Dear Efstathia Lazaridi ,

I'm afraid that the daily light length would not suffice.

The irradiance of PAR can be expressed as the Photosynthetic Photon Flux Density (PPFD), the number of photons in the 400-700 nm range received by a surface for a defined amount of time.

In order to do it you need the absorption surface and the number of photons.

To calculate the number of photons you need the total energy and the energy of a photon. The latter is dependent on its wavelength (see formula below), but you have only a range of 300 nm.

E (photon) = (hc)/λ

where h is the Planck constant, c is the light speed and λ is the wavelength.

If you have also the diurnal variation and the variation in day length, you could achieve a more accurate result using the Daily Light Integral (DLI), which is the daily flux of photons per ground area, and includes both diurnal variation as well as variation in day length.

This is similar as trying to estimate the total irradiance from just daylength. It might be feasible with large uncertainty if you limit yourself to cloudless conditions. Otherwise, it's a waste of time!

For example, in the south of our country (Uzbekistan, φ=41.32), the sunshine duration is more then 3050 hours, and 3050/4380≈70%. So, if the latitude is even farther south, then, may be, the consideration of cloudless conditions will give some estimation.

Thank you all for your answers! Actually my final target is to include photosynthesis effect in Growing Degree Days calculation.

Probably APSIM model could be a solution as I am missing PAR data.

Any other suggestions?