CHAPTER XX 



CONTROL OF MOISTURE 

 III. DRY-LAND AGRICULTURE 



THE distribution of rainfall over the surface of the earth is very 

 irregular, so that extensive areas are deficient in moisture and 

 special cultural methods and special crops must be used. Many 

 regions are so poorly supplied with moisture that crops cannot be 

 grown, even under the best conditions, without irrigation. It will 

 be well to consult the table, page 189, on the annual precipitation 

 on the earth's land surface. The map, figure 89, may be of further 

 help in giving a correct idea as to the location of the humid and 

 dry areas. 



From the table, page 189, it is seen that approximately 65 per 

 cent of the land area of the earth receives less than 30 inches of 

 rainfall. About 25 per cent receives less than 10 inches, while 40 

 per cent has from 10 to 30 inches. In the United States alone the 

 dry-land region covers about one-half the entire area, and 1,135,000 

 square miles of this is suitable for dry farming. Australia has 

 about the same amount, and extensive areas are found in Africa 

 and Asia and smaller ones in Europe and South America. 



Adaptation of a Region to Dry Farming. In dry-land 

 farming, while the amount of moisture supplied by the rainfall 

 is by far the most important factor, yet there are secondary 

 ones4hat must be taken into consideration. These are frequently 

 of sufficient importance to bring absolute failure if overlooked or 

 neglected. These include evaporation and the character of the 

 soil, which are of almost equal significance with the rainfall. 



(a) Rainfall. The adaptation of a region to dry farming de- 

 pends upon several factors, one of the principal ones being the 

 amount of rainfall and its distribution through the year (Pig. 

 103). Dry farming is not practical with less than 10 inches of 

 rainfall annually, but there are modifying factors. With this 

 amount the moisture must be carefully stored and conserved for 

 the crops. The margin is so narrow that a year or two with a 

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