i.evapo.pt(1)

💧 i.evapo.pt — the GRASS GIS manual page, but make it make sense

okay bestie, gather round, because this one is about evapotranspiration and it's genuinely so cool once you get it. This is i.evapo.pt, a GRASS GIS module that computes evapotranspiration using the Priestley and Taylor formulation from 1972. ✨

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🌸 KEYWORDS 🌸

imagery, evapotranspiration

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✨ SYNOPSIS ✨

Like, here's the shape of it:

i.evapo.pt
i.evapo.pt --help
i.evapo.pt [-z] net_radiation=name soil_heatflux=name
air_temperature=name atmospheric_pressure=name
priestley_taylor_coeff=float output=name  [--overwrite]  [--help]
[--verbose]  [--quiet]  [--ui]

🎀 Flags 🎀

• -z — sets negative ETa to zero. bye negativity 💅
• --overwrite — lets your output files overwrite files that already exist
• --help — prints the usage summary
• --verbose — verbose module output (she's chatty)
• --quiet — quiet module output (she's mysterious)
• --ui — forces the GUI dialog to launch

🦋 Parameters 🦋

• net_radiation=name [required] — the name of your input net radiation raster map, in [W/m2]
• soil_heatflux=name [required] — the name of your input soil heat flux raster map, in [W/m2]
• air_temperature=name [required] — the name of your input air temperature raster map, in [K]
• atmospheric_pressure=name [required] — the name of your input atmospheric pressure raster map, in [millibars]
• priestley_taylor_coeff=float [required] — the Priestley-Taylor coefficient. Default: 1.26
• output=name [required] — the name of your output evapotranspiration raster map, in [mm/d]

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💖 DESCRIPTION 💖

So i.evapo.pt calculates the diurnal evapotranspiration after Prestley and Taylor (1972) — and yes, the manpage spells it "Prestley" right there, we're keeping it, character for character, no notes. 🫶

The Priestley-Taylor model (Priestley and Taylor, 1972) is a modification of Penman's more theoretical equation. It's giving glow-up. It's giving theoretical but make it practical. 😌

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🔥 NOTES 🔥

RNETD is an optional output from i.evapo.potrad, and ngl it's giving good results as input for net radiation in this module. So if you were wondering what to feed her, that's the tea. ☕

Now, the Alpha values — this is the part where you actually pick your coefficient, so pay attention bestie:

• 1.32 for estimates from vegetated areas, as a result of the increase in surface roughness (Morton, 1983; Brutsaert and Stricker, 1979)
• 1.26 is applicable in humid climates (De Bruin and Keijman, 1979; Stewart and Rouse, 1976; Shuttleworth and Calder, 1979), and temperate hardwood swamps (Munro, 1979)
• 1.74 has been recommended for estimating potential evapotranspiration in more arid regions (ASCE, 1990). This worked well in Greece with University of Thessaloniki.

Alpha values extracted from: Watflood manual. 🧚

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🌈 SEE ALSO 🌈

i.evapo.mh, i.evapo.pm, i.evapo.time, i.eb.netrad, r.sun

like, the whole evapo cinematic universe fr fr 👑

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👑 AUTHOR 👑

Yann Chemin, GRASS Development Team, 2007-08

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🍓 SOURCE CODE 🍓

Available at: i.evapo.pt source code (history)

Accessed: Saturday Jan 17 05:41:40 2026

Main index | Imagery index | Topics index | Keywords index | Graphical index | Full index

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and that's the whole page, bestie 💜 This little masterpiece lives in the GRASS GIS 8.4.2 Reference Manual, © 2003-2025 GRASS Development Team — so yeah, she's official, she's iconic, and the version you're reading is GRASS 8.4.2. periodt. 💅✨🌸