r.flowgrass - Online in the Cloud

PROGRAM:

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 thec 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.

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