236 Irrigation and Drainage 



sionally, four times, two being the usual number ; in 

 New Mexico, the ground is irrigated once before and 

 once after seeding and five times later, making seven 

 times in all ; while in Utah the number of waterings is 

 three to five. 



The average number of irrigations appears to be 

 from three to five for wheat in all parts of the world. 

 But it should be understood that these irrigations are, 

 in all cases, supplemented more or less with natural 

 rainfall. In Colorado, for example, where the usual 

 number of irrigations is two, the rainfall from April 1 

 to July 1 is often as great as 8 inches, or two-thirds 

 the amount of water required for a yield of 40 bushels 

 per acre, thus making the number of irrigations amount 

 practically to six rather than two, and the mean interval 

 16% days, instead of 33 to 20. 



It must be remembered, further, that while the 

 irrigations of wheat are in all cases supplemented with 

 natural rainfall, the yield per acre does not average 40 

 bushels ; hence the agreement of the theoretical fre- 

 quency of irrigation, 33 to 20 days, with that actually 

 practiced is more apparent than real. 



In Egypt, maize is irrigated every 15 days, which 

 would make seven waterings for the crop. Barker states 

 that six irrigations are given to a crop in the Mesilla 

 valley, New Mexico; while in Italy three is the usual 

 number. But here, again, the spring and early summer 

 rainfall is quite large; so large, indeed, that much maize 

 is grown without irrigation. It appears, therefore, that 

 where this crop must really depend upon irrigation 

 for the water needed, it must be applied as often as 



