RESPIRATION AND OXYGEN. 



109 



value of the crop that could be produced with the superfluous water, 

 while in seasons of drought it may often reach the total value of the 

 average crop. In the third place, excessive irrigation works injury 

 to the fertility of the soil, largely as a matter of defective aeration. 

 Thus, Main (1916 : 47) has shown that continuous cropping to 

 wheat with several irrigations has reduced the yield per acre at 

 Mirpurkhas from 759 pounds in 1908-09 to 372 pounds in 1913-14, 

 in spite of the use of fertilizers. 



TABLE 28. Water-saving experiments on wheat, 1916-17. 



QUETTA. 



NOTE. In Mirpukhas one watering yielded 1,483 pounds, two yielded 1,471. 



It appears almost certain that the common practice in irrigated 

 regions involves the use of too much water, with the consequent 

 economic losses. Not the least of these is the necessary restriction 

 of irrigation systems to a smaller territory than should be the case, 

 which results in a serious limitation of opportunity and production. 

 Throughout the western United States it is a fortunate system that 

 does not face an annual or periodic deficit, while the extent of new 

 reclamation projects must unfortunately be determined by the exist- 

 ing practice rather than by the optimum duty of water. As a con- 

 sequence, it would seem an indispensable task of every great system, 

 installed or to be installed, to determine the optimum use of water 

 and to take steps to see that everyday practice conforms to the 

 findings. Moreover, while annual crops quickly show the effect of 



