1914] 



FULLER— EVAPORATION AND SOIL MOISTURE 



199 



by Gleason (8) and others, but the evaporation thus controlled 

 and changed is one of the principal factors in causing the develop- 

 ment of a different vegetation, or, in other words, the decreased 

 rate of evaporation caused by the heavier vegetation is the direct 

 cause of succession between different associations. It seems sur- 

 prising that Gleason has reached an opposite conclusion from 

 somewhat similar data. 



An average of the records of stations 1, 2, and 3 is plotted for 

 comparison with similar records from the other associations (fig. 4) . 



Fig. 2. — Dune with Populus dcltoidcs and Salix syrticola; station 3; Miller, IikI. 



Here it will be seen that the maximum average evaporation for 

 any week of 19 10 was just above 32 cc. per day, and the minimum 

 only once fell below 12 cc. per day. The average for the entire 

 season of 178 days was 21 . 1 cc. per day. In 19 11 several days in 

 May, with a temperature above 90 F., caused a remarkable maxi- 

 mum of 47 cc. per day in that month (fig. 5), and serves to empha- 

 size the importance of records extending through several seasons. 

 The midsummer maximum was 42 cc. per day, the minimum 11 . 5 

 cc, and the average 24.6 cc. per day. The season of 191 2, on the 



