Vapour-pressure deficit

To compute the VPD,[2] 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

${\displaystyle vp_{\text{sat}}=e^{A/T+B+CT+DT^{2}+ET^{3}+F\ln T},}$

where:

${\displaystyle vp_{\text{sat}}}$ is the saturation vapor pressure in PSI,

${\displaystyle A=-1.0440397\times 10^{4}}$,

${\displaystyle B=-11.29465}$,

${\displaystyle C=-2.7022355\times 10^{-2}}$,

${\displaystyle D=1.289036\times 10^{-5}}$,

${\displaystyle E=-2.4780681\times 10^{-9}}$,

${\displaystyle F=6.5459673}$,

${\displaystyle T}$ is temperature of the air in the Rankine scale.

To convert between Rankine and degrees Fahrenheit: ${\displaystyle T[{\text{R}}]=T[^{\circ }{\text{F}}]+459.67}$

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 [%]:

${\displaystyle vp_{\text{air}}=vp_{\text{sat}}\times ({\text{relative humidity}})/100}$

and finally VPD using ${\displaystyle vp_{\text{sat}}-vp_{\text{air}}}$ or ${\displaystyle vp_{\text{canopy sat}}-vp_{\text{air}}}$ when the canopy temperature is known.

Or simply

${\displaystyle VPD=vp_{\text{sat}}\times (1-{\text{relative humidity}}/100)}$

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.[3]

The vapour pressure deficit can be utilized when predicting behaviour of a wildfire. Such predictions are an essential tool of wildfire suppression.[4]

• Water vapour