,  University  of  Illinois at
226       Urbana-Champaign with support from USA CERL.  GMSL/University of  Illi‐
227       nois at Urbana-Champaign
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229       Last changed: $Date: 2018-10-18 21:05:15 +0200 (Thu, 18 Oct 2018) $
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SOURCE CODE

232       Available at: r.flow source code (history)
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234       Main  index  | Raster index | Topics index | Keywords index | Graphical
235       index | Full index
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237       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.0 Reference Manual
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241GRASS 7.6.0                                                          r.flow(1)
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