Evaporation and Evapotranspiration

Evaporation and transpiration together return most of the water that falls on land back to the atmosphere, making evapotranspiration the largest outgoing term of the terrestrial water balance.

Definition

Evapotranspiration is the combined transfer of water from the land surface to the atmosphere by direct evaporation from soil and water surfaces and by transpiration through plants, governed by available energy, atmospheric demand, and water supply.

Scope

This topic covers the physics of evaporation from water and soil, transpiration by plants, the energy and aerodynamic controls on these fluxes, and the standard methods for estimating potential and actual evapotranspiration. It excludes plant-physiological detail of transpiration, treated in plant science, focusing on the hydrological flux.

Core questions

• What energy and atmospheric factors control evaporation and transpiration?
• How do potential and actual evapotranspiration differ?
• How is evapotranspiration estimated from meteorological data?
• Why is evapotranspiration the dominant loss term in many land water balances?

Key concepts

• Evaporation and transpiration
• Potential versus actual evapotranspiration
• Energy balance and net radiation
• Aerodynamic and surface resistance
• Penman and Penman-Monteith equations
• Reference and crop evapotranspiration

Key theories

Penman combination equation

Penman combined the energy balance with aerodynamic transfer to compute evaporation from standard meteorological variables, removing the need to measure surface temperature and forming the basis of modern evaporation estimation.

Penman-Monteith equation

Monteith extended Penman's approach to vegetated surfaces by introducing surface (stomatal) and aerodynamic resistances, yielding the Penman-Monteith equation that underlies standardized reference and crop evapotranspiration.

Mechanisms

Evapotranspiration is driven by the energy available to vaporize water (mainly net radiation) and by the capacity of the air to remove vapor (a function of humidity deficit and wind), modulated by the resistance of the surface, including plant stomata, and by the supply of water; when water is limiting, actual evapotranspiration falls below the potential rate.

Clinical relevance

Evapotranspiration estimates are central to irrigation scheduling and crop water requirements, to closing catchment water balances and estimating water yield, and to assessing how land-cover change and warming alter the partitioning of precipitation between the atmosphere and runoff.

History

Penman's 1948 combination equation gave evaporation a physical basis from routine weather data; Monteith's 1965 extension added surface resistance for vegetation. The FAO-56 standardization of the Penman-Monteith reference evapotranspiration made the approach a global operational standard for irrigation and water management.

Key figures

• Howard L. Penman
• John L. Monteith
• Wilfried Brutsaert

Seminal works

• penman1948
• monteith1965
• allen1998

Frequently asked questions

What is the difference between evaporation and evapotranspiration?

Evaporation is the change of water to vapor from open water and soil surfaces, while evapotranspiration also includes transpiration, the water drawn up by plants and released through their leaves; over vegetated land the two are combined because they are hard to separate.

What is potential evapotranspiration?

Potential evapotranspiration is the rate that would occur if water supply were unlimited, set by energy and atmospheric demand; actual evapotranspiration equals it only when water is freely available and is lower when soil moisture limits the supply.