i.evapo.pmgrass - Online in the Cloud

PROGRAM:

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 [--overwrite] [--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 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)"

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