DESCRIPTION

i.eb.h_SEBAL95 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 sensible heat flux map (h0).

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>0 vegetation is present and evapotranspiration is calculated;
- where Vh=0 bare ground is present and evapotranspiration is calculated;
- where Vh<0 water surface is present and evaporation is calculated;

For more details on the algorithms see [1].

OPTIONS

The program will run non-interactively if the user specifies program arguments and flag settings on the command line using the following form:

i.eb.h_SEBAL95 [ -qzd ] DEM=name T=name RH=name WS=name NSR=name Vh=name ETP=name

Alternatively, the user can simply type i.eb.h_SEBAL95 on the command line and the program will ask for parameter values and flag settings interactively, using the standard GRASS parser interface.

Flags:

-q
Run quietly (do not display status messages). By default r.evapo.PM is run verbosely.
-z
Set negative calculated evapotranspiration values to zero.
-n
Calculate soil heat flux for night time. By default r.evapo.PM calculate it for day time.

Parameters:

DEM=name
Input elevation raster [m a.s.l.]. Required.
T=name
Input temperature raster [�C]. Required.
RH =name
Input relative humidity raster [%]. Required.
WS =name
Input wind speed at two meters raster [m/s]. Required.
NSR =name
Input net solar radiation raster [MJ/(m2*h)]. Required.
Vh =name
Input vegetation heigth raster [m]. Required.
ETP =name
Output evapotranspiration raster [mm/h]. Required.

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->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 r.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

AUTHORS

Yann Chemin, International Rice Research Institute, Los Banos, The Philippines.

Contact: Yann chemin

REFERENCES

[1] Bastiaanssen, W.G.M., 1995. Estimation of Land surface paramters by remote sensing under clear-sky conditions. PhD thesis, Wageningen University, Wageningen, The Netherlands.

[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.

Last changed: $Date$