8 DRY FARMING IN RELATION TO RAINFALL AND EVAPORATION. 



drawn on these maps where sufficient observations were available. 

 Maps of this kind have not before been available for these Western 

 States. 



CLASSIFICATION OF DRY-FARMING REGIONS. 



Dry-farming regions are naturally classified on the basis of rain- 

 fall. In this country dry farming is usually considered to be con- 

 fined to those regions in which the annual rainfall is less than 20 

 inches and more than 10 inches. This classification must be con- 

 sidered as only a rough approximation, since dry-farming districts 

 can not be outlined on the basis of the annual rainfall alone on account 

 of the great influence of the evaporation and the monthly distribution 

 of the rain in growing dry-farm crops. Furthermore, it is impossible 

 to say just where ordinary methods of farming leave off and dr}^- 

 farming methods begin. Dry-farming methods are usually under- 

 stood to mean those which lead to the conservation of moisture, but 

 the conception of what these methods actually are varies greatly in 

 different sections of the country. The dry farmer in the Great Plains, 

 where the rainfall is from 15 to 20 inches, usually gets his best results 

 with annual cropping methods combined with good tillage. On the 

 other hand, the dry farmers in that part of the Great Basin where the 

 rainfall is less than 15 inches follow the method of alternate cropping 

 and summer tillage almost exclusively. Between these two classes 

 there is a third operating under conditions where summer tillage may 

 be necessary to insure returns on every crop, but where it frequently 

 pays to take chances on producing a crop on the land each year. 



The method of alternate cropping and summer tillage is gener- 

 ally recognized as the most highly developed dry-farming method 

 because it gives better returns with a lower annual rainfall than other 

 methods. There are considerable areas in central Utah that are cul- 

 tivated in this way where the annual rainfall does not exceed 13 inches. 

 It must be remembered, however, that this is a region of winter rain- 

 fall, and that the method of alternate cropping and summer tillage 

 is particularly adapted for such regions, as we shall show later. 

 Therefore, while the method of alternate cropping and summer fallow- 

 ing is recognized as the most highly developed dry-farming method, 

 it does not follow that it is the best method for all dry-farming 

 regions or that it will always give the best returns. In dry-farming 

 sections where the rainfall is not so limited as in Utah, and espe- 

 cially in regions having a summer rainfall, other methods give as 

 good or better returns. 



IMPORTANCE OF KNOWING THE NORMAL RAINFALL. 



The rainfall in all parts of the United States is subject to wide 

 fluctuations from year to year. The West does not appear to be 

 more subject to these changes than the East, but the fluctuations 



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