THE FUNCTION OF WATER IN AGRICULTURE 249 



the estimates made for the irrigated regions in Idaho. 

 These data, however, are sufficient to show not only the 

 dominating influence of water in crop production, but 

 the enormous quantity of it required. They also show 

 that where the precipitation is about equally distributed 

 over the entire year with a rainfall of approximately 

 40 inches, only about one-third of the precipitation on 

 a field of growing grain is used. 



THE WEIGHT OF THE SOIL TO THE ACBE AND AVEBAGK0 



* 



RAINFALL. 



Very many determinations have been made of the 

 specific gravity of the soil. When this is ascertained 

 it is easy to calculate the weight of the soil per acre 

 to any given depth. Ordinary soil is about 2^/2 times 

 heavier than water, and the weight of soil, one foot 

 deep, over an acre of land is approximately 2,000 tons. 

 The amount of water, therefore, necessary to produce 

 a crop is many times heavier than the weight of the soil 

 to the ( depth of one foot in which the crop grows. 



Over the so-called humid region of the United States 

 the mean rainfall is about four feet. The mean rain- 

 fall over the States in the regions bordering the arid 

 area of the United States is about thirty inches. The 

 mean rainfall in the semi-arid regions of the United 

 States is about twelve inches ; while the mean rainfall 

 for the driest portions of the United States varies from 

 nothing at all to about six or seven inches. 



Of the total amount of water that falls on the sur- 

 face of the soil, about one-third reaches the sea. A 

 small quantity of the water is worked up into organic 

 union with growing crops, or in the crystallization of 

 minerals, or else sinks so deep into the earth as to escape 

 any further measurement. All the rest of the rainfall, 



