134 



IRRIGATION PRACTICE 



rule. Further, the transpiration ratios vary considerably, 

 even under similar conditions of humidity or aridity, 

 varying from 247 to 870 pounds of water for one pound 

 of dry matter. As would be expected, the evapo-transpira- 

 tion ratios are higher than the transpiration ratios. The 

 variation among the evapo-transpiration ratios is also 

 large, varying from 270 to 1,118 pounds of water for one 

 pound of dry matter. 



The data in the above table may well be used to show 

 the average limits of the magnitudes of the transpiration 

 and evapo-transpiration ratios on good soils of the stand- 

 ard crops in different parts of the world; for, of the thou- 

 sands of determinations, not included in the table, nearly 

 all fall within the limits here given. Yet, in a given locality, 

 the transpiration ratio is not even approximately constant, 

 unless the many factors concerned in plant-growth and in 

 evaporation are constant. The variability of the water- 

 cost of dry matter is well brought out in the following 

 table, which shows the range of transpiration ratios for 

 certain standard crops in India and in Utah: 



TRANSPIRATION RATIO 



For wheat, the range was in India from 422 to 1,133 

 pounds of water, and in Utah from 258 to 2,017 pounds of 

 water for each pound of dry matter produced. Other 



