USE OF THE RAINFALL 233 



142. Results of dry-farming. This theoretical demon- 

 stration has been well borne out by the results of the 

 modern art of dry-farming. Dry-farming, as a system of 

 agriculture, attempts to produce profitable crops without 

 irrigation on soils that receive an annual rainfall of 

 between 10 and 20 inches. Where there are high winds or 

 other water-dissipating factors, a rainfall of from 20 to 30 

 niches a year also requires dry-farming methods. The 

 two-thirds of the area of the earth's surface receiving 

 annually less than 20 inches of rain, are the so-called arid 

 and semi-arid regions of the earth on which irrigation 

 has been held, until lately, to be a necessity for success- 

 ful crop-production. Yet, successful dry-farming, during 

 the last few years, has been practised on great areas that 

 receive between 10 and 20 inches of rainfall annually. 

 This confirms the accuracy of the theoretical deduction 

 of the crop-producing power of the rainfall. 



143. Crop value of rainfall in irrigation. Experi- 

 ments have also been performed to discover what propor- 

 tion of a crop grown under irrigation may properly be 

 credited to the natural precipitation. In the Utah work, 

 the same crop was planted on two neighboring plots. 

 One was irrigated; the other dry-farmed. Both plots 

 yielded crops, and it was assumed that the yield on the 

 plot receiving no irrigation was the same as the part of 

 the yield under irrigation, due to the natural precipitation. 

 Some of the results thus obtained are given in the follow- 

 ing table. In reading the table, it should be remembered 

 that the average precipitation under which the work was 

 done was in the neighborhood of 15 inches a year. The 

 soils were deep and of splendid water-holding power, and 

 had been carefully tilled according to the best dry-farm 

 methods, so as to conserve the natural precipitation. 



