r.water.outlet - GRASS GIS manual
Author: GRASS Development Team
Description: r.water.outlet: Creates watershed basins from a drainage direction map.
Keywords: raster, hydrology, watershed

## NAME

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

## KEYWORDS

raster, hydrology, watershed

## SYNOPSIS

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

### Flags:

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

## DESCRIPTION

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.

## NOTES

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.

## EXAMPLE

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
    r.report map=basin_A30 units=h

Figure: Watershed draped over flow accumulation

## SEE ALSO

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

## AUTHOR

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

## SOURCE CODE

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

Accessed: Saturday May 09 11:17:41 2026

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