232 IRRIGATION PRACTICE 



thought in irrigation practice. Modern irrigation was 

 founded in a very arid country, with almost rainless 

 summers, at a time when there was no science of agricul- 

 ture, and by men who had had no previous irrigation 

 experience. The practice of irrigation was, therefore, 

 founded on the assumption that irrigation was a primary 

 art, practically independent of the natural precipitation. 

 As practical irrigation experience was gathered it became 

 clear that any rainfall, even a small one, if conserved in 

 the soil, has crop-producing power, and that irrigation 

 is always a supplementary art. Irrigation is and always 

 should be supplementary to the rainfall. Consequently, 

 the first big irrigation problem is to conserve the rain- 

 fall in the soil for crop use, so that the available irriga- 

 tion water may be made to cover as much ground as 

 possible. The beginning of irrigation wisdom is the con- 

 servation of the natural precipitation for the use of 

 crops. 



141. Crop-producing power of rainfall. The theoreti- 

 cal productive power of the natural precipitation shows 

 that even a low annual rainfall, properly conserved, may 

 produce fair crops without irrigation. It has been shown 

 previously that to produce one pound of dry matter on 

 a fertile soil under arid conditions, more than 750 pounds 

 of water are seldom required. If 750 pounds of water 

 are required to produce one pound of dry matter, one 

 bushel of wheat will require for its production 90,000 

 pounds, or forty-five tons, of water. On this basis, each 

 acre-inch of water weighing about 113 tons should pro- 

 duce about two and one-half bushels of wheat. An annual 

 precipitation of 10 inches, if fully conserved, should then 

 produce twenty-five bushels of wheat per acre, which is a 

 high average crop. 



