Tesfalidet,
You should be aware of units. You can express radiation in energy units or
in quantum units. Conversion is not simple, because energy of light is
wavelength dependent. Weather inputs in DSSAT models use Solar radiation in
energy units (MJ/m2 d); optionally you can input PAR, but in quantum (or
mole) units (mole/m2 d).

Now the conversion of solar radiation (SRAD) into photosynthetically active
radiation (PAR) when both are expressed in energy units is aprox 0.5 (0.5
MJ PAR/MJ SRAD). This is what CERES uses. (We found that 0.43 works better
for mid-Iowa).

Photosynthetic Photon Flux Density (PPFD), a measure of PAR in quantum
units, normally is expressed as micro Einstein/m2 s (uE/m2 s) or micro mole
/m2 s (umol/m2 s). The standard conversion to energy units calculated by
McCree (1981) is 4.6 uE/J or 4.6 umol/J or 4.6 mol/MJ.

Finally, to convert directly SRAD in MJ/m2 d to PAR in mol/m2 s the
conversion is aprox 2 (2 mol PAR/MJ SRAD). Of course 4.6 * 0.5 approximates
2.  This is what CROPGRO uses.

McCree, K. J. 1981. Photosynthetically active radiation. pp 41-55. In: O.
L. Lange, P. S. Nobel, C. B. Osmond and H. Ziegler (Ed). Encyclopedia of
Plant Physiology. New Series Vol 12-A. Springer-Verlag. Berlin.


At 04:50 PM 8/22/2002 +0200, you wrote:
>I am aware that CERES wheat converts inputs of solar radiation to
>Photosynthetically active radiation (PPFD) by multiplyinhg the radiation
>values by the constant 0.5 assuming that 50% of the incoming solar
>radiation is in the photsynthetically active range. Does anyone know how
>CROPGRO model does this?
>
>Tesfalidet A. Ghebreab
>University of Natal
>School of Applied Environmental Sciences
>Department of Soil Science and Agrometeorology
>Post bag 3210
>South Africa


Jon I. Lizaso
Dep. Agricultural & Biosystems Engineering
124 Davidson Hall
Iowa State University    515 294 7350 (Voice)
Ames IA 50011            515 294 2552 (Fax)