i.evapo.pm(1grass) GRASS GIS User's Manual i.evapo.pm(1grass)
NAME
i.evapo.pm - Computes potential evapotranspiration calculation with
hourly Penman-Monteith.
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 [--over-
write] [--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
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 poten-
tial 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 nega-
tive values motivated by the condensation of the air water vapour con-
tent, 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-Mon-
teith 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 cal-
culated.
For more details on the algorithms see [1,2,3].
NOTES
Net solar radiation map in MJ/(m2*h) can be computed from the combina-
tion 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 genera-
tion 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)"
SEE ALSO
The HydroFOSS project at IST-SUPSI (Institute of Earth Sciences - Uni-
versity 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
REFERENCES
[1] Cannata M., 2006. GIS embedded approach for Free & Open Source Hy-
drological 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 Evap-
otranspiration: Guidelines for computing crop water requirements. Ir-
rigation 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.
SOURCE CODE
Available at: i.evapo.pm source code (history)
Accessed: unknown
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