{{Short description|Difference between moisture in air and moisture capacity}} [[File:VPD wiki.png|thumb|alt=vpd|Global distribution of Vapour-pressure deficit averaged over the years 1981-2010 from the CHELSA-BIOCLIM+ data set<ref>Brun, P., Zimmermann, N.E., Hari, C., Pellissier, L., Karger, D.N. (2022): Global climate-related predictors at kilometre resolution for the past and future. Earth Syst. Sci. Data Discuss. https://doi.org/10.5194/essd-2022-212</ref>]]
'''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.
In equation form:<ref name=":0">{{Cite journal |last=Seager |first=Richard |last2=Hooks |first2=Allison |last3=Williams |first3=A. Park |last4=Cook |first4=Benjamin |last5=Nakamura |first5=Jennifer |last6=Henderson |first6=Naomi |date=2015-06-01 |title=Climatology, Variability, and Trends in the U.S. Vapor Pressure Deficit, an Important Fire-Related Meteorological Quantity |url=https://journals.ametsoc.org/view/journals/apme/54/6/jamc-d-14-0321.1.xml |journal=Journal of Applied Meteorology and Climatology |language=EN |volume=54 |issue=6 |pages=1121–1141 |doi=10.1175/JAMC-D-14-0321.1 |issn=1558-8424}}</ref>
<math>VPD = e_s(T_a) - e_a</math>
<math>e_a</math> = actual [[vapor pressure]]
<math>e_s(T_a)</math> = [[saturation vapor pressure]] at temperature T<sub>a</sub>
Once air becomes saturated, water will condense 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 and 0.95 kPa.{{citation needed|date=September 2018}}
In [[ecology]], it is the difference between the [[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. Also, vapor pressure deficit is a more concrete measurement of the difference of the moisture content in the air, while relative humidity is a ratio of the actual vapor pressure to the saturation vapor pressure at the given temperature.<ref name=":0" />
== Computing VPD for plants in a greenhouse ==
To compute the VPD,<ref>[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)"] {{Webarchive|url=https://web.archive.org/web/20210119031221/http://www.ecaa.ntu.edu.tw/weifang/class-cea/Greenhouse%20Condensation%20Control%20VPD,%20AEX-804-01.htm |date=2021-01-19 }}. Ohio State University Extension Fact Sheet. Retrieved November 7, 2017.</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
:<math> vp_\text{sat} = e^{A/T + B + CT + DT^2 + ET^3 + F\ln T},</math>
where :<math>vp_\text{sat}</math> is the saturation vapor pressure in PSI, :<math>A = -1.0440397 \times 10^4</math>, :<math>B = -11.29465</math>, :<math>C = -2.7022355 \times 10^{-2}</math>, :<math>D = 1.289036 \times 10^{-5}</math>, :<math>E = -2.4780681 \times 10^{-9}</math>, :<math>F = 6.5459673</math>, :<math>T</math> is temperature of the air in the [[Rankine scale]].
To convert between Rankine and degrees Fahrenheit: <math>T[\text{R}] = T[^\circ\text{F}] + 459.67</math>
<!-- derived from vpsat = exp(''A''/''T'' + ''B'' + ''CT'' + ''DT''² + ''ET''³ + ''F''ln''T'')
where:
''A'' = -1.044x104 ''B'' = -1.129x101 ''C'' = -2.702x10-2 ''D'' = 1.289x10-5 ''E'' = -2.478x10-9 ''F'' = 6.456
''T'' – Temperature of the air in °R, ''T''(°R) = ''T''(°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 [%]:
:<math>vp_\text{air} = vp_\text{sat} \times (\text{relative humidity})/100</math>,
and finally VPD using <math>vp_\text{sat} - vp_\text{air}</math> or <math>vp_\text{canopy sat} - vp_\text{air}</math> when the canopy temperature is known, or simply
:<math>VPD = vp_\text{sat} \times (1-\text{relative humidity}/100)</math>.
It can easily be seen from this formula that if <math>T</math> rises (which raises <math>vp_\text{sat}</math>), but relative humidity remains constant, <math>VPD</math> will increase.
== Climate == 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.<ref>{{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}}</ref><ref>{{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}}</ref>
VPD is also a strong influencer of [[potential evapotranspiration]], which has important effects in the warming climate.
== Vapor pressure deficit management in agriculture == In controlled environments, such as greenhouses, vapor pressure deficit can be managed to maximize production. Excessive VPD can be reduced with misting or irrigation, or low VPD can be boosted by dehumidification or ventilation.<ref>{{cite book | last=Goldhammer | first=Ted |title=Greenhouse management |chapter=Managing Vapor Pressure Deficit in Greenhouses |date=2025 |publisher=Apex Publications |isbn=979-8-89766-348-4 |url=https://www.greenhouse-management.com/greenhouse_management/managing_vapor_pressure_deficit_greenhouse_crops/vapor_pressure_deficit_greenhouse_crops.htm}}</ref>
== Application in contexts of wildfire == As the vapor pressure deficit increases, the amount of moisture in the vegetation and soil decreases, leading to drier conditions. VPD correlates strongly with the area that has been burned by wildfires in the southwest United States.<ref name=":0" /> Warming temperatures and drops in actual vapor pressure have contributed to an increase of VPD in the southwest U.S. Conversely, in the northern Plains and midwest, an increase in actual vapor pressure has caused a decrease in VPD.<ref name=":0" />
The vapour pressure deficit can be utilized when predicting behaviour of a wildfire. Such predictions are an essential tool of [[wildfire suppression]].<ref>{{cite web |last1=Gabbert |first1=Bill |date=26 January 2015 |title=The role of vapor pressure deficit in wildland fire |url=https://wildfiretoday.com/the-role-of-vapor-pressure-deficit-in-wildland-fires/ |access-date=24 August 2020 |website=Wildfire Today}}</ref>
==See also== * [[Water vapour]]
==References== {{Reflist}}
[[Category:Psychrometrics]]