r.water.outlet(1grass)

💧 r.water.outlet — the GRASS User's Manual page, but make it make sense 💧

okay bestie, welcome to r.water.outlet, straight out of the GRASS User's Manual. it makes watershed basins from a drainage direction map. yes she's a hydrology girlie. yes she's iconic. let's go 🫶

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✨ NAME ✨

r.water.outlet — Creates watershed basins from a drainage direction map. that's it, that's the whole job, and she does it flawlessly 💅

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🏷️ KEYWORDS 🏷️

raster, hydrology, watershed

the holy trinity. if you're deep in the flow-accumulation trenches, this is your girl. iykyk 🌊

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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]

look at her. three required inputs, five optional flags, no drama ✨

🎀 Flags 🎀

• --overwrite — Allow output files to overwrite existing files. she WILL steamroll your old file, so be normal about it.
• --help — Print usage summary. when in doubt, ask the girl herself.
• --verbose — Verbose module output. she tells you everything, lowkey oversharing.
• --quiet — Quiet module output. silent treatment mode 😌
• --ui — Force launching GUI dialog. for when clicking > typing.

🦋 Parameters 🦋

• input=name [required] — Name of input drainage direction map. this is the drainage direction map, bestie, you gotta bring it.
• output=name [required] — Name for output watershed basin map. where your precious little basin gets written 💖
• coordinates=east,north [required] — Coordinates of outlet point. the exact spot where the water peaces out. east, then north, comma, no space. periodt.

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💖 DESCRIPTION 💖

so basically, 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. that's the whole plot 🎬

the input drainage direction map indicates the "aspect" for each cell. and yes, multiplying positive values by 45 will give you the direction in degrees that the surface runoff will travel from that cell. math is serving 💅

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. and where does this raster map come from? she's generated from r.watershed. the origin story 🫶

output raster map values of 1 (one) indicate the watershed basin. values of 0 (zero) are not in the watershed basin. clean, simple, no crumbs left 🌈

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📝 NOTES 📝

in the context of this program, a watershed basin is the region upstream of an outlet point. so if the user chooses an outlet point on a hill slope, the resulting map will be a thin sliver of land representing the overland slope uphill of the point. just a lil skrunkly strip of terrain. it's giving ribbon 🎀

not a bug, bestie — that's just geometry doing its thing.

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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

she ate and left no crumbs 😭🔥

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👀 SEE ALSO 👀

• r.wateroutlet.lessmem (addon) — the addon cousin, holding it down when memory is tight
• d.where
• r.basins.fill
• r.watershed
• r.topidx

run the whole friend group. family reunion energy 🫶

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👑 AUTHOR 👑

Charles Ehlschlaeger, U.S. Army Construction Engineering Research Laboratory. giving legendary, giving main character, giving credit where it's due 💜

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📦 SOURCE CODE 📦

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

Latest change: Tuesday Jul 01 10:05:22 2025 in commit: 041dfcd0bde6bac18b7065ee697e80f0e5620073

Main index | Raster index | Topics index | Keywords index | Graphical index | Full index

© 2003-2026 GRASS Development Team, GRASS 8.5.0 Reference Manual

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okay that's the whole page, bestie ✨ you came in for a watershed and you left with the entire lore. this was GRASS 8.5.0, r.water.outlet(1grass) — stay hydrated, stay flowed-in-the-right-direction, and go map something gorgeous 🌸💧🦋