[GRASS-dev] Changes to r.sun made Feb 2006

Dylan Beaudette dylan.beaudette at gmail.com
Tue Nov 28 16:50:03 EST 2006

Hi everyone,

I am repeating an experiment, originally conducted Feb 7th 2006, comparing the 
output from r.sun with real data from a weather station. 

On Feb 7th, 2006 the output from r.sun (computed with customized linke 
turbidity values) very closely matched the weather station data. 

As of today the exact same experiment gives slightly different results:
1. the correlation between r.sun and the weather station is nearly identical
2. the output from r.sun (specifically the beam component) is about 1500 
Wh/sq. meter/day higher than it should be.

The only thing that has changed since I last conducted this experiment was 
r.sun. From the CVS server it looks like r.sun/main.c was altered shortly 
after I did my original experiment:

Revision 2.14 / (as text) / (view) - annotate - [select for diffs] , Tue Feb 
21 10:00:52 2006 UTC (9 months ago) by markus
Branch: MAIN
Changes since 2.13: +28 -174 lines
Diff to previous 2.13

Jaro Hofierka: new shadow algorithm; dist param changed to fixed value; 
correction factor for shadowing to account for the earth curvature

In summary, the diffuse component of the modeled solar radiation is nearly the 
same as it was when computed by r.sun Feb 7 2006. The beam component is now 
about 1500 Wh/sq m / day higher than it was as on Feb 7 2006. 

This brings up an interesting question: which results from r.sun where the 
most representative of reality ? I noticed that the changes made to r.sun 
involved a shadow algorithm- yet my weather station (and modeled area) are 
not greatly affected by the shadowing of adjacent terrain. 

Any thoughts on how to best proceed would be greatly appreciated.

 the original results were mentioned in a previous message to the GRASS list 
on Feb 8 2006.

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

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