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i.evapo.pm

Computes potential evapotranspiration calculation with hourly Penman-Monteith.

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

Example:

i.evapo.pm elevation=name temperature=name relativehumidity=name windspeed=name netradiation=name cropheight=name output=name

grass.script.run_command("i.evapo.pm", elevation, temperature, relativehumidity, windspeed, netradiation, cropheight, output, flags=None, overwrite=False, verbose=False, quiet=False, superquiet=False)

Example:

gs.run_command("i.evapo.pm", elevation="name", temperature="name", relativehumidity="name", windspeed="name", netradiation="name", cropheight="name", output="name")

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]
-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
--qq
    Very quiet module output
--ui
    Force launching GUI dialog

elevation : str, required
    Name of input elevation raster map [m a.s.l.]
    Used as: input, raster, name
temperature : str, required
    Name of input temperature raster map [C]
    Used as: input, raster, name
relativehumidity : str, required
    Name of input relative humidity raster map [%]
    Used as: input, raster, name
windspeed : str, required
    Name of input wind speed raster map [m/s]
    Used as: input, raster, name
netradiation : str, required
    Name of input net solar radiation raster map [MJ/m2/h]
    Used as: input, raster, name
cropheight : str, required
    Name of input crop height raster map [m]
    Used as: input, raster, name
output : str, required
    Name for output raster map [mm/h]
    Used as: output, raster, name
flags : str, optional
    Allowed values: z, n
    z
        Set negative evapotranspiration to zero
    n
        Use Night-time
overwrite: bool, optional
    Allow output files to overwrite existing files
    Default: False
verbose: bool, optional
    Verbose module output
    Default: False
quiet: bool, optional
    Quiet module output
    Default: False
superquiet: bool, optional
    Very quiet module output
    Default: False

DESCRIPTION

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:

  • 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].

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)"

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)
Latest change: Friday Feb 07 19:16:09 2025 in commit a82a39f