234 IRRIGATION PRACTICE 



PERCENT OF A CROP RAISED WITH ABOUT 7% INCHES OP IRRI- 

 GATION, DUE TO THE NATURAL PRECIPITATION 



Per cent 



Wheat (grain) 83.99 



Wheat (straw) 86.42 



Oats (grain) 85.67 



Oats (straw) 98.19 



Corn (grain) 81.14 



Corn (stover) 83.03 



Alfalfa (all crops) 77.18 



Potatoes 66.89 



The results show that between 80 and 90 per cent 

 of the yield of grain of wheat, oats and corn, grown with 

 about iy% inches of irrigation water, was really produced 

 by the natural precipitation. Even larger proportions 

 of straw and stover were so produced. With the same 

 degree of irrigation, 77 per cent of a crop of alfalfa, and 

 67 per cent of a crop of potatoes, were produced by the 

 natural precipitation. 



Bark and Welch carried on similar experiments on the 

 Gooding Experiment Station, Idaho, with Blue Stem, 

 Sonora and Little Club wheats. As an average of three 

 years of work with these wheats it was found that of a 

 crop raised with about 6 inches of irrigation water, about 

 75 per cent of the yield should be credited to the natural 

 precipitation. This confirms the Utah work. True, on 

 different soils, the same precipitation would produce dif- 

 ferent results, and these figures are probably maximum 

 because of the excellent treatment given the soil and 

 crops. However, a considerable proportion, usually from 

 40 to 70 per cent, roughly one-half, of the crop obtained 

 under irrigation may be safely credited to the natural 

 precipitation, wherever the rainfall is over 12 inches 

 annually and proper methods of cultivation are practised. 

 On the other hand, when the rainfall is not conserved, the 



