IRRIGATION 



117 



grow crops, even though the actual rainfall is quite small, 

 inasmuch as it may be stored in the soil and conserved by 

 cultivation for use during the dry season. 



Frequency of Applying Water. In like manner the fre- 

 quency of applying irrigation water is a factor which deter- 

 mines the duty of water. One good thorough irrigation, 

 under most conditions, is preferable to several light appli- 

 cations. 



The Amount of Water Used in Irrigation. It is to be 

 expected that the student is anxious to know how much 

 water must be applied to the soil to supply the plants where 

 the rainfall is not sufficient, or where the rainfall is too slight 

 to be considered. The amount of water is usually designated 

 in inches or feet. This means that the water applied is 

 sufficient to cover the entire surface to a depth indicated in 

 inches or feet as occasion may require. The actual amount 

 of water varies largely, as may be expected. 



Mr. H. M.Wilson, in " Manual of Irrigation Engineering," 

 gives the following table setting forth the amount of water 

 used in irrigation in different countries. 



Amounts of water used in irrigation in various countries. 



*See Chapter XXI for definition of this unit. 



