236 SOILS 



In general, a country is said to be arid when 

 it has an average rainfall of less than twenty 

 inches. The arid region of North America 

 extends into Canada and Mexico. Only a small 

 portion of this, about 70,000,000 acres, is desert, 

 contrary to the common notion in the East. 

 Most of it supports more or less vegetation; 

 120,000,000 acres are lightly timbered and 

 470,000,000 acres are grazing land. 



The number of acres of arid land in the United 

 States which it is possible and practicable to irri- 

 gate can be stated only approximately. Mr. Meade 

 writes: "A few years ago 75,000,000 acres was a 

 quite common estimate, but most of those familiar 

 with the arid West now make their estimates 

 smaller. There is at present a strong tendency to 

 use less water than was formerly used and as the 

 demand for agricultural products increases greater 

 expense in securing water can be borne. These 

 two influences would tend to increase the area 

 which can be irrigated with the existing water 

 supply under present practice, and any statement 

 as to the ultimate extent of land which can be 

 irrigated is little more than a guess." 



OBJECTS OF IRRIGATION 



The chief occasions for irrigation are an irregular 

 or an insufficient rainfall. The former is char- 

 acteristic of most all parts of the United States; 

 the latter is found mainly in western United States. 

 Most of the irrigation in this country is for the pur- 

 pose of remedying an actual deficiency in rainfall, 

 out much of it would be unnecessary if the rainfall 

 came at an opportune time. The time when 

 crops need water most is in summer and if it is not 



