i.evapo.pm(1)

📦
version 8.4.2 · 8 versions
, in fedora 44

✨ i.evapo.pm ✨

okay bestie, welcome to i.evapo.pm — it's a GRASS GIS User's Manual page and it's giving hydrology main character energy. 💧👑

KEYWORDS: imagery, evapotranspiration 🌱

🌈 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. bye bye negative numbers 💅
  • -n — Use Night-time 🌙
  • --overwrite — Allow output files to overwrite existing files (no cap, it just goes for it)
  • --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]

💖 DESCRIPTION 💖

So like, i.evapo.pm takes 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, and it literally calculates the potential evapotranspiration map (EPo). slay. 🔥

Optionally you can activate a flag (-z) that lets you set all of the negative evapotranspiration cells to zero. and honestly? those negative values are motivated by the condensation of the air water vapour content and they're sometime undesired because they can produce computational problems 😭 so yeah, the flag is there for a reason. Using the flag -n detects that the module is run in night hours and the appropriate soil heat flux is calculated. 🌙✨

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. iconic behaviour, no notes.

Land and water surfaces are idenfyied by Vh (yes, "idenfyied", that's what it says, we don't gatekeep):

  • 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 📚

🌸 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. math girlies rise up 🧮💅

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)"

👑 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.

🫶 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

💜 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

⭐ SOURCE CODE ⭐

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

Accessed: Saturday Jan 17 05:41:39 2026

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