EKS-HUM-041 / Agriculture and controlled environments

Plant Transpiration

作物蒸腾

Plant transpiration is the vaporisation of water within plant tissues and the release of that water vapour to the surrounding air, mainly through stomata.

Aliases: crop transpiration

QUANTITY, SYMBOL AND UNIT

Quantity / objectPlant transpiration rate

Symbolṁ_tr, E

SIkg/skilogram of water per second

Commonkg/hkilogram of water per hour

Commonmm/daymillimetre per day: Water depth over a stated crop area

Definition

Plant transpiration is the vaporisation of water within plant tissues and the release of that water vapour to the surrounding air, mainly through stomata.

Water taken up by roots moves through the plant before most of it is lost as vapour. Radiation, leaf temperature, air humidity, airflow, stomatal behaviour, crop development and root-zone water all affect the rate.

Formula and variables

ṁ_tr = Δm_w,plant / Δt

Conditions: Average water-vapour loss from the stated plant or crop boundary over the stated interval. Soil or substrate evaporation must be separated if the quantity is intended to represent transpiration alone.

Symbol	Meaning	Unit
`ṁ_tr`	plant transpiration rate	kg/s or kg/h
`Δm_w,plant`	water mass lost from plant tissues as vapour	kg
`Δt`	observation interval	s or h

Relations and distinctions

Transpiration and evaporation

Transpiration passes through plant tissues and stomata; evaporation can occur directly from substrate, nutrient solution and wet surfaces.

Transpiration and evapotranspiration

Evapotranspiration combines plant transpiration with evaporation from soil or other exposed surfaces.

Engineering meaning

Treat crop transpiration as a dynamic internal moisture source in a CEA load schedule.
Use crop- and stage-specific measurements or validated models when moisture removal capacity depends on the peak rate.

Common misconceptions

MisconceptionVPD alone determines the crop's transpiration rate.

Accurate explanationEnergy, stomatal response, leaf area, airflow and root-zone water can limit or change the response at the same VPD.

Application boundaries

Field evapotranspiration coefficients are not direct indoor transpiration rates without a compatible area, radiation and crop basis.
Transpiration does not include leaks, irrigation evaporation, wet floors or other facility moisture sources.

Supporting sources

Food and Agriculture Organization of the United Nations / SRC-FAO-56-1998

Crop Evapotranspiration - Guidelines for Computing Crop Water Requirements - FAO Irrigation and Drainage Paper 56 ↗

FAO Irrigation and Drainage Paper 56 / Chapter 1, Transpiration and factors affecting evapotranspiration
Supports: Transpiration is vaporisation of water in plant tissues and removal of vapour to the atmosphere; Water is lost mainly through stomata; Radiation, temperature, humidity, wind, crop and soil water affect the rate
United States Department of Agriculture, Agricultural Research Service / SRC-USDA-ARS-VPD-2002

The Effect of Vapor Pressure Deficit on Maize Transpiration Response to a Drying Soil ↗

Plant and Soil 239: 113–121 / Abstract, methods and discussion of maize transpiration response
Supports: Transpiration response to atmospheric demand interacts with drying soil; Crop-specific results cannot be universalised across species and systems

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