Vapour-pressure deficit

In ecology, it is the difference between the 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.

Computing VPD for plants in a greenhouse

To compute the VPD, 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

${\displaystyle v{p}_{\text{sat}}=e^{A/T+B+CT+D{T}^2+E{T}^3+F\ln T},}$

where

${\displaystyle v{p}_{\text{sat}}}$ 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 partial pressure of the water vapour in the air by multiplying by the relative humidity [%]:

${\displaystyle v{p}_{\text{air}}=v{p}_{\text{sat}}\times (\text{relative humidity})/100}$,

and finally VPD using ${\displaystyle v{p}_{\text{sat}}-v{p}_{\text{air}}}$ or ${\displaystyle v{p}_{\text{canopy sat}}-v{p}_{\text{air}}}$ when the canopy temperature is known, or simply

${\displaystyle VPD=v{p}_{\text{sat}}\times (1-\text{relative humidity}/100)}$.

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