[GRASS-dev] Re: r.sun units

[Jaro Hofierka]

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