Water vapor pressure deficit

The commonly used expression Vapor Pressure Deficit or VPD is the partial pressure of water vapor in the leaf intercellular air spaces, minus the partial pressure of water vapor in the turbulent air outside the boundary layer, P 0,. Often P af is calculated as the saturation water vapor partial pressure at the temperature of the leaf (for a leaf water potential of -1.4 MPa at 20°C, this leads to an error of only 1% in Pj. af Table 2-1). P a, equals the air relative humidity times the saturation water vapor partial pressure (P. w) at the air temperature (values of P , in kPa, which can be used to calculate P f and P 0, are given at the end of Appendix I). 

Gollan T., Turner N. C., and Schulze E. D. (1985) The responses of stomata and leaf gas exchange to vapor pressure deficits and soil water content III. In the sclerophyllous woody species Nerium oleander. Oecologia 65, 356-362. 

Vapor Pressure Deficit. The water-holding capacity of saturated wood changes when it cools below the freezing point, with lowering of the fiber-saturation point (ii). When saturated wood is frozen, the vapor pressure of ice, is lower at the same temperature than the vapor pressure of supercooled water, e, bound to internal surfaces. The resulting saturated vapor pressure diflFerence between the ice and the supercooled water is given by and may be referred to as a vapor pressure deficit (VPD). 

Figure 2. Vapor pressure deficit (VPD) between supercooled water and ice at the same temperature.

Ap p is the vapor pressure deficit = saturated vapor pressure - vapor pressure of the air, N/m is the water efficiency coefficient, N/m ... 

Fig. 5 Annual dynamics of (a) partial pressure of water vapor (gPa), (b) saturation deficit (gPa), and (c) relative humidity (%). Names of HMS are given in the figure...

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