Provided by: grass-doc_8.3.0-1_all 
NAME
r3.flow - Computes 3D flow lines and 3D flow accumulation.
KEYWORDS
raster3d, hydrology, voxel
SYNOPSIS
r3.flow
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]
Flags:
-a
Create and fill attribute table
--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:
input=name
Name of input 3D raster map
vector_field=name[,name,...]
Names of three 3D raster maps describing x, y, z components of vector field
seed_points=name
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
flowline=name
Name for vector map of flow lines
flowaccumulation=name
Name for output flowaccumulation 3D raster
sampled=name
Name for 3D raster sampled by flowlines
Values of this 3D raster will be stored as attributes of flowlines segments
unit=string
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)
step=float
Integration step in selected unit
Default step is 0.25 cell
Default: 0.25
limit=integer
Maximum number of steps
Default: 2000
max_error=float
Maximum error of integration
Influences step, increase maximum error to allow bigger steps
Default: 1e-5
skip=integer[,integer,...]
Number of cells between flow lines in x, y and z direction
direction=string
Compute flowlines upstream, downstream or in both direction.
Options: up, down, both
Default: down
DESCRIPTION
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.
Attributes
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.
NOTES
r3.flow uses Runge-Kutta with adaptive step size (Cash-Karp method).
EXAMPLES
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.
SEE ALSO
r.flow, r3.gradient, r3.gwflow
AUTHOR
Anna Petrasova, NCSU OSGeoREL, developed during GSoC 2014.
SOURCE CODE
Available at: r3.flow source code (history)
Accessed: Tuesday Jun 27 11:13:38 2023
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