Take a look at my first posting in the other thread where I include a link to my code. There’s a stand-alone simulation that shows the approach I’m taking. I adjust the frequencies of the waterings, not their durations.

The WeatherReach station map is here: http://access.weatherreach.com/map_stations. I see they have a station in Buffalo, NY. Perhaps this would be close enough to you? The key piece of data is the solar radiation, so if you can get that from WeatherReach or some other source, you can calculate ET from other weather data that is widely available (temps, humidity, wind). The solar radiation value on the ExactET site might be correct even if the rain measurement isn’t. Also, Weather Underground can store solar radiation, but most home weather stations don’t report this info. Still, might be worth checking WU stations near you to see if any do. Cloud cover has a big effect on solar radiation, so the main consideration would be whether the sky conditions at the station usually match those at your location.

Davis sells a solar radiation sensor for their weather stations for around $150. I wonder if it would be possible to put together a low cost sensor for the Pi that would generate “good enough” estimates for solar radiation, for cases where budget matters more than total accuracy. Even a simple solar cell might do the trick when combined with formulas that estimate the clear sky radiation value based on location and date.

I just found another Python program to calculate ET, via Wikipedia:

An aside: Not to get too pedantic, but I just realized that in the various papers the subscript following ET is the letter O, not the number zero. I had assumed it was a zero since it’s a reference value, and other values like ETc – the crop ET – are calculated from it. Anyone know what gives?