Vapour-pressure deficit

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Vapour-pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated, water will condense out to form clouds, dew or films of water over leaves. It is this last instance that makes VPD important for greenhouse regulation. If a film of water forms on a plant leaf, it becomes far more susceptible to rot. On the other hand, as the VPD increases, the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.[1] The optimality degrees of VPD for specific greenhouse crop are defined by a series of membership function model at the different light condition and growth stages [2] which are used in adaptive management framework for adjusting optimum microclimate parameters inside the greenhouse [3].

In ecology, it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.

Computing VPD for plants in a greenhouse

To compute the VPD,[4] we need the ambient (greenhouse) air temperature, the relative humidity and, if possible, the canopy air temperature. We must then compute the saturation pressure. Saturation pressure can be looked up in a psychrometric chart or derived from the Arrhenius equation, a way to compute it directly from temperature is


is the saturation vapor pressure in PSI,
is temperature of the air in Rankines.

To convert between Rankine and degrees Fahrenheit:

We compute this pressure for both the ambient and canopy temperatures.

We then can compute the actual partial pressure of the water vapour in the air by multiplying by the relative humidity [%]:

and finally VPD using or when the canopy temperature is known.

See also

Water vapour is closely related to this subject.


  1. ^ Autogrow Systems Ltd. web site, humidity page, accessed October 13, 2006.
  2. ^ Shamshiri et al., (2017). Membership function model for defining optimality of vapor pressure deficit in closed-field cultivation of tomato. Acta Hortic. 1152, 281-290, DOI: 10.17660/ActaHortic.2017.1152.38
  3. ^ Shamshiri et al. 2017. Adaptive Management Framework for Evaluating and Adjusting Microclimate Parameters in Tropical Greenhouse Crop Production Systems. INTECH Publication. Book title: Plant Engineering. ISBN 978-953-51-3608-8.
  4. ^ Ohio State University Extension Fact Sheet, Greenhouse Condensation Control, accessed November 7, 2017.

External links

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  • [2]
  • [3]
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