Ubuntu Manpage: r.flow - Construction of slope curves (flowlines), flowpath lengths, and flowline

NAME

       r.flow   -  Construction  of  slope  curves  (flowlines),  flowpath  lengths, and flowline
       densities (upslope areas) from a raster digital elevation model (DEM).

KEYWORDS

       raster, hydrology

SYNOPSIS

       r.flow
       r.flow help
       r.flow   [-u3m]   elevin=string     [aspin=string]      [barin=string]      [skip=integer]
       [bound=integer]      [flout=string]      [lgout=string]     [dsout=string]     [--verbose]
       [--quiet]

   Flags:
       -u
           Compute upslope flowlines instead of default downhill flowlines

       -3
           3-D lengths instead of 2-D

       -m
           Use less memory, at a performance penalty

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       elevin=string
           Input elevation raster map

       aspin=string
           Input aspect raster map

       barin=string
           Input barrier raster map

       skip=integer
           Number of cells between flowlines
           Options: 1-360
           Default: 7

       bound=integer
           Maximum number of segments per flowline
           Options: 0-1609
           Default: 1609

       flout=string
           Output flowline vector map

       lgout=string
           Output flowpath length raster map

       dsout=string
           Output flowline density raster map

DESCRIPTION

This program generates flowlines using a combined raster-vector approach (see Mitasova and
Hofierka 1993 and Mitasova et al. 1995) from an input elevation raster map elevin (integer
or floating point), and optionally an input aspect raster map aspin and/or an input
barrier raster map barin. There are three possible output maps which can be produced in
any combination simultaneously: a vector map flout of flowlines, a raster map lgout of
flowpath lengths, and a raster map dsout 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 GRASS
programs (see v.surf.rst or r.slope.aspect).

Flowline output is given in a vector map flout, (flowlines 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=val, indicates that only the flowlines from every val-th cell are to be
included in flout. The default skip is max(1, /50, /50). A high skip usually speeds up
processing time and often improves the readability of a visualization of flout.

Flowpath length output is given in a raster map lgout. 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 dsout. 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 barin parameter is a raster map name with non-zero values
representing barriers as input.

NOTES

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.

The program 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
1. Construction of flow-lines (slope-lines): 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 both r.flow and r.flowmd (GRASS 5 only)
algorithms (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.

2. Computation of contributing areas: r.flow uses a single flow algorithm, i.e. all flow
is transported to a single cell downslope.

Differences between r.flow and r.flowmd
1
r.flow has an option to compute slope and aspect internally thus making the
program capable to process much larger data sets than r.flowmd. It has also 2
additional options for handling of large data sets but it is not known that they
work properly.

2
the programs handle the special cases when the flowline passes exactly (or very
close) through the grid vertices differently.

3
r.flowmd has the simplified multiple flow addition so the results are smoother.

In conclusion, r.flowmd produces nicer results but is slower and it does not support as
large data sets as r.flow.

Diagnostics
"ERROR: r.flow: " input " file's resolution differs from current" region resolution

The resolutions of all input files and the current region must match.

"ERROR: r.flow: resolution too unbalanced (" val " x " val ")" 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.

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.

AUTHORS

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

       The current version of the program (adapted for GRASS5.0):
       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: 2008-05-15 12:30:26 -0700 (Thu, 15 May 2008) $

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