[GRASS-user] Clarification on units used in r.sun

[Hamish]

Dylan Beaudette wrote:
> Hi everyone,
> 
> Quick question on the units used for the ouput of r.sun in mode 2
> (daily sums):
> 
> In the manual pages for r.sun, the following 'unit' is included in the
> 
> description:
> ----------------------------
> The solar radiation maps for given day are computed integrating the
> relevant  irradiance between sunrise and sunset times for given day.
> The user can set  finer or coarser time step step used for all-day
> radiation calculations. A  default value of step is 0.5 hour. Larger
> steps (e.g. 1.0-2.0) can speed-up  calculations but produce less
> reliable results. The output units are in Wh  per squared meter per
> given day [Wh/(m*m)/day].
>  -------------------------
> 
> Is one to interpret this as "watt-hour per square meter per day" ? 
> This would seem a little odd, as the the unit 'watt-hour' is not an SI
> unit.  Hopefully I am reading this incorectly.

If you want SI, 1 Wh = 3600 watt-sec (joules)

 
> An alternative interpretation of these units (one which I hope to be
> the  correct one):
> 
> Wh.m-2.day-1 = "watts per square meter per day, as estimated at some
> fraction  of an hour" ?


mode 1 is instantaneous flux

mode 2 is cumulative energy / area, summed for the day

ie watt-hours per square meter, summed over enitre day. The "dt"
timestep for the numerical intergration is set by the step= parameter
(default 30 minutes).


To convert to micro-moles (micro-Einsteins) you have to pick an energy
level (light wavelength) and convert using that. (moles of photo quanta)


Hamish