@ZipLine-- There are businesses that calculate evapotranspiration for farmers for a fee. Not just farmers, but large estate owners and golf club owners need this information as well.
Farmers can calculate it on their own using the evapotranspiration formula, but like you said, a lot of factors have to be considered. Farmers need to find coefficients for different crops and then they need to consider the season and how much water the crop needs at that time.
Crops need different amounts of water when they are first planted and throughout their development. Of course factors like the weather also effect this calculation. For example, evapotranspiration is higher when there is rainfall.
I think many farmers have someone else calculate all this for them, especially if they plant more than one crop.