[GRASS-user] Re: [GRASS-dev] Changes to r.sun made Feb 2006 [resolved]

[Dylan Beaudette]

On 12/5/06, Hamish <hamish_nospam at yahoo.com> wrote:
> Dylan Beaudette wrote:
> >
> > > Get local Linke values from the SoDa Solar radiation Databases
> > >   http://www.soda-is.com/
> >
> > Tried this, and got unrealistic values for my site.
>
> how so? Have you calculated good Linke values from your light meters?
> (formula for that, please?)

The soda-is values ranged from 2.8-3.8 - however through
experimentation, and the finally a calculation (reference in a minute)
the real values should be closer to the range 2-6 : smallest values in
the middle of summer and fall, highest values in the winter and
spring.

I have calculated Linke turbidity for my site with the equation
presented in [1]. an image of the equation is attached to this
message. If you would like I can send a snippet of the article to you
off-list. Unfortunately, there are some aspects to this equation which
I do not understand: namely the two constants present. I am currently
working with an atmospheric scientist to track down the sources of
error in this equation - apparantly there are some issues.

Hopefully, by the end of the week I will have a more appropriate
equation for connecting r.sun (mode 1) + local pyranometer data to
calculate Linke turbidity.

1. Becker, S. Caclulation of Direct Solar And Diffuse Radiation in
Israel International Journal of Climatology, 2001, 21, 1561 - 1576


> beware some LICOR meters return PAR (400-700nm) and some full spectrum.
> The PAR measurements should be adjusted before comparison with r.sun
> results to compensate for the missing wavelengths (but fortunately
> evolution has calibrated plants (and eyes) pretty well to match the
> sun's dominant energies).
>

Good point. Looking at the literature associated with this sensor
(LI-200SZ) the pyranometer is integrating over energy from 400nm -
1000nm . This is a subset of the entire solar spectrum, but covers a
bt more than the PAR subset.


Mostly, I am looking for a relatively simple approach to calculate
local linke values. Since these will be estimates, based on other
estimates, I am certainly not expecting perfection. :)

Cheers,

Dylan
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