EKS-HUM-040 / Agriculture and controlled environments

Vapour Pressure Deficit

水汽压亏缺

Vapour pressure deficit is the difference between saturation water-vapour pressure at a stated temperature and the actual water-vapour pressure of the air.

Abbreviations: VPD
Aliases: vapor pressure deficit, saturation deficit

QUANTITY, SYMBOL AND UNIT

Quantity / objectVapour pressure deficit

SymbolVPD, e_s − e_a

SIPapascal

CommonkPakilopascal

Definition

Vapour pressure deficit is the difference between saturation water-vapour pressure at a stated temperature and the actual water-vapour pressure of the air.

Air VPD uses air temperature for the saturation term. Leaf-to-air VPD uses leaf temperature and can differ when leaves are warmer or cooler than the air, so the temperature basis belongs with every value.

Formula and variables

VPD = e_s(T_basis) − e_a

Conditions: The saturation-pressure temperature basis must be named. Air VPD uses air temperature; leaf-to-air VPD uses leaf temperature. Actual vapour pressure and saturation pressure must use compatible units and pressure conventions.

Symbol	Meaning	Unit
`VPD`	vapour pressure deficit	Pa or kPa
`e_s`	saturation vapour pressure at the stated temperature basis	Pa or kPa
`T_basis`	air or leaf temperature used for saturation pressure	°C or K
`e_a`	actual water-vapour pressure of the air	Pa or kPa

Relations and distinctions

VPD and RH

VPD is a pressure difference; RH is a saturation ratio. Both depend on temperature but communicate different information.

Air VPD and leaf VPD

Air VPD uses air temperature; leaf-to-air VPD uses leaf surface temperature with the same actual air vapour pressure.

Engineering meaning

Label sensor location, temperature basis and calculation method when VPD is used for crop control.
Coordinate humidity, temperature, airflow and irrigation decisions because plant response is not governed by VPD alone.

Common misconceptions

MisconceptionOne VPD target is valid for every crop and every stage.

Accurate explanationUseful ranges depend on crop, stage, light, leaf temperature, airflow, root-zone water and production objectives.

Application boundaries

A calculated VPD is only as representative as the temperature and humidity measurements used.
VPD alone does not provide transpiration rate, crop health, moisture load or dehumidifier capacity.

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 3, air-humidity concepts and vapour pressure deficit, especially Equations 11–19 and the VPD section
Supports: VPD is saturation vapour pressure minus actual vapour pressure; VPD is expressed as pressure; Temperature basis and averaging method affect the result
ASHRAE / SRC-ASHRAE-F25-CH01

ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗

2025 SI edition / Chapter 1, saturation and partial-pressure relationships
Supports: Saturation vapour pressure depends on temperature; Actual water-vapour pressure can be related to humidity quantities; Pressure and phase conventions must be consistent
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 under soil drying
Supports: Plant response to VPD interacts with soil water status; One crop experiment does not establish a universal control target

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