PAPERS BY MEMBERS 93 



During: the summer of 1911 a series of evaporation and 

 soil moisture determinations were carried out at Chicago 

 Lawn, an undisturbed prairie area within the city limits of 

 Chicago. The work was made to conform more or less with 

 similar, though more extensive studies of Fuller (2) on the 

 forest associations of the south shore dune region of Lake 

 Michigan, and those of McXutt and Fuller (3) on the oak- 

 hickory forest at Palos Park. 111. 

 II. Evaporation. 



Evaporation determinations were made by means of the 

 Livingston standard atmometer (4) and the methods of oper- 

 ation suggested by him were carefully followed. During the 

 earlv part of the season, including the months of May and 

 June, the cups were equipped with the rain-correcting de- 

 vice.! 5) 



Two stations were established and maintained through- 

 out the season, while at times, three, and during one week, 

 four stations were in operation. The two principal stations. 

 Xos. one and two. were located in the midst of the association, 

 about 250 meters apart, the instruments being placed in sim- 

 iliar relation to the vegetation with the cups just below the 

 general level of the grasses at a height of about 30 cm., above 

 the surface of the soil. During the latter part of the season 

 a third instrument was set up with the cup placed about 23 

 cm., above the level of the surrounding vegetation ( i. e. about 

 6o cm. above the surface of the soil). Readings were made 

 at intervals of one week or ten days throughout the season. 

 The average daily loss for the intervening days was computed 

 from these data. And finally the mean was taken from the 

 results of stations one and two. 



The records show the maximum evaporation, for the sea- 

 son, of Z7 c. c. per day to occur about the 20th of May. This 

 extreme rate of evaporation might be considered very abnor- 

 mal on the supposition that May 1911 was an unusually hot 

 and dry month, as noted by ^IcXutt and Fuller (3) in their 

 work of the same spring. However. !Miss Xewlon finds the 

 same situation in her studies on Chicago Lawn during the 

 season of 1912. It seems not improbable that this high rate 

 of evaporation is the usual condition at the beginning of the 

 season before vegetation becomes well developed, and may 

 be a factor of considerable importance in the growth of these 

 plants. An examination of graph a, fig. 1. will show the 

 .presence of two summer maxima, one in July, of 15.8 c. c. 

 per dav. and the other in August, of 16.0 c. c- per day. The 

 minimum evaporation recorded is 5.9 c. c. per day. occuring 

 in the latter part of September. The mean daily rate for the 

 entire season of 168 days is 12-5 c. c. Also from fig. lb, it 



