Evapotranspiration
Soil & evaporation ET
Evapotranspiration is the combined water loss through evaporation and release by plants. It is an important part of a soil or garden water balance.
Two routes back into the air
- Evaporation: Water leaves soil, open water or wet leaves.
- Transpiration: Plants take up water through roots and release it mainly through small leaf pores.
Together these processes are evapotranspiration. Turning liquid water into vapour requires energy and can cool the surroundings.
Millimetres make comparison easier
Values are commonly given in millimetres per day. 1 mm equals 1 litre per square metre. A daily loss of 4 mm means four litres per square metre have entered the air. With precipitation in the same unit, this supports a simplified water balance.
What ET₀ means
Reference evapotranspiration, ET₀, describes a specified, well-watered grass surface. It allows the evaporative demand of different weather conditions to be compared. Actual loss may be lower when soil is dry or plants close their stomata.
Not an automatic watering instruction
An ET₀ of 4 mm does not mean every plant needs four litres of irrigation per square metre. Rain, soil reserves, root depth, plant type and irrigation losses belong in the calculation. Lettuce and an apple tree do not share one drinking schedule.