Enter the saturated vapor pressure and the vapor pressure of the air into the calculator to determine the vapor pressure deficit (VPD).

VPD Formula

The following formula is used to calculate a vapor pressure deficit. 

VPD = SVP - AVP
  • Where VPD is the vapor pressure deficit
  • SVP is the saturated vapor pressure
  • AVP is the actual vapor pressure

To calculate the saturated vapor pressure, visit the water vapor pressure calculator linked above.

The VPD can also be calculated using the saturated vapor pressure and the relative humidity with this formula:

VPD = SVP * (1-RH/100) = VPD

  • Where RH is the relative humidity (%)

Vapor Pressure Deficit Definition

A vapor pressure deficit, or VPD for short, is the difference between the saturated vapor pressure and the actual current vapor pressure.

The saturated vapor pressure is the maximum amount of water vapor a body of air at a given temperature can hold.

Example Problem

How to calculate VPD?

  1. First, determine the actual water vapor pressure.

    For this example, the air is found to have a water vapor pressure of 1.45 kPa (kilo-pascals).

  2. Next, determine the saturated water vapor pressure.

    For this problem, the saturated vapor pressure is found to be 2.14 kPa.

  3. Finally, calculate the VPD.

    Using the formula above, the VPD is calculated as:
    VPD = SVP – AVP
    VPD = 2.14 – 1.45
    VPD = .69 kPa

FAQ

What is the significance of measuring Vapor Pressure Deficit (VPD) in agriculture?

Measuring VPD is crucial in agriculture because it helps in understanding the moisture demand of the air around plants. A higher VPD indicates dry air, prompting plants to transpire and cool themselves more, which can enhance nutrient uptake and growth. Conversely, a low VPD suggests moist air, reducing transpiration, potentially leading to issues like fungal diseases. Thus, monitoring VPD allows for better irrigation and climate control strategies to optimize plant health and yield.

How does temperature affect Vapor Pressure Deficit?

Temperature plays a significant role in determining VPD. As temperature increases, the capacity of air to hold water vapor (saturated vapor pressure) also increases, potentially raising the VPD if the actual vapor pressure (amount of moisture in the air) does not increase proportionally. Conversely, a decrease in temperature reduces the air’s capacity to hold moisture, which could lower the VPD if the actual vapor pressure remains constant. This relationship underscores the importance of temperature control in managing VPD in controlled environments like greenhouses.

Can VPD be used to predict weather conditions?

While VPD is primarily used in agricultural and environmental science to assess plant water stress and manage irrigation, it can also offer insights into weather conditions. A high VPD is often associated with dry, sunny weather, indicating a greater potential for evaporation and transpiration. On the other hand, a low VPD can signal high humidity levels, cloudiness, or precipitation, as the air is closer to saturation with water vapor. However, predicting weather conditions accurately requires a comprehensive analysis of various meteorological factors alongside VPD.