Evapotranspiration

Evapotranspiration (ET) is the combined transfer of water from land to atmosphere through direct evaporation and plant transpiration. It is one of the main outgoing terms in the water balance and a key link between climate, vegetation, and water availability.

Components

• Evaporation from soil, open water, intercepted rain, and wet surfaces
• Transpiration from plants as water moves through vegetation and is released through stomata

Why it sits between the water and energy budgets

Evapotranspiration determines how much incoming water remains available for runoff, recharge, and ecosystem storage. It also links water and energy exchange, because evaporation and transpiration consume energy and cool surfaces.

Main controls

• available energy and net radiation
• air temperature, humidity, and wind
• soil moisture and root-zone water availability
• vegetation type, density, and phenology

In hydroclimate

In hydroclimate studies, ET is important because wet and dry conditions are shaped not only by how much water arrives, but also by how quickly water is returned to the atmosphere. Actual evapotranspiration can differ strongly from potential demand when soils are dry or vegetation is stressed.

Where it is hardest to constrain

Evapotranspiration is the term in the water balance that is almost never measured directly at the scale it is needed. Eddy covariance gives a flux for a footprint of a few hundred metres; catchment-scale estimates come from closing the balance, which loads every other term’s error into this one.

That matters most where hydrological observations are sparse, which is most of the places I work. In the Galapagos highlands the problem compounds, because fog interception adds water that gauges do not record while low cloud suppresses the radiation driving evaporation — so both sides of the balance are mis-estimated by conventional instruments, in opposite directions.

The practical consequence is that modelled evapotranspiration is usually validated indirectly, through the soil moisture and near-surface temperature it produces, rather than against the flux itself.

See also: Water balance, Hydroclimate, Precipitation