r.water.outlet(1)

💧 r.water.outlet — the GRASS GIS manual page, bestie edition

okay so welcome to r.water.outlet, a GRASS GIS manual page and honestly? she's giving hydrology main character energy. this one lives in the GRASS GIS User's Manual and she creates watershed basins from a drainage direction map. that's the whole vibe. let's get into it 💖

KEYWORDS: raster, hydrology, watershed 🦋

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✨ 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 — lets your output files just go ahead and overwrite files that already exist. she's fearless.
• --help — prints the usage summary, aka the tl;dr.
• --verbose — verbose module output. she's a talker, love that for her.
• --quiet — quiet module output. shhh. 🤫
• --ui — forces the GUI dialog to launch. pop off, graphical interface.

🌸 Parameters 🌸

• input=name [required] — name of the input drainage direction map. mandatory, no skipping.
• output=name [required] — name for the output watershed basin map.
• coordinates=east,north [required] — coordinates of the outlet point. east and north, in that order, periodt.

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💅 DESCRIPTION 💅

so r.water.outlet generates a watershed basin from a drainage direction map plus a set of coordinates that represent the outlet point of the watershed. two inputs, one basin, iconic.

the input drainage direction map is indicating the "aspect" for each cell. here's the tea: multiply the positive values by 45 and you get the direction in degrees that the surface runoff will travel from that cell. the value -1 means that cell is a depression area. any other negative values mean surface runoff is leaving the boundaries of the current geographic region, and for those negative cells the absolute value tells you the direction of flow. this raster map is generated from r.watershed, iykyk.

on the output side: raster map values of 1 mean the cell is in the watershed basin. values of 0 mean it's not in the watershed basin. that's it, that's the whole binary.

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🫶 NOTES 🫶

in the context of this program, a watershed basin is the region upstream of an outlet point. so bestie, listen — if you choose 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. tiny. microscopic. a little sliver of destiny. choose your outlet wisely 🙈

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⭐ 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 🏞️

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👑 SEE ALSO 👑

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

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💜 AUTHOR 💜

Charles Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory. absolute legend, we stan.

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🌈 SOURCE CODE 🌈

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

accessed: Saturday Jan 17 05:41:05 2026

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and that's the whole page, bestie! this was the GRASS GIS 8.4.2 Reference Manual, © 2003-2025 GRASS Development Team, and the page itself is r.water.outlet(1) from GRASS 8.4.2. go make some basins, you got this 💖✨🎀🦋🌸💅