110 Irrigation and Drainage 



to permit the largest crops the soil is capable of carrying. This 

 statement is founded upon the fact that with plenty of water the 

 same soils did produce much larger crops, the differences being 

 such as are given in the table below: 



Table shotving differences in yield when the natural rainfall in Wisconsin is 

 supplemented by irrigation 



These figures show very clearly the insufficiency of rain in 

 these four years to produce the largest possible yields, and they 

 show to what extent irrigation in a climate such as that which 

 has occurred during the years 1894 to 1897 in Wisconsin is likely 

 to increase the average yields. 



CONDITIONS WHICH MODIFY THE EFFECTIVENESS OF 

 RAINFALL 



The rains which fall upon a given area are not equally effec- 

 tive under all conditions of soil and topography, and hence it 

 happens that irrigation may be desirable in localities where the 

 amount of rain which falls may be both large and uniformly dis- 

 tributed throughout the growing season. It has been pointed out, 

 in the study aiming to measure the amount of water required to 

 produce a pound of dry matter, that it was necessary to water the 

 sandy soils of coarse texture once in three to four days in order 



