MEANING OF IRRIGATION 5 



cation of water to lands for the purpose of producing large 

 and steady crop yields whenever the rainfall is insufficient 

 to meet the full water requirements of crops. Irrigation for 

 the purpose of disposing of sewage is of limited extent, 

 and is not always true irrigation. 



8. Geographical need of irrigation. The field of 

 irrigation, as above denned, is very large. About 25 per 

 cent of the earth's surface receives 10 inches or less of 

 rainfall annually, and, with our present knowledge, can be 

 reclaimed only by irrigation. Another 30 per cent of the 



FIG. 2. The limited water supply makes it unlikely that more than one-tenth of 

 the land will be irrigated. The shaded area is irrigated. 



earth's surface receives between 10 and 20 inches of rain- 

 fall annually. Over this vast area the chief extensive 

 crops may be grown without irrigation, but the intensive 

 crops demand the help of irrigation. That is, nearly 

 six-tenths of the earth's surface will be reclaimed, if at 

 all, by irrigation and dry-farming. The remaining four- 

 tenths will be helped materially by a system of irrigation. 

 9. Possible extent of irrigation. The great rivers 

 which, with their numberless tributaries, flow from the 



