VPD - Rwema25/AE-project GitHub Wiki

Vapor Pressure Deficit (VPD) as an Agricultural Stress Indicator

Vapor pressure deficit (VPD) measures the difference between the moisture in the air and the maximum moisture the air can hold when saturated. It is a key indicator of atmospheric water demand, directly influencing plant transpiration, water uptake, and photosynthesis, making it critical for assessing plant water stress.

VPD and Plant Physiology

High VPD means drier air, which increases transpiration and water consumption by plants, potentially causing water stress if root water uptake is insufficient.
Plants respond to high VPD by reducing stomatal conductance to limit water loss, which may also reduce photosynthesis, nutrient uptake, growth, and yield.
Monitoring VPD helps optimize irrigation scheduling and greenhouse climate control to prevent water stress and reduce disease risk caused by leaf wetness.

Research Highlights

High VPD reduces crop productivity by lowering stomatal conductance and photosynthesis. Rising VPD increases transpiration rates, causing plants to lose more water through stomata. When VPD exceeds about 1.5 kPa, many crops begin to experience physiological stress shown by reduced stomatal conductance, lower photosynthesis, and reduced growth and yield.
In some protected vegetables, high VPD can improve fruit color and flavor quality.
VPD thresholds have been identified where plants show stress; for example, strawberry plants experience stress when VPD exceeds about 8.0 hPa under certain conditions.
Standardized VPD indices and sensors are increasingly used for drought and water stress monitoring across crops and climates.

Practical Implications

Optimal VPD for most crops ranges between 0.4 and 1.6 kPa, depending on species and growth stages.
Managing VPD in greenhouses promotes healthy transpiration rates without encouraging pathogen growth due to condensation.
VPD is a more effective stress indicator than relative humidity as it directly relates to evaporation and transpiration potentials.

Formulas to Calculate Vapor Pressure Deficit (VPD)

VPD is calculated by the difference between the saturation vapor pressure at the leaf or canopy temperature and the actual vapor pressure of the air:

$$ VPD = e_s(T_{leaf}) - e_a $$

Where:

$(e_s(T_{leaf}))$ = saturation vapor pressure at leaf temperature (kPa)
$(e_a)$ = actual vapor pressure of air (kPa)

The saturation vapor pressure at a given temperature (T) (in °C) is calculated using the Tetens formula:

$$ e_s(T) = 0.61078 \times \exp\left(\frac{17.27 \times T}{T + 237.3}\right) $$

Due to the non-linearity of the above equation, the mean saturation vapour pressure for a day, week, decade or month should be computed as the mean between the saturation vapour pressure at the mean daily maximum and minimum air temperature for the period:

$$ e_s = \frac{e_s(T_{max}) + e_s(T_{min})}{2} $$

Using mean air temperature instead of daily minimum and maximum temperatures results in lower estimates for mean saturation vapour pressure. The corresponding vapour pressure deficit will also be smaller, and the result will be some underestimation of the reference crop evapotranspiration. Therefore, the mean saturation vapour pressure should be calculated as the mean between the saturation vapour pressure at both the daily maximum and minimum air temperatures.

The actual vapor pressure of the air is:

$$ e_a = \frac{e_s \times RH}{100} $$

Where:

$(RH)$ = relative humidity (expressed as a decimal, e.g., 0.55 for 55%)
The actual vapour pressure $(e_a)$ can also be derived from dewpoint temperature $T_{dew}$. As the dewpoint temperature is the temperature to which the air needs to be cooled to make the air saturated, the actual vapour pressure is the saturation vapour pressure at the dewpoint temperature $T_{dew} [°C]$

$$ e_a= 0.61078 \times \exp\left(\frac{17.27 \times T_{dew}}{T_{dew} + 237.3}\right) $$

Thus, VPD can also be expressed as:

$$ VPD = e_s - e_a $$

Summary: VPD is a valuable, widely researched indicator for agricultural water stress. It provides actionable insights for irrigation management, crop productivity improvement, and environmental optimization in agricultural systems.

Vapor Pressure Deficit (VPD) from Adaptation Atlas

Core formula

(1)

$$ e_s(T)= 0.61120 \times \exp\left(\frac{17.67 \times T}{T + 243.5}\right) $$

where

$e_s(T)$ : Saturation vapor pressure
$T$: Temperature in [°C]

(2)

$$ VPD = vpd_{cte} \times \left(e_s(T_{max}) - e_s(T_{min}) \right) $$

where

$vpd_{cte}$: Empirical constant for scaling VPD (set to 0.7)

$$ VPD = vpd_{cte} \times 0.61120 \times \left[ \exp\left(\frac{17.67 \times T_{max}}{T_{max} + 243.5}\right) - \exp\left(\frac{17.67 \times T_{min}}{T_{min} + 243.5}\right) \right] $$

VPD Range (kPa)	Crop Stage / Context	Effects on Crops	Effects / Relevance to Humans	Source / Notes
0.3 – 0.7	Propagation / Young plants	Low transpiration stress, reduced leaf wetness	Generally comfortable humidity range	Greenhouse Management 1
0.4 – 1.0	Early vegetation / propagation	Optimal stomatal function, healthy growth	Comfortable indoor environment	Dimlux Lighting 2
0.8 – 1.2	Vegetative growth	Balanced transpiration and photosynthesis	Suitable for human comfort	Pulse Grow 3, J. Huete Greenhouses 4
1.2 – 1.6	Flowering / fruiting stages	Increased transpiration, possible stress onset	High evaporation demand, may feel dry	Dimlux Lighting 2, Maxapress 5
>1.6	Stress threshold (high)	Stomatal closure, reduced photosynthesis, yield reduction	Discomfort due to dryness, stress on respiratory system	Hortidaily 6
<0.3	Very low VPD	Reduced transpiration, risk of pathogen growth	Possible dampness, mold growth indoors	Greenhouse Management 1, Pessl Guide 7

Notes:

Optimal VPD varies by species and growth stage; young plants prefer lower VPD to reduce water stress.
Very high VPD causes water stress and reduced crop yields; very low VPD leads to poor transpiration and disease risk.
For humans, VPD relates indirectly to comfort via relative humidity and temperature balance.

References: