Chapter X — 183 — Loss and Retention 



Orstrand and Dewey, 1915). According to Leighly (1937), air mov- 

 ing over an evaporating surface has two effects: -Z) a turbulence or eddy 

 diffusion at some small distance from the surface and 2) a laminar move- 

 ment of air very near the surface in which region the velocity decreases to 

 zero at the surface layer. In this boundary layer the gradients of vapor 

 concentration, air velocity, and temperature are linear. 



The thickness of this boundary layer of even simple evaporimeters is a 

 function of at least five factors, namely: 1) wind velocity in free air, 2) 

 length of evaporating surface in the direction of the wind, 3) density of the 

 air, 4) molecular viscosity of the air, and 5) length of evaporating surface 

 at right angles to the wind (Martin, 1943). In the case of evaporation 

 from leaves the qualitative character of the surface must be added. Cor- 

 rugation, creasing, occurrence of hairs, and close proximity of adjacent 

 leaves all tend to increase the thickness of the boundary layer. 



Leighly's formula for evaporation into moving air, modified by Mar- 

 tin to include the length of the evaporating surface at right angles to the 

 direction of wind flow is as follows, 



V = c k (p— p") R-O-2 L-0.3 w+O-5 where (6) 



V = amount of water evaporated per unit area, 

 c = a proportionality constant. 



k = the diffusion coefficient. 



p' = vapor pressure at the surface. 



p" = vapor pressure at the edge of the boundary layer. 



R = length of the evaporating surface at right angles to the wind direction. 



L = length of the evaporating surface parallel to the wind direction. 



W = wind velocity. 



The exponents for the factors R, L, and W were determined experi- 

 mentally using blotting-paper evaporimeters and are in good agreement 

 with data reported by Renner (1910, 1911&), Thomas and Ferguson 

 (I917a,b), Jeffreys (1918), Gallenkamp (1919), Sierp and Seybold 

 (1927), and Seybold (1929). 



The above equation may be summarized by stating that evaporation from 

 small areas varies as the square root of the wind velocity and inversely as 

 the 0.3 and 0.2 powers of the length and breadth of the area. Certain pre- 

 sumed variations of this law are, according to Martin, probably due to 

 failure to determine the actual temperature of the evaporating surface (see 

 Walter, 1925). 



It should be noted that this expression of the rate of evaporation into 

 moving air is very different from the formula given above expressing the 

 rate of evaporation into still air where diffusion is molecular and "eddy 

 diffusion" is not involved. The first holds approximately where a steady 

 horizontal wind is present and cannot be reduced to the second which ap- 

 plies only in the absence of air movement. 



Transpiration: — Because of the complex nature of the process of 

 evaporation and the added complexities of leaf structure, transpiration is an 

 intricate process that presents many experimental difiiculties. On the other 

 hand gross measurements of the rate and volume of water loss by plants 

 may be readily made and many methods for studying water relations of 

 plants both in the greenhouse and in the field have been devised. But hav- 

 ing made measurements on water loss, the experimenter often meets his 

 most difficult task in attempting to interpret their meaning, and unless a 

 rigid control has been exercised over most or all of the external factors, 

 their explanation may be difficult or even impossible. Of the immense 



