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
KPA may refer to:
* Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA)
* Kensington (Olympia) station, London, England, National Rail station code
* Kenya Ports Authority
* ''Kiln phosphoric acid'', a ...
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.
In
ecology
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
, it is the difference between the actual
water vapour pressure and the saturation water vapour pressure at a particular
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
. Unlike
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
and other measures of evaporation.
Computing VPD for plants in a greenhouse
To compute the VPD, we need the ambient (greenhouse) air temperature, the
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
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
In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 1 ...
; a way to compute it directly from temperature is
:
where:
:
is the saturation vapor pressure in PSI,
:
,
:
,
:
,
:
,
:
,
:
,
:
is temperature of the air in the
Rankine scale
The Rankine scale () is an absolute scale of thermodynamic temperature named after the University of Glasgow engineer and physicist Macquorn Rankine, who proposed it in 1859.
History
Similar to the Kelvin scale, which was first proposed in 1848 ...
.
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.
Or simply
:
Climate
VPD can be a limiting factor in plant growth.
Climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
is predicted to increase the importance of VPD in plant growth, and will further limit growth rates across ecosystems.
Application in contexts of wildfire
The vapour pressure deficit can be utilized when predicting behaviour of a wildfire. Such predictions are an essential tool of
wildfire suppression
Wildfire suppression is a range of firefighting tactics used to suppress wildfires. Firefighting efforts in wild land areas require different techniques, equipment, and training from the more familiar structure fire fighting found in populated ...
.
See also
*
Water vapour
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
References
{{Reflist
Psychrometrics