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

Dylan Beaudette dylan.beaudette at gmail.com
Thu Nov 30 16:54:49 EST 2006

On Wednesday 29 November 2006 12:31, Glynn Clements wrote:
> Dylan Beaudette wrote:
> > 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.
> More odd (to me) is having (different) units of time in both the
> numerator and denominator. I would have thought it more logical to
> divide the result by 24 to give Watts per square metre [W/(m^2)].

I need to check with some local experts, but judging from some recent tests - 
the output from r.sun is comparible to that from a weather station, with 
*hourly* averaged data in W/(m^2) which i think is analogous to the Wh/(m^2) 
units that r.sun uses.

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

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