NAME

(GRASS Raster/Vector Program)

SYNOPSIS

DESCRIPTION

Flowlines are generated from each cell uphill by default, they can be generated downhill using the flag -d. Flowline stops when it reaches a point with lower or equal (for uphill case), higher or equal (for downhill case) elevation value, or a barrier (e.g.road). Barriers are optionally defined by non-zero values in an input raster map barierin.

Flowpath length flout for each grid cell is computed as a plan distance summing lengths of linear parts of the flowline between intersections. Flowline densities dsout represent a number of flowlines passing through the given cell. Flowline lengths and densities can be computed for both uphill and downhill flowlines.

It is suggested to use input elevation values in centimeters because integer values in meters on flat areas will cause ending the flowline.

For the better visualization of the flowlines, it is suggested in the case of large areas with hundreds of columns and rows, to use a skip option skip and write coordinates of the flowlines into the file only from each n-th cell (e.g. from each fifth cell).

OPTIONS

r.flow [-d] elevin = name aspin =name [barrierin = name] [skip = val] [flout = name] [lgout = name] [dsout = name]

Alternately, the user can simply type r.flow on the command line without program arguments. In this case, the user will be prompted for parameter values and flag settings using the standard GRASS parser interface.

Flags:

[-d]

Generate flowlines downhill (default generates flowlines uphill)

Parameters:

elevin=name

Use the existing raster file with elevationsname as input.

aspin=name

Use the existing raster file with aspectname as input.

barrierin=name

Use the existing raster file with non-zero values representing barriers with name as input.

skip=val

Set the the number of cells skipped for origins of flowlines to val.

flout=name

Output coordinates of flowlines to vector file named name.

lgout=name

Output flowpath length values to raster file named name.

dsout=name

Output flowline density values to raster file named name.

NOTES

Flowlines generated uphill are used for computation of the flowpath length for each grid cell. This length is the distance of the given cell from upland flat or singular point. This can be used, for example, in water erosion modeling for the computation of LS factor in standard form of USLE. Upslope flowlines are merging on ridge lines and therefore by redirecting the order of points defining the flowline dispersed waterflow can be simulated. The map of their accumulation can be used for the extraction of ridge lines. Flowlines can be also generated downhill from each grid point to simulate the actual flow (known also as a raindrop method). These flowlines merge in valleys and therefore they can also be used for localization of areas with waterflow accumulation and for the extraction of valley lines. Downslope flowline density multiplied by resolution is equal to the upslope contributing area which is defined as area from which water goes into a given cell. This area is a measure of potential water flux and can be used in the modeling of water erosion for the computation of unit stream power based LS factor. The program has been designed for modeling erosion on hillslopes and has a very strict condition for ending the flowline. It is therefore not very suitable for the extraction of stream network or delineation of watersheds unless a DEM without pits or flat areas is available.

SEE ALSO

AUTHOR

Modified program (adapted for GRASS):
Helena Mitasova, US Army CERL ,

REFERENCES