[GRASS-user] r.sun Direct/Diffuse problem

[Collin Bode]

I have a research site in Mendocino County, California where I have taken several months of insolation measurements using two Hukseflux LP02 black body Pyranometers.  One is equiped with a shade ring.   Measurements were taken in an open field with no topographic shading.  Surrounding area is hilly and forested.

I have run r.sun for every week of the year, and compared the values that match my measurement times.   The global radiation values match within 5%.   However, the ratio of direct to diffuse radiation is off.   r.sun is reporting 20-22% diffuse radiation, while the field measurements are reporting 7-10% diffuse.   

I am using Linke Turbidity Index values from www.helioclim.org, which look too high.   However, when I tried arbitrarily changing the Linke Turbidity index and rerunning r.sun, I could get the diffuse/direct ratio to match field values, but then the global radiation was much too high.  

Am I missing a parameter to help with this?  Any suggestions would be appreciated.

Collin