WHAT IS VAPOUR PRESSURE DEFICIT?
If you have a METOS® device with hygroclip sensor (air temperature and relative humidity) now you can also see Vapour pressure deficit values and chart in FieldClimate. 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 occurs when water pressure in leaves is higher than air vapour pressure).
It is the difference between the amount of moisture in 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 appear.
If we have low VPD, this means that RH is high and transpiration 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:
Figure 1: Psychrometric chart
Figure 2: VPD Chart on FieldClimate showing vapour pressure deficit (mBar), air temperature (°C) and relative humidity (%).
Rate of evapotranspiration from the leaves can exceed the supply of water through the roots – the stomata will close and photosyntesis will slow down or stop. Leaves are at risk of high temperature injury, since evaporative cooling is reduced.
To avoid injury and death from wilting, many plant species will either curl their leaves or orient them downwards in attempt to expose less surface area to the sun. This can downgrade the quality of potted and foliage plants and can also reduce the growth rate and quality of vegetable crops.
- Plants are unable to evaporate enough water to enable the transport of minerals (calcium) to growing plant cells, even that the stomata may be fully open.
- At extremly low VPD water may condens onto leaves, fruits and other plant parts. This can provide a medium for fungal growth and disease.
- At low VPD also gutation can occur (plant exude water from their leaf cells).
- When plants are unable to evaporate water, excessive turgor pressure within cells can cause splitting and cracking of fruits (for example tomato).
- In cases where VPD alternates between too high and too low, fruit quality can be adversely affected by shrink cracks in the skin of the fruit, as turgor pressure alternately expands and contracts the water-filled cells in the fruit.
- Newly rooted cutlings or just germinated seedlings or young plants, with limited foliage and a small root system. This plants should have low transpiration, so we need to keep them at low VPD (4 – 8 mbar) in order to achive that high RH must be established (depending on temperature).
- Good established plants, with developed foliage and root system. This plants should have higher VPD (8 – 12 mbar), that means we need to keep low RH (depending on temperature), and that we have high transpiration. We achive with that:
- More healthy plants, because of lower diseases pressure (low RH).
- More nutrients uptake, because of more root system activity (high transpiration), also more water uptake.
- If we keep high VPD at lower temperature (higher RH), we will avoid transpiration stress.