The plants' water requirements are defined here as the quantity of water from the soil replaced through the crop evapotranspiration. Based on the climatic parameters, the model computes the reference evapotranspiration (ETo), which is defined as the evapotranspiration rate of a well irrigated, growing grass surface. It is based on the Penman-Monteith method providing daily evapotranspiration values in mm (l/m²).

The Penman Monteith method of computing ETo is based on two main equations: an energy equation (solar radiation) and an aerodynamic equation (wind and air humidity). A typical ETo station is therefore composed of the following components:

- A733 addWAVE RTU with solar panel
- SEN-R Combi sensor for air temperature and humidity
- SP-Lite Silicium-Pyranometer from Kipp&Zonen
- WIN Combi sensor for wind speed and direction
- RG Rain gauge
- Pole set to mount the station

If the station's data is also to be used for computing disease models, a leaf wetness sensor is usually added.

ETo and direct soil moisture monitoring
For a long time it was either or. Nowadays the technologies more and more complement one another, and soil moisture and other sensors (like dendrometers) are being used to fine tune ETo and get better site specific data.

Especially for deep rooting crops direct soil moisture monitoring is very much recommended, while for many field crops, covering literally thousand of acres of farm land, ETo often is the better and more economical alternative.