Very Cool, I love this. What are you using to control your irrigation?
Right now, an old controller that HS turns on and off. I think I'm going to check out your EtherRain referral, though.
Do you need the UV sensor to calculate ET? Can you give me an example on how you would use the watering index % to determine how much to water your grass?
All you need is the solar sensor, not UV. The Davis ET calc uses solar irradiance, wind, RH, and BP to calculate the running daily ET value from midnight.
For people with newer controllers than mine, it's pretty easy. The controller is set to water 100% for the highest weekly average ET. Here in California, that's usually in July.
Then, once a week, check the Watering Index, and set the knob or whatever on the controller to that percentage. The controller automatically adjusts each station accordingly.
I haven't gone overboard with getting my adjustments just right yet, because we only water about 100 square feet of turf for the dogs, some raised beds for veggies and flowers, and a native plant bird-and-butterfly garden that requires almost no supplemental water in the summer.
In HomeSeer, it's easy to pick up the station data from VWS and then script to control relays directly, or, like I do, just keep the controller from opening the solenoids.
I also notice you are comparing 10 years worth of ET. Are those the numbers you have collected?
The State of California has a listing somewhere on-line of highest average ETs for various locations in the state. I got my info from there.
EDIT: Found the map... It's big, so here's the link:http://www.cimis.water.ca.gov/cimis/cimiSatEtoZones.jsp