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relative humidity
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Generic crop (0.5 to 1.2 kPa)

The VPD chart calculator (or VPD calculator) is an important tool in horticulture to help optimize the growing environment for plants. But what is vapor pressure deficit and why is it important?

Read on to learn more about:

  • The definition of VPD (Vapor Pressure Deficit);
  • How to calculate vapor pressure deficit;
  • The difference between relative humidity and vapor pressure;
  • How to interpret our VPD chart; and
  • Some example calculations.

This calculator was created with the consultation of Dr. Erik Runkle, who specializes in horticultural research at Michigan State University, as well as David Llewellyn from the University of Guelph.

What is vapor pressure deficit (VPD)?

Vapor pressure deficit (VPD) is the difference between the amount of moisture in the air and the amount of moisture in the leaf of a crop, and we typically represent it in pressure units, such as kilopascals. The vapor pressure deficit is a useful metric in horticulture because it roughly correlates to how quickly plants transpire (the loss of water through the leaf stomata).

If you've ever been in a dry climate, you may have realized that dry air causes us to lose our bodily moisture more quickly, whereas humid climates may cause us to feel sticky or sweaty due to slower evaporation rates.

While plants are able to regulate water loss to some extent through the opening and closing of the stomata in their leaves, the vapor pressure deficit still affects the rate of plant transpiration. This is why growers use VPD to maintain a healthy environment for plant growth.

What does the vapor pressure deficit mean for plants?

A low vapor pressure deficit indicates a small difference between the vapor pressure of the air and of the leaf. In other words, the air is humid, and so plants tend to transpire more slowly.

Lower vapor pressure deficits can be desirable during the early stages of propagation where small seedlings or cuttings can dry out easily. However, at very low vapor pressure deficits, plants may be at a greater risk of mildew and other diseases.

A high vapor pressure deficit indicates that there is a greater difference between the vapor pressures of the air and leaf. This indicates that the air is more dry, and so plants tend to transpire more quickly.

At very high vapor pressure deficits, if there is a risk of dehydration, many plants will employ a defensive mechanism by closing their stomata to conserve water. As a consequence, both transpiration and gas exchange are slowed, ultimately leading to slower plant growth.

Relative humidity vs. vapor pressure

A common question when discussing vapor pressure deficit (VPD) is, why can't we just use relative humidity? One may reasonably argue that relative humidity is also a measure of moisture in the air, and it is much easier to measure than vapor pressure.

The answer is that relative humidity is what its name indicates: relative. Relative humidity tells you the amount of moisture in the air, expressed as a percentage of how much moisture the air can hold.

Warm air can "hold" more water vapor than cold air, so the absolute difference of vapor pressure between the relative humidities of 60% and 100% will change depending on temperature. As a consequence, relative humidity is a poor indicator of the "drying power" of air. Air with 60% relative humidity at 35 °C would feel much drier than air with the same relative humidity but at a colder temperature (such as 18 °C).

In contrast, vapor pressure is an absolute measure of the water vapor in the air and does not change with temperature. This means that vapor pressure deficit can be reliably used as a gauge for plant transpiration at different temperatures.

How to calculate vapor pressure deficit?

Vapor pressure deficit can be calculated as:

vapor pressure deficit = vapor pressure of leaf - vapor pressure of air

where we commonly represent vapor pressure using kilopascals, millibars, or pounds per square inch. For the standard calculation, you will need:

  • Relative humidity;
  • Air temperature; and
  • Leaf temperature (or canopy temperature).

You can get an estimate of leaf temperature by measuring canopy temperature with an infrared sensor, or an air temperature sensor (aspirated is best) located near the plant canopy. Leaf temperature is normally a few degrees cooler than air temperature due to evaporative cooling.

There are various equations that we can use to calculate vapor pressure. Our VPD calculator uses the equation set out by Tetens in 1930 to calculate vapor pressure of liquid water, which is equal to the vapor pressure of air at saturation. The Tetens equation is accurate to within 0.1% over the range of 0 to 50 °C.

The Tetens equation for saturation vapor pressure uses the exponent function:

saturation vapor pressure = 0.61078 * exp[17.27 * T / (T + 237.3)]

where T is temperature in °C and saturation vapor pressure is in kilopascals. However, because air in a growing environment is usually not saturated, we calculate the actual vapor pressure of air by multiplying the saturation vapor pressure by relative humidity:

vapor pressure of air = saturation vapor pressure of air * relative humidity

where relative humidity is written a as a decimal (e.g., 60% is written as 0.6).

For the vapor pressure in the leaf, we assume that the air is saturated (100% relative humidity) so the Tetens equation is unmodified.

Thus we can also write the VPD formula as:

vapor pressure deficit = saturation vapor pressure of leaf - saturation vapor pressure of air * relative humidity

where we use the leaf or canopy temperature measurement to calculate the saturation vapor pressure of the leaf and the air temperature measurement to calculate the saturation vapor pressure of the air.

What if I don't know canopy temperature?

If you haven't got a temperature sensor for the crop canopy set up, you can still get a rough idea about the vapor pressure deficit by assuming that leaf temperature is equal to air temperature. This essentially tells us the vapor pressure deficit of air compared to saturated air, instead of the vapor pressure deficit of the crop.

Of course, crop temperature isn't necessarily equal to air temperature. Well-watered plants have cooler canopies due to evaporative cooling. Conversely, if light levels are high, the radiation can cause crop temperature to increase.

Nonetheless, knowing the vapor pressure deficit of air is still useful as it gives you a general idea about your growing environment. If you know the air temperature, try playing around with the VPD calculator, using different canopy temperatures to see how the VPD chart changes when canopy temperature is cooler or hotter than the air.

Calculate VPD using dew point

If you want to set up an environmental control system to automatically regulate vapor pressure deficit, the above equation requires you to regulate both relative humidity and air temperature. To simplify the control system, the dew point can replace both the air temperature and relative humidity in the vapor pressure deficit equation. By using dew point, you only need to control one variable (dew point) instead of two (air temperature and relative humidity). We also measure leaf temperature but do not control it in both cases.

How does dew point replace relative humidity and temperature?

The dew point is the temperature at which the moisture present in the air will begin to condense. In other words, if air were to be cooled exactly to its dew point, the relative humidity would have just reached 100%. Because of the fact that cooling air in a closed system doesn't affect the absolute content of moisture in the air (provided that the water vapor is in the gas phase), using the dew point and 100% relative humidity to calculate the vapor pressure of air would result in the same vapor pressure as if we used air temperature and relative humidity.

The equation for vapor pressure deficit using dew point is:

vapor pressure deficit = saturation vapor pressure of leaf - saturation vapor pressure at dewpoint

where we use the leaf temperature to calculate the saturation vapor pressure of the leaf, and the dew point temperature to calculate the saturation vapor pressure at dew point.

The VPD chart calculator allows you to use either relative humidity & air temperature or dewpoint to calculate your vapor pressure deficit.

Calculate VPD using wet-bulb temperature

If you are using dry- and wet-bulb temperature measurements instead of relative humidity, you can still calculate vapor pressure deficit.

Using the formulas from Practical Meteorology by Roland Stull, we've implemented the equations to find the dewpoint based on dry- and wet-bulb temperature.

From here, the steps you need to follow to calculate VPD are the as same steps we used in the previous section to calculate VPD from dewpoint.

Example calculation of vapor pressure deficit

Let's say we're in a greenhouse. The air temperature is 22 °C (72 °F), the relative humidity is 55%, and the temperature at the crop canopy is 20 °C (68 °F). Follow the steps to calculate vapor pressure deficit:

1. Calculate vapor pressure in the leaf

Use the leaf temperature of 20 °C as T in the Tetens equation to calculate the saturation vapor pressure (in kilopascals) in the leaf:

vapor pressure in the leaf = 0.61078 * exp[17.27 * T / (T + 237.3)]

= 0.61078 * exp[17.27 * 20 / (20 + 237.3)]

= 2.338 kPa

2. Calculate vapor pressure of air

Use the air temperature of 22 °C as T in the Tetens equation to calculate the saturation vapor pressure of air, and multiply it by the relative humidity of 55% (0.55):

vapor pressure of air = 0.61078 * exp[17.27 * T / (T + 237.3)] * relative humidity

= 0.61078 * exp[17.27 * 22 / (22 + 237.3)] * 0.55

= 1.454 kPa

3. Calculate vapor pressure deficit

Finally, subtract the vapor pressure of air from the vapor pressure in the leaf to determine the vapor pressure deficit:

vapor pressure deficit = 2.338 kPa - 1.454 kPa

= 0.88 kPa

Excellent, now you know how to calculate vapor pressure deficit! Try it yourself with different numbers, then check your result with our VPD chart calculator.

Interpreting the VPD chart (vapor pressure deficit chart)

Use the VPD chart as a quick way to understand what ranges of air temperature and relative humidity you should aim for to produce your desired vapor pressure deficit.

Follow these basic steps to use the VPD chart:

  1. Select which method you will use to calculate VPD and enter your values. This is important because the curves on the chart are affected by the difference between leaf and air temperature. If you don't know the crop temperature, the calculator assumes the temperature difference is 0 °C. For this example, we'll say that we know the crop temperature, and add a leaf temperature that is 1 °C less than air temperature.

  2. In the settings below the chart, ensure that the minimum desired VPD and maximum desired VPD are appropriate for your plants. For this example, we will use the generic crop setting to target a VPD between 0.5 and 1.2 kPa. Also select your desired temperature units to °C or °F.

  3. Looking at the chart, identify your air temperature on the x-axis. Draw an imaginary vertical line on the chart. Note locations where the line intersect with the desired VPD range (the light blue portion of the vapor pressure deficit chart).

VPD chart interpretation step 3
  1. Draw horizontal lines from the boundaries of the desired VPD range to the y-axis. Note down the relative humidities. In the following example the identified humidities were 46% and 74%.
VPD chart interpretation step 4
  1. To interpret, we would say that for an air temperature of 21 °C where the crop is 1 °C cooler than the air, we must keep the relative humidity between 46% and 74% to achieve a VPD in the range of 0.5 to 1.2 kPa.
Jasmine J Mah
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