i.evapo.pm(1grass)

💧 i.evapo.pm — the vibe ✨

okay bestie, gather round, because we're talking about i.evapo.pm from the GRASS User's Manual, and she is that girl. This little icon computes potential evapotranspiration calculation with hourly Penman-Monteith. no cap. 💅

KEYWORDS: imagery, evapotranspiration 🌸

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✨ SYNOPSIS ✨

i.evapo.pm
i.evapo.pm --help
i.evapo.pm [-zn] elevation=name temperature=name relativehumidity=name
windspeed=name netradiation=name cropheight=name output=name
[--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

🎀 Flags

• -z — Set negative evapotranspiration to zero 💀
• -n — Use Night-time 🌙
• --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

• elevation=name [required] — Name of input elevation raster map [m a.s.l.]
• temperature=name [required] — Name of input temperature raster map [C]
• relativehumidity=name [required] — Name of input relative humidity raster map [%]
• windspeed=name [required] — Name of input wind speed raster map [m/s]
• netradiation=name [required] — Name of input net solar radiation raster map [MJ/m2/h]
• cropheight=name [required] — Name of input crop height raster map [m]
• output=name [required] — Name for output raster map [mm/h]

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💖 DESCRIPTION 💖

okay so i.evapo.pm, given the vegetation height (hc), humidity (RU), wind speed at two meters height (WS), temperature (T), digital terrain model (DEM), and net radiation (NSR) raster input maps, calculates the potential evapotranspiration map (EPo). slay. literally all that input, one gorgeous output. 👑

Optionally the user can activate a flag (-z) that allows him setting to zero all of the negative evapotranspiration cells; in fact these negative values motivated by the condensation of the air water vapour content, are sometime undesired because they can produce computational problems. so like, negative numbers are giving chaos and sometimes you just want them gone, no judgement. 🙈 The usage of the flag -n detect that the module is run in night hours and the appropriate soil heat flux is calculated. night shift energy only bestie. 🌙

The algorithm implements well known approaches: the hourly Penman-Monteith method as presented in Allen et al. (1998) for land surfaces and the Penman method (Penman, 1948) for water surfaces.

Land and water surfaces are idenfyied by Vh:

• where Vh gt 0 vegetation is present and evapotranspiration is calculated;
• where Vh = 0 bare ground is present and evapotranspiration is calculated;
• where Vh lt 0 water surface is present and evaporation is calculated.

For more details on the algorithms see [1,2,3]. iykyk 📚

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📝 NOTES 📝

Net solar radiation map in MJ/(m2*h) can be computed from the combination of the r.sun , run in mode 1, and the r.mapcalc commands.

The sum of the three radiation components outputted by r.sun (beam, diffuse, and reflected) multiplied by the Wh to Mj conversion factor (0.0036) and optionally by a clear sky factor [0-1] allows the generation of a map to be used as an NSR input for the i.evapo.PM command.

Example:

r.sun -s elevin=dem aspin=aspect slopein=slope lin=2 albedo=alb_Mar \
      incidout=out beam_rad=beam diff_rad=diffuse refl_rad=reflected \
      day=73 time=13:00 dist=100;
r.mapcalc "NSR = 0.0036 * (beam + diffuse + reflected)"

like look at her. math but make it fashion. ✨

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📚 REFERENCES 📚

[1] Cannata M., 2006. GIS embedded approach for Free & Open Source Hydrological Modelling. PhD thesis, Department of Geodesy and Geomatics, Polytechnic of Milan, Italy.

[2] Allen, R.G., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop Evapotranspiration: Guidelines for computing crop water requirements. Irrigation and Drainage Paper 56, Food and Agriculture Organization of the United Nations, Rome, pp. 300

[3] Penman, H. L. 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. London, A193, pp. 120-146.

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👀 SEE ALSO 👀

The HydroFOSS project at IST-SUPSI (Institute of Earth Sciences - University school of applied science for the Southern Switzerland)

i.evapo.mh, i.evapo.time, r.sun, r.mapcalc 🫶

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💜 AUTHORS 💜

Original version of program: The HydroFOSS project, 2006, IST-SUPSI. (http://istgis.ist.supsi.ch:8001/geomatica/index.php?id=1) Massimiliano Cannata, Scuola Universitaria Professionale della Svizzera Italiana - Istituto Scienze della Terra Maria A. Brovelli, Politecnico di Milano - Polo regionale di Como

Contact: Massimiliano Cannata

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🌈 SOURCE CODE 🌈

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

Latest change: Friday Apr 17 01:04:26 2026 in commit: 32a143358df26b2e518b03c7e0d79541e8d1e5b1

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

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and that's the whole page bestie 🎀 that was i.evapo.pm from the GRASS 8.5.0 reference manual — go forth and evapotranspire responsibly, mwah 💋✨