[GRASS-user] use r.sun with monthly cloud cover data

[Dylan Beaudette]

On Wednesday, January 18, 2012, Hamish wrote:
> Simone wrote:
> > I  would like to use the r.sun module to produce a map
> > of montly solar
> > radiation for a certain area. I would like to incorporate
> > cloud cover
> > data into the calculations, as well as information on local
> > topography
> > (i.e. aspect and slope).
> 
> slope and aspect are automatically taken into
> account even if you don't use the slope and aspect
> input map options.
> 
> actually due to a bug you shouldn't use them:
>  https://trac.osgeo.org/grass/ticket/498#comment:8
>  "r.sun2 commissioning trials"
> 
> just use the -s shading flag and the rest will
> happen automatically.

Is this still the case? I get very odd results when I do not specify a slope 
and aspect map.

https://trac.osgeo.org/grass/ticket/498#comment:18

Dylan


> 
> > I was wondering whether you could
> > help me to
> > solve two problems: i) I have not quite yet understood what
> > I should
> > do to specify a raster containing data on cloudiness when
> > using the module
> > eg. r.sun elevin=altmap aspin=aspmap slopein=slopemap +
> > cloudmap??
> 
> from the help page:
> >  Besides clear-sky radiations, the user can compute a real-sky
> >  radiation  (beam,  diffuse)  using  coefbh  and  coefdh input
> >  raster maps defining the fraction of the respective clear-sky
> >  radiations  reduced by atmospheric factors (e.g. cloudiness).
> >  The value is between 0-1. Usually these coefficients  can  be
> >  obtained  from  a long-terms meteorological measurements pro-
> >  vided as raster maps with spatial distribution of these coef-
> >  ficients  separately for beam and diffuse radiation (see Suri
> >  and Hofierka, 2004, section 3.2).
> 
> 
> I've now updated the module in trunk and devbr6 to
> (a little) better explain that.
> 
> 
> > ii)
> >  I am interested in a montly average of daily sum of solar
> > radiation,
> > and I only have montly cloud cover data.  I suspect
> > that this is not
> > ideal when calculations by r.sun are done on daily basis.
> > Do you confirm? Is there a way around this?
> 
> If it's all you have to work with, it's all you have
> to work with & you'll have to pretend like every
> day is like the climactic average.
> 
> some LiCOR light data or even battery voltage from an installed
> solar panel at a weather station can give you an
> idea about which days are cloudy, and by how much.
> 
> 
> Hamish
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> 


-- 
Dylan E. Beaudette
USDA-NRCS Soil Scientist
California Soil Resource Lab
http://casoilresource.lawr.ucdavis.edu/