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

Dylan Beaudette dylan.beaudette at gmail.com
Tue Nov 28 18:31:29 EST 2006

Hi everyone,

Quick question on the units used for the ouput of r.sun in mode 2 (daily 

In the manual pages for r.sun, the following 'unit' is included in the 
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.

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" ?

Any thoughts would be appreciated,


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

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