Arizona Agricultural Experiment Station 489 



readily the water can be run parallel to the steeper side of the field, 

 Vv'hile on the heavy clay soils the flatter grades are used. 



With the extension of agriculture to lands farther from the 

 rivers, steeper slopes are encountered, and during the past year 

 much new land with slopes of over 25 feet per mile has been devel- 

 oped. The question has arisen as to the desirable slope in which to 

 lay out the borders or rows. In one locality 8 feet per mile has 

 been adopted arbitrarily, although it has entailed much greater ex- 

 pense for grading than would a steeper grade for the direction of 

 irrigation. Moreover, the flatter grade introduces a heavy side-fall, 

 or cro.'^s-slope, which may become very troublesome during irri- 

 gation. 



The question involves the possibility of irrigating in furrows or 

 lands with steep gradient. Can the field be wetted uniformly? The 

 efficiency of irrigation depends upon the uniformity of distribution 

 even more than upon the average depth applied. If a large unit 

 head of water is turned into each land or furrow, the soil may be 

 washed or eroded, or, on clayey soil, the water may reach the lower 

 end and be turned off before much has been absorbed. If the head 

 is too small, then on sandy soil it will be absorbed in the upper 

 part of the field and the lower part of the field will receive a rela- 

 tively light irrigation. But the head can be varied between quite 

 wide limits, and it should be the dependent variable. On each field 

 it is necessary to ascertain by trial the optimum quantity for the 

 unit head. 



The length of the furrow, likewise, can be varied if taken in 

 time. One rancher in the Salt River Valley during the past sum- 

 mer ran water in furrows 330 feet long and his neighbor ran the 

 same stream, when it came his turn in the rotation, in furrows one- 

 half mile long. Many cases have been noted where water has been 

 wasted downward by deep percolation below the reach of plant 

 roots in the upper part of fields, and the cause assigned has been 

 that the lands were too long. But in most cases the waste of water 

 could be prevented by turning the direction of irrigation so as tcj 

 get more slope or, the more practical way, by increasing the unit 

 head of water. On the other hand, sometimes the rancher finds 

 that the lower part of his field is getting the larger proportion of 

 the water. Usually he wishes to regrade his field so as to have less 

 fall in the lands, but his more rational solution is to reduce the unit 

 head. 



The character of the soil, the crop, the slope, the length of run 



