Originally Posted by darryl gesner
It's not a simple calculation and you may have a hard time finding the data to calculate it. You really need a meteorological station in order to do it, and even then the best you're going to get is an estimate. You could directly measure the evaporation rate relatively easily, but that would only give you information of what it under the specific conditions at the time and would not give you transpiration. My best advice would be to try to do a search and see if you can at least find a range from already existing data/calculations.
This is not entirely true. There are modifications to the Penman-Monteith
equation which only require a limited amount of data (see Blaney-Criddle and Hargreaves-Samani equations). This is how the temp based smart controllers calculate ET.