Vapour-pressure deficit

To compute the VPD,[http://www.ecaa.ntu.edu.tw/weifang/class-cea/Greenhouse%20Condensation%20Control%20VPD,%20AEX-804-01.htm "Greenhouse Condensation Control: Understanding and Using Vapor Pressure Deficit (VPD)"]. Ohio State University Extension Fact Sheet. Retrieved November 7, 2017. 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

:$vp\_\backslash text\{sat\}\; =\; e^\{A/T\; +\; B\; +\; CT\; +\; DT^2\; +\; ET^3\; +\; F\backslash ln\; T\},$

where

:$vp\_\backslash text\{sat\}$ is the saturation vapor pressure in PSI,

:$A\; =\; -1.0440397\; \backslash times\; 10^4$,

:$B\; =\; -11.29465$,

:$C\; =\; -2.7022355\; \backslash times\; 10^\{-2\}$,

:$D\; =\; 1.289036\; \backslash times\; 10^\{-5\}$,

:$E\; =\; -2.4780681\; \backslash times\; 10^\{-9\}$,

:$F\; =\; 6.5459673$,

:$T$ is temperature of the air in the Rankine scale.

To convert between Rankine and degrees Fahrenheit:

$T[\backslash text\{R\}]\; =\; T[^\backslash circ\backslash text\{F\}]\; +\; 459.67$

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

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

:$vp\_\backslash text\{air\}\; =\; vp\_\backslash text\{sat\}\; \backslash times\; (\backslash text\{relative\; humidity\})/100$,

and finally VPD using $vp\_\backslash text\{sat\}\; -\; vp\_\backslash text\{air\}$ or

$vp\_\backslash text\{canopy\; sat\}\; -\; vp\_\backslash text\{air\}$

when the canopy temperature is known, or simply

:$VPD\; =\; vp\_\backslash text\{sat\}\; \backslash times\; (1-\backslash text\{relative\; humidity\}/100)$.

It can easily be seen from this formula that if $T$ rises (which raises $vp\_\backslash text\{sat\}$), but relative humidity remains constant, $VPD$ will increase.

VPD can be a limiting factor in plant growth. Climate change is predicted to increase the importance of VPD in plant growth, and will further limit growth rates across ecosystems.{{Cite journal |last1=Novick |first1=Kimberly A. |last2=Ficklin |first2=Darren L. |last3=Stoy |first3=Paul C. |last4=Williams |first4=Christopher A. |last5=Bohrer |first5=Gil |last6=Oishi |first6=A. Christopher |last7=Papuga |first7=Shirley A. |last8=Blanken |first8=Peter D. |last9=Noormets |first9=Asko |last10=Sulman |first10=Benjamin N. |last11=Scott |first11=Russell L. |date=2016 |title=The increasing importance of atmospheric demand for ecosystem water and carbon fluxes |journal=Nature Climate Change |language=en |volume=6 |issue=11 |pages=1023–1027 |bibcode=2016NatCC...6.1023N |doi=10.1038/nclimate3114 |issn=1758-6798 |hdl-access=free |hdl=10150/622526}}{{Cite journal |last=Grossiord |first=Charlotte |last2=Buckley |first2=Thomas N. |last3=Cernusak |first3=Lucas A. |last4=Novick |first4=Kimberly A. |last5=Poulter |first5=Benjamin |last6=Siegwolf |first6=Rolf T. W. |last7=Sperry |first7=John S. |last8=McDowell |first8=Nate G. |date=2020 |title=Plant responses to rising vapor pressure deficit |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.16485 |journal=New Phytologist |language=en |volume=226 |issue=6 |pages=1550–1566 |doi=10.1111/nph.16485 |issn=0028-646X |access-date=13 March 2024}}

The vapour pressure deficit can be utilized when predicting behaviour of a wildfire. Such predictions are an essential tool of wildfire suppression.{{cite web |last1=Gabbert |first1=Bill |title=The role of vapor pressure deficit in wildland fire |url=https://wildfiretoday.com/2015/01/26/the-role-of-vapor-pressure-deficit-in-wildland-fires |website=Wildfire Today |access-date=24 August 2020 |date=26 January 2015}}

• Water vapour

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Category:Psychrometrics