122 Irrigation and Drainage 



all soils to produce maximum crops of all kinds, then irrigation 

 will be unnecessary in them. 



It has been shown that, under conditions in which no water 

 can be lost by surface or under -drain age: 



Clover uses 5.089 acre-inches in producing one ton of dry matter. 



Oats " 4.447 



Barley " 4.096 



Maize " 2.391 % ** " " " " 



Potatoes use 3.399 " " " " " " " 



These figures are an approximate measure of the demands of 

 those crops for water, and if one, two or three tons of dry matter 

 per acre are to be produced by these crops, then the amount of 

 available rainfall needed will be given by multiplying the figures 

 in this table by the yield which is expected per acre from 

 the soil. 



Let us see what the available rainfall is in various parts of 

 the eastern and central United States. To make the discussion as 

 pointed as possible, let us draw our data from the states of Illi- 

 nois, Indiana, Iowa, eastern Kansas, Maine, Michigan, Missouri, 

 Minnesota, New York, Ohio, Pennsylvania, Vermont, and Wiscon- 

 sin. In these states, what is the amount of rainfall available for 

 crop production? 



In the map, Fig. 23, is represented the mean annual rainfall of 

 the United States, as given by the Weather Bureau. Such a map, 

 however, does not show the amount of water which is available for 

 crop production, because, as shown on the map, Fig. 22, a large 

 part of this rain is carried to the sea in the rivers, and cannot, 

 therefore, be used in producing crops. But if the rains which 

 would drain away were subtracted from the mean annual rainfall, 

 the difference would still be too large, for we have many showers 

 which are too slight to be of any service whatever. Not only this, 

 but very light rains often do positive injury by destroying the 

 effectiveness of earth mulches which have been developed by till- 

 age, thus causing a loss of a part of the water already in the soil, 

 with that which fell as rain. 



It is further necessary, in discussing this problem, to consider 



