44 



IRRIGATION PRACTICE 



To stop the upward movement of soil water due to 

 surface evaporation is a chief consideration of a system 

 of irrigation-farming in which economy in the use of 

 water is a vital factor. 



34. Intensity of evaporation. It is well known that 

 water evaporates into the air whenever the air is not 

 fully saturated with water vapor. Under the conditions 



FIG. 10. Evaporation usually exceeds rainfall. 



prevailing over the earth's surface, the air is always 

 unsaturated. In arid regions the air is very dry, as is 

 well shown by the high heat that may be endured in 

 such places, due to the rapid evaporation of perspiration 

 into the dry air. 



The rate of evaporation from water surfaces is much 

 larger than commonly believed. Briggs and Belz have 

 sought out all available records in various sections of the 

 United States, of the quantities of water that will evapo- 

 rate from a free-water surface during the six summer 

 months, April to September inclusive. A summary of 

 their findings is presented in the following table: 



