Provided by: grass-doc_7.8.2-1build3_all 
NAME
r.flow - Constructs flowlines.
Computes flowlines, flowpath lengths, and flowaccumulation (contributing areas) from a elevation raster
map.
KEYWORDS
raster, hydrology
SYNOPSIS
r.flow
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]
Flags:
-u
Compute upslope flowlines instead of default downhill flowlines
-3
3D lengths instead of 2D
-m
Use less memory, at a performance penalty
--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
aspect=name
Name of input aspect raster map
barrier=name
Name of input barrier raster map
skip=integer
Number of cells between flowlines
bound=integer
Maximum number of segments per flowline
flowline=name
Name for output flow line vector map
flowlength=name
Name for output flow path length raster map
flowaccumulation=name
Name for output flow accumulation raster map
DESCRIPTION
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 map.
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).
NOTES
Aspect used for input must follow the same rules as aspect computed in other modules (see v.surf.rst 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 segment.
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
capacity.
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 D8.
r.flow uses a single flow algorithm, i.e. all flow is transported to a single cell downslope.
Diagnostics
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.
EXAMPLE
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 dataset):
g.region raster=elevation -p
r.flow elevation=elevation skip=3 flowline=flowline flowlength=flowlength \
flowaccumulation=flowaccumulation
Figure: Flow lines with underlying elevation map; flow lines with underlying flow path lengths (in map
units: meters); flow accumulation map (zoomed view)
REFERENCES
• 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) p.18-19.
SEE ALSO
r.basins.fill, r.drain, r.fill.dir, r.water.outlet, r.watershed, v.category, v.to.rast
AUTHORS
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
SOURCE CODE
Available at: r.flow source code (history)
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