218 Bulletin 88 



of the soil. Any one of these live factors can be taken as a function 

 of the other four factors. The problem is complex and should be 

 solved separately for each crop and for each locality. In some 

 communities the lands are graded level or on a very slight gradient 

 at an additional expense of $20 to $40 per acre. This outlay is of 

 doubtful utility. The lands should be graded down the natural 

 slope or aj^proximately so. Surely any lands w^ith slope from 3 to 

 40 feet per mile can be laid out and irrigated without material 

 change in the general direction of the slope. The other factors, 

 then, can be determined so as to give the most uniform distribution 

 of water. Thus, on light soil with a grade of 20 feet per mile, 

 where a large head of water is available, perhaps the lands can be 

 laid out 50 feet w^ide and 880 feet long. If the head of water is 

 small, as from a No. 5 centrifugal pump, then the lands should be 

 not over 30 feet wide and 440 feet long. If, however, the grade is 

 only 10 feet per mile, the lands, perhaps, should be 660 feet long 

 for the large head and 330 feet long for the small head. These 

 values are intended to be suggestive ; on shallow soils underlain 

 by caliche the lands can be longer; in some cases lands 1300 feet 

 long are irrigated successfully. For heavy loams the lands can be 

 considerably longer than for sandy soil, and in general the flatter 

 the grade, the shorter should be the runs and the larger should be 

 the head of water. 



The final adjustment to obtain an even distribution should be 

 made by varying the head of water in each land or in each furrow. 

 This adjustment should be made last because it is the easiest to 

 make. Recently an irrigator near Mesa complained that the stand 

 of alfalfa was better in the lower part of his field than in the upper 

 part. He wished to regrade the field so as to reduce the fall. But 

 the remedy was much simpler than that. His head of water de- 

 livered by the Reclamation Service was 300 miner's inches. By 

 changing his order and obtaining 275 miner's inches he would get a 

 uniform irrigation and uniform crop. Many irrigators have difB- 

 culty in getting the water across their land. They require a larger 

 unit head. They should order more water, or concentrate it in 

 fewer lands or furrows, or if this cannot be done without increasing 

 the unit head to a point where it will erode the soil, then the length 

 of run should be reduced. 



In cases where the distances between head ditches, especially 

 cement pipe lines, prove to be too great, special methods of irrigat- 

 ing can be used. One method is to open an intermediate head 

 ditch midway between the permanent ditches. The intermediate 



