118 Irrigation and Drainage 



available for irrigation purposes during the irrigation 

 season, it is capable of watering but .092 of the catch- 

 ment area at the rate of 2 inches of water once in 10 

 days. 



It is true that the mean run -off for the whole 

 basin is less than is found in much of the United 

 States ; but, taking a district where the mean drainage 

 to the sea is 30 inches instead of 6.7, and supposing 

 that this is collected into canals, so as to be used for 

 irrigation, then it would be able to supply only about 

 .4 of the area at the rate assumed above. It is 

 safe to say that these estimates of the area which 

 might be irrigated with such amounts of water is too 

 large, for the summer discharge, when irrigation is 

 needed, is in most drainage basins much less than 

 the mean values which have been taken in making 

 the calculations. 



Newell has made as close an estimate of the mean 

 annual run -off for the United States as the then ex- 

 isting data would permit, and has expressed the 

 results in a map, which is reproduced in Fig. 22. An 

 inspection of this map will make it plain, in connec- 

 tion with what has been said, that however great irri- 

 gation developments may become in the future, it is 

 not possible for the practice to be extended so as to 

 displace the methods of "dry farming." Hence the 

 question, How far may tillage compensate for a defi- 

 cient rainfall ? will long remain a pertinent one in 

 agricultural practice. 



Since much less than one -half of agricultural lands 

 can be irrigated under any efforts which can be made, 



