From eedomus - Documentation

The Evapotranspiration module computes the daily evapotranspiration.

It is the water loss of plants by transpiration and soil by evaporation.

Methodology

The method uses the Hargreaves and Samani formula. There are dozens of formula Wikipedia Evapotranspiration.

The advantages of this methodology is to use only outside temperature and latitutde (and day of year).

The result is given in mm of water per day (increasing during the course of de day).

Deducting the irrigation

If you with to irrigate with this formula, you have do subtract the rain, and irrigate a pat of the difference (eg 50%), which is better for plantes.