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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 [[Cutting (plant)|cuttings]], the plant may [[Desiccation tolerance|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 to 0.95 kPa <ref>[http://www.hydro.co.nz/1_information/1_vpd/info_vpd.html Autogrow Systems Ltd. web site, humidity page, accessed October 13, 2006]</ref>
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| In [[ecology]], it is the difference between the actual [[vapour pressure of water|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.
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| == Computing VPD for plants in a greenhouse ==
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| To compute the VPD<ref>[http://ohioline.osu.edu/aex-fact/0804.html Ohio State University Extension Fact Sheet, Greenhouse Condensation Control, accessed October 13, 2006]</ref> 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 [[Psychrometrics|psychrometric chart]] or derived from the [[Arrhenius equation]], a way to compute it directly from temperature is
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| <math> vp_{sat} = e^{A/T + B + CT + DT^2 + ET^3 + F\ln T}</math>
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| where:
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| <math>vp_{sat}</math> is the saturation vapor pressure in kPa
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| <math>A = -1.88 \times 10^4</math>
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| <math>B = -13.1</math>
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| <math>C = -1.5 \times 10^{-2}</math>
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| <math>D = 8 \times 10^{-7}</math>
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| <math>E = -1.69 \times 10^{-11}</math>
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| <math>F = 6.456</math>
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| <math>T = </math><big>Temperature of the air in Kelvin</big>
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| To convert between Kelvin and Celsius:
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| <math>T(K) = T(C) + 273.15</math>
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| <!-- derived from
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| vpsat = exp(''A''/''T'' + ''B'' + ''CT'' + ''DT''² + ''ET''³ + ''F''ln''T'')
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| where:
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| ''A'' = -1.044x104
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| ''B'' = -1.129x101
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| ''C'' = -2.702x10-2
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| ''D'' = 1.289x10-5
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| ''E'' = -2.478x10-9
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| ''F'' = 6.456
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| ''T'' – Temperature of the air in °R,
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| ''T''(°R) = ''T''(°F) + 459.67
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| --->
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| We compute this pressure for both the ambient and canopy temperatures.
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| We then can compute the actual [[partial pressure]] of the water vapour in the air by multiplying by the relative humidity [%]:
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| <math>vp_{air} = vp_{sat}*</math>relative humidity/100
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| and finally VPD using <math>vp_{sat} - vp_{air}</math> or
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| <math>vp</math><sub>canopy sat</sub><math> - vp_{air}</math>
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| when the canopy temperature is known.
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| ==See also==
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| [[water vapor|water vapour]] is closely related to this subject
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| ==References==
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| {{Reflist}}
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| ==External links==
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| * [http://www.autogrow.com/index.php?option=com_content&view=article&id=7&Itemid=141]
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| [[Category:Psychrometrics]]
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