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Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

NAME - Computes potential evapotranspiration calculation with hourly Penman-Monteith.


imagery, evapotranspiration

SYNOPSIS --help [-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

Table of contents

DESCRIPTION, 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)"


The HydroFOSS project at IST-SUPSI (Institute of Earth Sciences - University school of applied science for the Southern Switzerland), i.evapo.time, r.sun, r.mapcalc


Original version of program: The HydroFOSS project, 2006, IST-SUPSI. (
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


[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.


Available at: source code (history)

Latest change: Monday Nov 25 19:00:04 2019 in commit: 1e5de88ff59cb16b9d553f69b8b2ae2a5a24258d

Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

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