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WHAT IS VAPOUR PRESSURE DEFICIT?

If you have an iMETOS 3.3 device with hygroclip sensor (air temperature and relative humidity) now you can also see Vapour pressure deficit values and chart in FieldClimate.com. Vapour pressure deficit (VPD) is a value calculated from relative humidity and air temperature, and is in a close relationship to evapotranspiration.

VPD is an indication that takes into account the effect of temperatures on the water-holding capacity of the air, which is what drives transpiration of the leaf surface (transpiration occures when water pressure in leaves is higher then air vapour pressure).

It is difference from the amount of moisture from the air and how much moisture the air can hold when it‘s saturated (100 % RH). When the air is saturated (vapour starts to condensate) the clouds will form, dew will form, and leaf wetness will apper.

If we have low VPD, this means that RH is high and trasnpiration is low, we also have leaf wetness.

If we have high VPD, this means that RH is low, no leaf wetness, and plants need to draw more water with its roots - high transpiration.

Vpd Table  

Table 1: Vapour pressure (mBar) at various air temperatures (°C) and relative humidities (%).

WHAT IS NEEDED FOR CALCULATION?

  • Air temperature (from HC) 
  • Relative humidity (from HC)

We can then calculate 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:

Vpd Chart  

Figure 1: Psychrometric chart

HOW IS IT SHOWN IN FieldClimate.com

Vpd Fc  

Figure 2: Chart on FieldClimate.com showing vapour pressure deficit (mBar), air temperature (°C) and relative humidity (%).