78 



was in an intermediate position, and that in an adjacent white oak-ash 

 forest evaporation was about twice as much as in the swamp meadow. 

 Sherff used as standard the forest (D). This gave him for the center 

 of the swamp (A) 38 per cent., for the swamp meadow (C) 54 per 

 cent., and for the outer swamp margin (B) 105 per cent. In Figure 

 7, I have used his swamp meadow as 100 per cent., and by recalcula- 

 tion this gives the forest (D) 185 per cent., for the swamp margin (B) 

 105 per cent., and for the center of the swamp (A) 70 per cent. These 

 figures indicate a concentric arrangement of the conditions of evap- 

 oration about the swamp. 



Intensity of evaporation ■ 



1907: 



Sta. A. Above vegetation. 4 feet, 6 

 inches above soil 



Sta. B. Middle of vegetation. 2 feet, 

 2 inches above soil 



Sta. 0. Lower vegetation, 

 above soil 



5 inches 



1908: 



Sta. A. Above vegetation. 5 feet, 6 

 inches above soil 



Sta. B. Middle of vegetation. 2 feet, 

 2 inches above soil 



Sta. C. Lower vegetation, 

 above soil 



S inches 



Pig. 8. Diagram showing the relative evaporation at different vertical levels in 

 a marsh in England, the evaporation in the lower layers of the vegetation being much 

 greater than in the upper strata or in the air above it. (Data from Yapp.) 



Thus far, attention has been devoted solely to the horizontal differ- 

 ences in evaporation. There are also important vertical ones, vary- 

 ing above the surface of the substratum. Important observations on 

 this subject have been made, by a porous-cup method, in an open 

 grassy marsh in England, by Yapp ('09). The vegetation grew to a 

 height of two to five feet. From his data the accompanying diagrams 

 (Figs. 8, 8a) have been prepared. This shows that when the stand- 

 ard was made the rate of evaporation above the general level of the 

 vegetation, within the grass layer evaporation was reduced from about 

 one half (Sta. B, 1908, 56.2 per cent.) to one third (Sta. B, 1907, 

 32.8 per cent. ) at 2 feet 2 inches above the soil ; and that at 5 inches 

 above the soil it was reduced to between one fourteenth (Sta. C, 1907, 

 6.6) and one seventh (Sta. C, 1908, 14.7) of that aboA'^e the vegeta- 

 tion. Yapp (1. c. : 298) concludes from his studies that "In general, 

 the results of the evaporation experiments show that the lower strata 

 of the vegetation possess an atmosphere which is continually very much 



