VPD Calculator

Vapour pressure deficit is the difference between the water vapour pressure a parcel of air would hold at saturation and the vapour pressure it actually holds, reported in kilopascals. It is the physical driving force for transpiration, because water evaporates from the wet cell walls inside the leaf into the substomatal cavity and then diffuses down the vapour pressure gradient to the outside air. Saturation vapour pressure is obtained here from the Tetens equation, 0.6108 times the exponential of 17.27T divided by T plus 237.3, which reproduces measured values to within about 0.1 percent between 0 and 50 degrees C. Because the leaf is the evaporating surface, the physically correct deficit uses saturation vapour pressure at leaf temperature and subtracts the actual vapour pressure of the air; a transpiring, well-watered leaf normally sits 1 to 3 degrees C below air temperature owing to evaporative cooling. The practical importance of the metric is that relative humidity alone is misleading, since the saturation point rises steeply with temperature. Air at 70 percent relative humidity carries a deficit of about 0.70 kPa at 20 degrees C but about 1.07 kPa at 27 degrees C, so a grower holding humidity constant while raising temperature is silently increasing evaporative demand by half. Horticultural targets typically run 0.4 to 0.8 kPa for unrooted cuttings and seedlings, 0.8 to 1.2 kPa in vegetative growth, and 1.0 to 1.6 kPa through flowering and ripening, where a drier canopy suppresses botrytis and powdery mildew. Above roughly 2 kPa many species close stomata to conserve water, which also shuts off CO2 uptake. The transpiration figure shown here is a relative indicator only, since it assumes a fixed stomatal conductance rather than modelling boundary layer resistance, radiation load or soil water supply.

The temperature dependence of saturation vapour pressure was described empirically in the nineteenth century, notably by Heinrich Gustav Magnus in 1844, whose exponential form underlies most of the equations still in use. The particular coefficients applied in this calculator come from O. Tetens, who published his formulation in 1930; Arden Buck offered refined coefficients in 1981, and modern meteorological practice uses variants of the same structure with separate constants for saturation over water and over ice. Saturation deficit became a working agricultural variable through evaporation research. Howard Penman combined an energy balance with an aerodynamic vapour transport term in 1948 to estimate evaporation from open water and grass, and John Monteith extended the approach in 1965 by introducing canopy resistance, producing the Penman-Monteith equation in which the deficit term appears explicitly. That formulation was adopted as the international reference method for crop water requirements in the Food and Agriculture Organization Irrigation and Drainage Paper 56, published in 1998. Commercial greenhouse growers began managing deficit rather than humidity from the 1970s and 1980s as climate computers became available, and the metric spread widely to indoor and controlled-environment horticulture more recently, once inexpensive temperature and humidity sensors, infrared leaf thermometers and programmable controllers put continuous calculation within reach of small operations.