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Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.


r3.flow - Computes 3D flow lines and 3D flow accumulation.


raster3d, hydrology, voxel


r3.flow --help
r3.flow [-a] [input=name] [vector_field=name[,name,...]] [seed_points=name] [flowline=name] [flowaccumulation=name] [sampled=name] [unit=string] [step=float] [limit=integer] [max_error=float] [skip=integer[,integer,...]] [direction=string] [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Create and fill attribute table
Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


Name of input 3D raster map
Names of three 3D raster maps describing x, y, z components of vector field
Name of vector map with points from which flow lines are generated
If no map is provided, flow lines are generated from each cell of the input 3D raster
Name for vector map of flow lines
Name for output flowaccumulation 3D raster
Name for 3D raster sampled by flowlines
Values of this 3D raster will be stored as attributes of flowlines segments
Unit of integration step
Default unit is cell
Options: time, length, cell
Default: cell
time: elapsed time
length: length in map units
cell: length in cells (voxels)
Integration step in selected unit
Default step is 0.25 cell
Default: 0.25
Maximum number of steps
Default: 2000
Maximum error of integration
Influences step, increase maximum error to allow bigger steps
Default: 1e-5
Number of cells between flow lines in x, y and z direction
Compute flowlines upstream, downstream or in both direction.
Options: up, down, both
Default: down

Table of contents


Module r3.flow computes 3D flow lines and 3D flow accumulation. It accepts either three 3D raster maps representing the vector field or one 3D raster map. In case of one map, it computes on-the-fly gradient field.

Flow lines

Flow lines are computed either from points (seeds) provided in seed_points vector map, or if there are no seeds, it creates seeds in a regular grid in the center of voxels (3D raster cells). Parameter skip then controls the step between the regularly distributed seeds. If skip is not provided, r3.flow decides optimal skip for each dimension based on current 3D region as one tenth of the number of columns, rows, and depths. Flow lines can be computed in upstream direction (in the direction of gradient or vector field), in downstream direction or in both directions.

Flow accumulation

Flow accumulation is computed as the number of flow lines traversing each voxel. Since the flow lines are computed for each voxel, the flow accumulation computation can be more demanding. Parameter skip does not influence the flow accumulation computation, parameter direction does.

Flow line integration

Flow line integration can be influenced by several parameters. Option step controls the integration step and influences the precision and computational time. The unit of step can be defined either in terms of the size of the voxel (3D raster cell), length in map units, or as elapsed time. Option limit specifies the maximum number of steps of each flow line.


Without using flag a, no attribute table is created and each flow line is represented by one vector line with one category. With a flag, an attribute table is created and each category (record) represents one segment of a flowline, so that attributes specific for segments can be written. In case of vector_field input, only velocity is written, in case of input option, also values of the input 3D raster are written. Option sampled allows sampling (query) given 3D raster by flow lines (computed from different 3D raster) and write the values of the given 3D raster as attributes of the flow line segments. Note that using a flag results in longer computation time, so consider increasing step and max_error parameter.


r3.flow uses Runge-Kutta with adaptive step size (Cash-Karp method).


First we create input data using example 1 from r3.gwflow manual page:
# set the region accordingly
g.region res=25 res3=25 t=100 b=0 n=1000 s=0 w=0 e=1000 -p3

# now create the input raster maps for a confined aquifer
r3.mapcalc expression="phead = if(row() == 1 && depth() == 4, 50, 40)"
r3.mapcalc expression="status = if(row() == 1 && depth() == 4, 2, 1)"
r3.mapcalc expression="well = if(row() == 20 && col() == 20 && depth() == 2, -0.25, 0)"
r3.mapcalc expression="hydcond = 0.00025"
r3.mapcalc expression="syield = 0.0001"
r.mapcalc  expression="recharge = 0.0"

r3.gwflow solver=cg phead=phead status=status hc_x=hydcond hc_y=hydcond  \
   hc_z=hydcond q=well s=syield r=recharge output=gwresult dt=8640000 vx=vx vy=vy vz=vz budget=budget
Then we compute flow lines in both directions and downstream flowaccumulation.
r3.flow vector_field=vx,vy,vz flowline=gw_flowlines skip=5,5,2 direction=both
r3.flow vector_field=vx,vy,vz flowaccumulation=gw_flowacc
We can visualize the result in 3D view:

We can store velocity values (and values of the input 3D raster map if we use option input) for each segment of flow line in an attribute table.

r3.flow -a vector_field=vx,vy,vz flowline=gw_flowlines skip=5,5,2 direction=both
v.colors map=flowlines_color@user1 use=attr column=velocity color=bcyr
Again, we visualize the result in 3D view and we check 'use color for thematic rendering' on 3D view vector page.


r.flow, r3.gradient, r3.gwflow


Anna Petrasova, NCSU OSGeoREL, developed during GSoC 2014.


Available at: r3.flow source code (history)

Latest change: Monday Nov 18 20:15:32 2019 in commit: 1a1d107e4f6e1b846f9841c2c6fabf015c5f720d

Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

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