EVAPORATION IN DRY-FARMING SECTIONS. 



17 



figures on the chart represent the total evaporation in inches from 

 April to September, inclusive. 



The circles represent stations where dry-farming experiments are 

 being conducted by the Bureau of Plant Industry. The figures for 

 these stations can be compared directly, since they have all been 

 obtained from tanks exposed in the same way. Most of these tanks 

 are 8 feet in diameter and 2 feet deep, and are sunk in the ground so 

 that the top of the tank is 4 inches above the surface of the ground. 

 (See PI. I, frontispiece.) The water level is kept about 4 inches 

 from the top of the tank. The amount of water that has to be added 

 to keep the water at this height, together with the rain that falls 

 into the tank, gives the total evaporation for any required time. 



Fig. 4.— Chart showing the evaporation in inches during the six summer months from April to September, 

 inclusive, for various points in the United States. The circles show stations where evaporation measure- 

 ments are being made in connection with the field experiments of the Bureau of Plant Industry. The 

 evaporation at these stations is measured in the same way and can be directly compared. 



Three years' records have been obtained for some of these stations, 

 and there is little change in the evaporation from year to year. 

 These figures representing the evaporation for equal periods then 

 become very useful in comparing the conditions in different sections 

 of the country from a dry-farming standpoint. 



Table I presents the results from which the accompanying map (fig. 

 4) was prepared and shows the total evaporation which took place 

 from tanks during the six months from April to September, inclusive, 

 for a number of points in the United States. The figures have been 

 obtained from widely scattered sources, and the measurements were 

 made with tanks of different sizes and exposed in different ways. 

 52102°— Bui. 188—10 2 



