77 



west and south winds, and, as might be expected, it has an excessive 

 evaporation — 184 per cent. Station i, also covered by prairie vegeta- 

 tion, and exposed to west and southwest winds but sheltered from 

 Tvinds from the south and southeast, also shows a very high evapora- 

 tion — 132 per cent. Station 4, which was made the standard, had been 

 cleared of forest, and was an open place protected by a ridge. Station 

 2 was apparently a dense grove composed of bur oak, basswood, elm, 

 and ash, with considerable undergrowth. Here the rate of evapora- 

 tion dropped considerably — to 36 per cent. The general character of 

 this forest calls to mind the denser oak forests on sand at Havana, 

 Illinois. An important feature of these observations is that they were 

 made far out upon the "prairie", bordering the plains, most other 

 studies on relative evaporation having been made much farther east. 



In Ohio, Dachnowski ('11) and Dickey ('09) have recorded the 

 relative evaporation of the air, using a campus lawn as unity. In the 

 central grass-like area of a cranberry bog the evaporation was 69.2 

 per cent., and in the marginal maple-alder forest it was 51.2 per cent. 



Harrington ('93:96-102), in summarizing European studies on 

 the relative evaporation (with a water-surface as standard) in the 

 open and in German forests shows that the "annual evaporation in the 

 woods is 44 per cent, of that in the fields." Compared with evapora- 

 tion in the open, that under deciduous trees is 41 per cent., and that 

 under conifers is 45 per cent. — a difference most marked in the sum- 

 mer. Ebermeyer's Austrian observations (1. c. : 99) show that the 

 "evaporation from a bare soil wet is about the same as that from a 

 water surface," both in the open and in the forest. A saturated soil 

 under forest litter gives an evaporation of only 13 per cent, of that 

 of a free-water surface in the open. Harrington (1. c. : 100) con- 

 cludes that "About seven-eighths of the evaporation from the forest 

 is cut off by the woods and litter together." Sherff ('13a, '13b) has 

 shown that in the Skokie Marsh, north of Chicago, the absolute 

 amount of evaporation near the soil was less at the center of a Phrag- 

 mites swamp than at its margin (Fig. 7), that a swamp meadow 



Intensity of evaporation. 



Sta. D. Wiiite oak-ash forest 



Sta. B. Phragmitea swamp, margin 



Sta. C. Swamp meadow 



Sta. A. Phragmites swamp, center 



Fig. 7. Diagram of relative evaporation in Skokie Marsh area, near Chicago, 

 at 10 inches (25 em.) above the soU. Becalculated. (Adapted from SherfE.) 



