[GRASS-user] calculate global horizontal solar irradiation for Clear-Sky-Situation Ghc/Gh

[Diana]

Hello,

for some calculation with r.sun i need to know if this method is
possible:

I've got the Cloud-Index n from satel-light.com aggergated in daily
values. To calculate the Clear-Sky-Index Kc against the Cloud-Index I
use the formulars on the basis of Fontoynont et al.

n =< -0,2, Kc=1,2
0,2 =< n =< 0,8 Kc=1-n
0,8 =< n =< 1,1 Kc=2,066-3,6667n+1,6667n^2
n >= 1,1 Kc=0,05

Now r.sun calculates the beam and diffuse irradiation unter cloudless
(clear) conditions.

I want to sum these two values in order received the global horizontal
solar irradiation Ghc. To get the global horizontal solar irradiation
for Clear-Sky-Situation, i convert the Kc=Gh/Ghc into Gh=Ghc*Kc with
values of the calculated Kc above.

The reason for this computation: I want to declare how much energy (only
global irradiation) with and wihtout cloudiness exists on a predfined
area. 

I choose this way because we have no other data excepting the
Cloud-Index.

Now the question: is this calculation possible AND valid or are there
some other parameters which have to be mentioned (and incorporate)?

Thanks a lot,
Diana