136 IRRIGATION PRACTICE 



yielding thirty bushels of grain to the acre, if 250 pounds 

 of water are required for one pound of dry matter, would 

 require throughout the season 5 acre-inches of water; if 

 500 is the transpiration ratio, 10 inches would be required, 

 and if 1,000 is the transpiration ratio 20 inches would be 

 required. A fair crop of wheat requires annually, con- 

 sidering the quantity that evaporates directly from the 

 surface, from 5 to 20 inches throughout the season. This 

 then gives a fairly safe basis on which to establish a 

 duty of water. 



97. The seasons. The seasons, including sunshine, 

 temperature, relative humidity and all other climatic 

 factors, are of first importance in determining the acre- 

 yield of crops. In fact, for the production of dry matter, 

 the seasons overshadow any other one natural factor, and 

 usually is as important as the cultural operations. In 

 irrigated sections, where water is added at will, the 

 influence of the season is usually underestimated, for on a 

 good and well-tilled soil, even though the season is unfa- 

 vorable, the application of a sufficient quantity of water 

 makes the crop sure. Nevertheless, in the irrigated sec- 

 tions, as elsewhere, the seasons determine the average 

 crop-yields for the season, whether they shall go high or 

 fall low. 



The seasons also determine in large measure the quan- 

 tity of water used in the production of one pound of dry 

 matter. At the Utah Station, in a series of experiments 

 covering several years, it was found that, under con- 

 ditions otherwise nearly identical, the transpiration 

 ratio for wheat varied from season to season, the range 

 being from 280 to 577. In 1902, the transpiration ratio 

 for wheat was 402; in 1903, 284; in 1904, 577; in 1905, 280, 

 and in 1908, 357. Leather, working under East Indian 



