[GRASS-dev] Re: r.sun units

[Dylan Beaudette]

On Friday 01 December 2006 02:50, Jaro Hofierka wrote:
> Hamish wrote:
> > Hi Jaro, Marcel, Tomas,
> >
> > I was wondering if you could contribute some insight WRT this thread on
> > the GRASS users' list:
> >
> >   http://thread.gmane.org/gmane.comp.gis.grass.devel/17118
> >
> >
> > I fear my advice so far is rather bad.
> >
> > thanks,
> > Hamish
>
> Hamish,
>
> Your advices were right. I am not sure if anything is still unclear, but
>
> I would add some minor clarifications:
> >> Is this essentially this same thing, as it is "integrated" over an
> >> hour when summed for the entire day?
> >
> > I would guess sum midnight to midnight hourly averages of W/m2 = mode2 ?
>
> Effectively, "per day" means from sunrise to sunset time (these are
> calculated internally), during night there is no solar radiation. Small
> amount of available diffuse radiation shortly before the sunrise and
> after the sunset are neglected. We use "per day" unit for situations
> where we need to calculate amount of available energy for a certain
> period of year (e.g. months). If you need radiation within one day you
> can use mode 1 - irradiance values [W/m^2] to calculate irradiance for a
> specific time and then multiply with your time interval.
>
> Some differences comparing with measurements may arrise from different
> time systems. r.sun uses local solar time, while practicaly we use
> "civic" times with time zones (and also modified with "summer/daylight
> saving time").
>
> Marcel can send you more information about this. I believe the JRC team
> has some modification of r.sun that uses civic time.
>
> > I was aware of the Joule - Watt relationship, but was not quite sure why
> > r.sun was using W*h instead of Joules. Since a Mega-Joule is a much more
> > readily used unit of enery, I think that I will do the conversion from
> > now on.
>
> This is solely for practical reasons. All calculations are done using
> watts (e.g. solar constant, irradiance), so expressing radiation in Wh
> is straightforward and gives a better idea how much energy is available,
> for example, for solar energy applications (photovoltaics, thermal
> systems). You can still make a simple conversion if you need joules.
>
> Jaro
>

Jaro, others;

Thank you for the quick clarifications. I would be happy to summarize this for 
the r.sun manual page- with your revision before posting.

Cheers,

-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341