You can calculate ET0 from the following weather data: temperature, humidity, wind speed, and solar radiation. Solar radiation is the tricky one since it’s generally not present in Weather Underground data. I use the solar radiation reading from a nearby WeatherReach station and calculate ET0 using the other weather data from my own weather station (via Weather Underground). I find the ET0 I calculate is close to the one at Weather Reach. I suspect I’d be fine using their value instead of my own. There are so many variables used in water balance scheduling that a little inaccuracy in ET0 should not be big deal.
Dan in CA previously posted links to a paper containing equations for calculating ET0 step-by-step, and to code implementing the calculation. I wrote code as well before discovering Dan’s, and you’re free to try it. I added it and the equations paper to an ET directory with my other code.
Dan in WA