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BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



Probably the variations in the measurements made with different 

 instruments are greater in the case of evaporation than in the 

 case of any other factor. Because of the lack of information as to 

 the accuracy of certain available instruments in the measurement 

 of evaporation, two different sets the free water surface pan of the 

 Weather Bureau pattern and the porous cup atmometer were used 

 in obtaining evaporation indices for comparison with plant activi- 

 ties, and it will be helpful in future field studies to compare the 

 results. 



As it was desirable to correlate evaporation and physiological 

 activities for short periods, the evaporation values obtained from 

 the readings of the spherical atmometer were first compared with those 

 obtained from the free water surface and the data from each were 

 then compared with the factors which chiefly determine the evapo- 

 ration rate, namely, air humidity, wind movement, and air tempera- 

 ture. By this means it should be possible to show which of the two 

 instruments is responding best to the conditions controlling evapora- 

 tion. The records as obtained in the aspen-fir type for August were 

 selected for this purpose. The data are presented in Table 6, and 

 as a matter of convenience in comparing, they are summarized 

 graphically in figures 5, 6, and 7. 



TABLE 6. Daily evaporation from spherical atmometer and from free water 

 surface, with corresponding relative humidity, temperature, and wind 

 velocity. 



