Glossary
Vapor-Pressure Deficit (VPD)
The difference between the saturation vapor pressure at the water surface and the actual vapor pressure of the overlying air. A larger deficit means the air is drier relative to the surface, so evaporation proceeds faster; it is the primary driving force behind evaporation.
VPD rises with higher water-surface temperature and with lower air humidity. Because it combines temperature and humidity into a single driving gradient, VPD appears in most physically based evaporation methods, including the aerodynamic mass-transfer and Penman-Monteith approaches.