79 



more humid than that of the upper strata, and farther, that the higher 

 and denser the vegetation the greater these differences are." This is 

 shown in Fig. 8a. 



Intensity of evaporation. 



Sta. A. 60 inches above ground, above 

 vegetation 



Sta. B. 12 inches above ground among 

 vegetation 



Sta. C. 3 inches above ground, among 

 vegetation 



I 



30 



40 



50 60 



70 



do go 



71-: 



50"; 



100' 



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

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

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



In America only a few records have been made on vertical gra- 

 dients in evaporation, two of these in marsh areas, one in Ohio by 

 Dachnowski ('ii), and the other near Chicago by Sherff ('13a, '13b). 

 The Ohio observations, made upon a small island in a lake, in a cran- 

 berry-sphagnum bog, show that the rate of evaporation above the vege- 

 tation is much greater than among it, and that this diminishes as the 

 soil is approached, these results agreeing with those obtained by Yapp. 

 Sherff's observations were made in Skokie Marsh, north of Chicago, 

 and show that the relative evaporation also varies with different kinds 

 of swamp vegetation. From his data a diagram has been made (Fig. 

 9) in which the rate of evaporation in the upper part of the reeds 



Intensity of evaporation 



Phragmites 

 Sta. A. Within vegetation, 198 cm. (77 

 inches) above soil. Standard. 



Sta. B. Within vegetation, 107 cm. (42 

 inches) above soil 



Sta. C. Within vegetation, 25 cm. (10 

 inches) above soil 



Sta. D. At soil surface 



Typha 



Sta. A. Within vegetation, 175 cm. (69 

 inches) above soil 



Sta. B. Within vegetation, 107 cm. (42 

 inches) above soil 



Sta. C. Within vegetation, 25 cm. (10 

 inches) above soil 



Sta. D. At soil surface 



Pig. 9. Diagram of relative evaporation at different vertical levels above the soil 

 within the vegetation of Skokie Marsh. (Adapted from Sherff.) 



