Dear Ayman:

I agree with you. In our early warning system for agricultural drought we use regressional equations derived by Borrajero et al. (1989). They were obtained using not the extraterrestrial radiation but other parameter called Rayleigh radiation, which is supposed to be not the radiation outside the atmosphere (without its influence) but at ground level assuming a clear atmosphere with specified parameters and without intervening clouds. I think this could be similar to what Joe and you did, but there could be differences between these two approaches. Borrajero et al. first calculated this Rayleigh radiation and after that they obtained regression equations for global, direct and diffuse solar radiation (in fact they obtained second order regressional equations) of the type,

Rg = R*(a + b*(n/N) + c*(n/N)*(n/N))

where R is the so called Rayleigh radiation (that you have to calculate first with a computer program "ad hoc")

It is a pity that these friends of us presented this paper at a meeting but (as far as I know) it hasn't been published. Truly speaking I don't have it myself and I doubt if it is available. But anyway, I am not sure if the equations would be valid for every place on earth.

That is why I think that if you and Joe Ritchie could make available your study it would be very well received by the universe of DSSAT users and agrometeorologists in general.

Sincereley yours,

Roger