Evapotranspiration

Evapotranspiration is the sum of two distinct physical processes occurring together across a cropped field: evaporation from soil and water surfaces, and transpiration of water vapor from plant leaves.

Evaporation

Water loss directly from soil or open water surfaces to the atmosphere.

Transpiration

Water loss from plants as it moves through roots, stems, and leaves and evaporates through stomata.

Reference evapotranspiration

A weather-driven estimate of water loss from a standardized reference surface, used as a baseline for crop water-use calculations.

Crop coefficient

A factor applied to reference evapotranspiration to estimate the water use of a specific crop at a given growth stage.

• Temperature — higher temperatures increase the rate of both evaporation and transpiration
• Humidity — drier air increases the rate of water loss
• Wind — moving air removes humid air from around leaves, increasing water loss
• Solar radiation — more incoming energy drives more evaporation and transpiration
• Canopy cover and growth stage — a fuller canopy generally transpires more than sparse or early-stage growth

Weather-station or modeled reference evapotranspiration, combined with crop coefficients, estimates daily or weekly crop water use. Growers and irrigation-scheduling tools use this estimate to time and size irrigation applications, including for deficit-irrigation strategies that deliberately apply less than the estimated need at certain growth stages.

When evapotranspiration is high and rainfall is low, soil moisture depletes faster, accelerating the onset of agricultural drought. Understanding evapotranspiration is therefore central both to irrigation scheduling in normal conditions and to anticipating drought stress during dry periods.