i.evapo.pm.1grass - Man Page

Computes potential evapotranspiration calculation with hourly Penman-Monteith.


imagery, evapotranspiration


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]



Set negative evapotranspiration to zero


Use Night-time


Allow output files to overwrite existing files


Print usage summary


Verbose module output


Quiet module output


Force launching GUI dialog


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]


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).

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. The usage of the flag -n detect 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.

Land and water surfaces are idenfyied by Vh:

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


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.


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)"


[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


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: Wednesday Jan 24 21:14:07 2024

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