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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.


r.water.outlet - Creates watershed basins from a drainage direction map.


raster, hydrology, watershed


r.water.outlet --help
r.water.outlet input=name output=name coordinates=east,north [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


input=name [required]
Name of input drainage direction map
output=name [required]
Name for output watershed basin map
coordinates=east,north [required]
Coordinates of outlet point

Table of contents


r.water.outlet generates a watershed basin from a drainage direction map and a set of coordinates representing the outlet point of watershed.

Input drainage direction map indicates the "aspect" for each cell. Multiplying positive values by 45 will give the direction in degrees that the surface runoff will travel from that cell. The value -1 indicates that the cell is a depression area. Other negative values indicate that surface runoff is leaving the boundaries of the current geographic region. The absolute value of these negative cells indicates the direction of flow. This raster map is generated from r.watershed.

Output raster map values of one (1) indicate the watershed basin. Values of zero (0) are not in the watershed basin.


In the context of this program, a watershed basin is the region upstream of an outlet point. Thus, if the user chooses an outlet point on a hill slope, the resulting map will be a thin silver of land representing the overland slope uphill of the point.


A watershed in the North Carolina sample dataset region:
g.region raster=elev_lid792_1m -p
# the watershed outlet position should be placed on a stream (from
# accumulation map):
r.watershed elev_lid792_1m threshold=5000 accumulation=accum_5K drainage=draindir_5K basin=basin_5K
r.water.outlet input=draindir_5K output=basin_A30 coordinates=638740.423248,220271.519225

d.mon wx0
d.rast map=accum_5K
d.rast map=basin_A30

# overlay with transparency
r.colors map=basin_A30 color=grey
d.his h=accum_5K i=basin_A30

# report outlet size in ha map=basin_A30 units=h

Figure: Watershed draped over flow accumulation


r.wateroutlet.lessmem (addon), d.where, r.basins.fill, r.watershed, r.topidx


Charles Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory


Available at: r.water.outlet source code (history)

Latest change: Thursday Oct 01 17:35:27 2020 in commit: 744fcaefa6aa37121e72a9530e90b48fa07bef3a

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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