Grafts et al. — 182 — Water In Plants 



„ kA , P — p" , ,,. 



V = — — log — , where (J) 



h P — p 



V := rate of total evaporation. 

 A = area of the tube. 



h = height of the tube from the liquid surface to the free air space, 

 p" = partial pressure of vapor at the free end of the tube, 

 p' = partial pressure of vapor at the evaporating surface. 



The equation becomes V = — — - — ■ (4) 



when p' p" are small in comparison to P. 



Factors Affecting Evaporation from Free Water Surfaces of Uni- 

 form Shape and Area : — Since evaporation involves the escape of mole- 

 cules from a body of fluid against internal forces of attraction and forces 

 resident in the surface film as well, many factors relating to intermolecular 

 forces in the liquid affect evaporation rates. The more important factors 

 will be discussed. 



a) Solutes. — Any solute dissolved in a liquid reduces the diffusion 

 pressure and hence the rate of evaporation of the liquid. Because they 

 dissociate, salts in aqueous solution are particularly effective in lowering 

 the evaporation rate of water. For example sea water evaporates about 5 

 per cent less rapidly than fresh water under similar conditions. 



b) Temperature. — Evaporation of a liquid increases with increased 

 temperature in approximately the same ratio as the vapor pressure at satura- 

 tion increases with temperature. When air temperature is above that of 

 water the latter will cease to evaporate before the air is saturated ; if the 

 temperature relations are reversed, evaporation will continue into the satu- 

 rated atmosphere and dew may be deposited. 



c) Dryness of the air. — The evaporation rate of a liquid depends on 

 the concentration of liquid molecules in the vapor layer in contact with the 

 liquid surface, and it is the gradient of concentration above, that determines 

 diffusion away. In still air evaporation is approximately equal to the diff- 

 erences in absolute dryness of the air rather than to relative humidity as is 

 shown by equations (2) and (4). That is, 



V = k P'-P" , where (5) 



V = amount of water evaporated per unit area, the area being large and the water 



flush with the surface, 

 k = the diffusion coefficient. 



pi = the vapor pressure of the liquid at saturation at the surface temperature. 

 pO = the vapor pressure of the liquid in air. 

 P = atmospheric pressure. 



Vapor pressure deficit is defined as the difference between the actual 

 vapor pressure at a given temperature and the maximum possible vapor 

 pressure (saturated) at the same temperature. It should be understood 

 that the above equation holds only where the temperatures of the evaporat- 

 ing surface and the air are the same. Leighly (1937) discusses vapor 

 pressure deficits stressing the fact that the temperature equality mentioned 

 above represents a special case seldom attained. 



d) Atmospheric pressure. — Evaporation rate changes approximately 

 in inverse proportion to the total barometric pressure (equation 5). This 

 follows from the kinetic law of gases. 



e) Wind velocity. — Evaporation increases with wind velocity due to 

 eddy diffusion of vapor near the liquid surface (Jeffreys, 1918; Van 



