286 



JOHN ERNST \\ EAVER 



unoccupied At the time of the third maximum period of evapora- 

 tion during the last week in July, the vegetation near station 

 No. 4 (except Hoorebekia racemosa and Solidago missouriensis) 

 was rapidly drying up, and the soil moisture for a depth of 25 cm. 

 had been reduced to its non-available point; while on the north 

 slopes similar conditions were not reached until the second 

 week in September, and, indeed, several plants, such as Gentiana 

 oregana, Aster fremonti, Carum gairdneri, and others, remained 

 in blossom until the frosts of late September. 



For comparison, the mean average daily evaporation graphs 

 of ail instruments in the bunchgrass-rimrock association, the 

 more xerophytic and less xerophytic prairies, and the pine associ- 

 ation are plotted upon the same chart in figure 5. • All show a 



Fig. 6. Diagram showing the comparative evaporation rates in the different 

 plant formations and associations of the plains on the basis of the average 

 daily amount from May 7 to September 10. 



maximum in early June and another in July after which time 

 evaporation is high until the middle of September. The greater 

 regularity in the highest and lowest graphs is due in part to 

 the longer intervals between readings, and this same explanation 

 probably accounts for the overlapping in places of the bunch- 

 grass and dry prairie graphs With this exception, however, the 

 evaporation in each association is decidedly lower than that of 

 the association which it replaces in the normal succession. In 

 figure 6 is shown a comparison of the evaporation rates in the 

 different associations on the basis of the average daily amount 

 from May 7 to September 10, a period of 126 days. This gives 

 the same result, only expressed in a different manner. 



